Humans could naturally imagine a scene according to a piece of text or audio description. However, the underlying processes are not that straightforward to neural networks due to the inherent modality gap. Targeting to mimic human imagination and creativity in the real world, the tasks of multimodal image synthesis and editing provide profound insights into how deep neural networks correlate cross-modal information with visual attributes in generative modeling.
Image synthesis and editing aims to create realistic images or edit real images with natural textures. In the last few years, it has witnessed very impressive progress thanks to the advance of deep learning especially Generative Adversarial Networks (GANs) . To achieve more controllable generation, a popular line of research focuses on generating and editing images conditioned on certain guidance. Typically, visual clues such as segmentation maps and image edge have been widely adopted with superior synthesis and editing performance [2, 3, 4]. Beyond these visual clues, cross-modal guidance such as texts, audios, and scene graph provides an alternative but often more intuitive and flexible way of expressing visual concepts. However, effective retrieval and fusion of heterogeneous information from data of different modalities remains a grand challenge for image generation and editing.
As one pioneer effort in multimodal image synthesis,  shows that recurrent variational auto-encoder could generate novel visual scenes conditioned on image captions. The research of multimodal image synthesis is then greatly advanced with the prosperity of generative adversarial networks [1, 6, 7, 3, 2, 8, 9, 10]. For example, Reed et al. extend conditional GANs  to generate natural images based on textual descriptions. Chen et al. introduce conditional GANs to achieve cross-modal audio-visual generation of musical performances. However, the two pioneer studies conduct synthesis on restricted datasets only (e.g., CUB-200 Birds  and Sub-URMP ) with relatively low image resolution (e.g., 64 64). In the last few years, this field has achieved notable improvements owe to the improved multimodal encodings [14, 15], novel architectures [16, 17], and cycle structure ). On the other hand, these early studies largely focus on multimodal image synthesis, while the task of multimodal image editing draws much less attention.
With the development of large scale GANs, a bunch of generative networks such as BigGAN  and StyleGAN [20, 21, 22] have been developed to synthesize images with high quality and diversity from random noise input. Recent studies show that GANs can effectively encode rich semantic information in the intermediate features  and latent space  as the result of image generation. Instead of synthesizing various images by varying the latent code, GAN inversion  is introduced to invert a given image back into the latent space of a pretrained GAN model, yielding an inverted code which can faithfully reconstruct the given image by the generator. Since GAN inversion enables to control attribute directions found in latent spaces, pre-trained GANs become applicable to real image editing, without requiring ad-hoc supervision or expensive optimization. Quite a number of studies [26, 27] have explored to vary the inverted code of real images along one specific direction to edit the corresponding attribute of the image. In terms of multimodal guidance, StyleClip  leverages the power of Contrastive Language-Image Pre-training (CLIP)  models to develop a text-based interface for StyleGAN image manipulation without requiring cumbersome manual effort. Talk-to-Edit  introduces an interactive facial editing framework that allows fine-grained attribute manipulation through dialog between the user and the system through a fine-grained attribute landscape on the semantic field.
With the prevalence of Transformer model 
which naturally allows cross-modal input, impressive improvements have been achieved in several domains such as language models, image generative pre-training , and audio generation . These recent advances fueled by Transformer suggest a possible route for multimodal image synthesis. Specifically, DALL-E  demonstrates that training a large-scale auto-regressive transformer on numerous image-text pairs can produce a high-fidelity generative model with controllable results through text prompt. Taming Transformer  introduces a VQGAN with discriminator and perceptual loss [37, 38, 39] to learn discrete image representation, and demonstrates the effectiveness of combining inductive bias of CNNs with expressivity of transformers in high-resolution image synthesis. ImageBART  tackles auto-regressive (AR) image synthesis by learning to invert a multinomial diffusion process which mitigates the well-known exposure bias of AR models by introducing contextual information. NUWA  presents a unified multimodal pre-trained model that allows to generate or manipulate visual data (i.e., images and videos) with an 3D transformer encoder-decoder framework and a 3D Nearby Attention (3DNA) mechanism.
With the development of generative models and neural rendering, several other models such as Neural Radiance Fields (NeRF)  and Diffusion models [43, 44] have also been explored for achieving multimodal image synthesis and editing.
The key contributions of this survey can be summarized in the following five points:
This survey covers the contemporary literature with respect to multimodal image synthesis and editing, and provides a comprehensive overview of the recent efforts in terms of the modalities, methods, datasets, evaluation metrics, and future research directions.
We provide a foundation of different types of guidance modality underlying image synthesis & editing tasks and elaborate the specifics of encoding approach associated with each guidance modality.
By focusing on the cross-modal guidance in image synthesis and editing, we develop a taxonomy of the recent approaches according to the essential architectures and highlight the major strengths and weakness of existing methods.
We provide an overview of various datasets and evaluation metrics in multimodal image synthesis & editing, and critically examine the performance of existing methods.
This survey summarizes the open challenges in the current research with an outlook towards promising areas and directions for future research.
The remainder of this survey is organized as follows. Section 2 presents the foundation of popular guidance modalities in image synthesis and editing. Section 3 provides a comprehensive overview and description of multimodal image synthesis & editing methods with detailed pipelines. Section 4 reviews the popular datasets and evaluation metrics, with quantitative experimental results of typical methods. In Section 5, we discuss the main challenges and future directions for multimodal image synthesis & editing. A few concluding remarks are drawn in Section 6.
2 Modality Foundations
Each source or form of information can be called a modality. For example, people have the sense of touch, hearing, sight, and smell; the medium of information includes voice, video, text, etc.; and a variety of sensors, such as radar, infrared, and accelerometer. Each above data form can be called a modality. In terms of image synthesis and editing, we group the guidance modality as visual guidance, text guidance, audio guidance, and other modalities. Detailed description with dedicated processing method will presented in the sub-sections.
2.1 Visual Guidance
Visual guidance has attracted broad attention in image synthesis and editing thanks to its wide applications. Typically, visual guidance represents certain image properties in pixel space, e.g., segmentation maps [3, 2], keypoints [49, 50, 51], edge maps [52, 53], and scene layouts [54, 55, 56] as illustrated in Fig. 1. Realistic images can be naturally induced from visual guidance, as visual clues can be regarded as certain type of image which allows directly encoding with convolution layers to yield the final generation or editing results. Thanks to the accurate and clear guidance in visual information, visual guidance can be paired or unpaired with real images in image synthesis, namely, paired image translation [2, 3] and unpaired image translation [57, 58].
By editing the visual guidance such as semantic maps, image synthesis methods can be directly adapted for image manipulation tasks. In addition, visual guided image synthesis and editing can be applied in many low-level vision tasks. For example, we can achieve image colorization by putting grayscale images as visual guidance and the corresponding color images as ground truth. Other tasks like image super-resolution, image de-haze, image de-rain, etc., can be formulated in the similar way.
2.2 Text Guidance
Compared to visual guidance such as edges and object masks, text prompt provides a more flexible way to express visual concepts. The text-to-image synthesis task aims to produce clear, photo-realistic scenes with high semantic relevance to the corresponding text guidance. This task is very challenging as text descriptions are often ambiguous and can lead to numerous images with correct semantics. In addition, images and texts come with heterogeneous features, which makes it hard to learn accurate and reliable mapping across the two modalities. Thus, learning an accurate embedding of text description plays an import for text-guided image synthesis and editing.
Text Encoding. Learning useful encodings from textual representations is a non-trivial task. There are a number of traditional text representations such as Word2Vec  and Bag-of-Words . With the prevalence of deep neural networks, Reed et al.
propose to encode texts with a character-level convolutional recurrent neural network (char-CNN-RNN), that is pre-trained to learn correspondences between texts and images. Instead of using char-CNN-RNN, AttnGAN learns text encoding with a bi-directional LSTM  by concatenating its hidden states. Instead of obtaining the embedding with a pre-trained network, StackGAN 
introduces Conditioning Augmentation (CA) which randomly samples latent variables from a Gaussian distribution defined by the text embedding. This encoding technique is widely adopted as it encourages smoothness over the textual guidance manifold. With the development of pre-trained models in natural language processing field, several studies[62, 63] also explore to perform text encoding by leveraging large-scale pre-trained language models such as BERT .
Recently, Contrastive Language-Image Pre-training (CLIP)  achieves SOTA image representation performance by learning the alignment of images and the corresponding captions from a large amount of (image, text) pairs. As illustrated in Fig. 2
, CLIP jointly optimizes an image encoder and a text encoder to maximize the cosine similarity between positive pairs and minimizing that of negative pairs, yielding accurate text embeddings.
2.3 Audio Guidance
Hearing helps human to sense the world. The relation between auditory contents and visual contents has been explored in previous cross-modal research , demonstrating that specific objects can be attended while the corresponding words are pronounced. In addition, Harwath et al. explore to learn neural network embeddings from natural images and the corresponding speech waveforms describing the images. With natural image embedding as an inter-lingual, the experiments in  show that the learnt models allow to perform cross-lingual speech-to-speech retrieval. Sounds can not only interact with visual contents but also capture rich semantic information. For example, by transferring the knowledge from other pre-trained scene and object recognition model, SoundNet  (a deep model for sound recognition) can learn to identify scenes and objects by using auditory contents only.
Audio Encoding Recently, there is research  generating sounds from given videos where deep convolution network is employed to extract features from video screenshots followed by LSTM  to generate waveform that is correspond to input video. There are some works [12, 70, 71] that generate images condition on sounds. Specially, Wang et al. first represent the input sound segment by a sequence of features which can be spectrograms, fbanks, and mel-frequency cepstral coefficients (MFCCs), and the hidden layer outputs of the pre-trained SoundNet model 
. Then all the features in the sequence are averaged into a single vector which is taken as the condition for image generation.
Audio-driven talking face generation is one of the important applications of audio guided image synthesis, which has attracted increasing interest in recent years [72, 73, 74, 75, 76, 47, 77]. For instance, Chung et al. propose an encoder-decoder CNN model that generates talking faces from a joint embedding of face and the corresponding audio. Song et al. introduce a conditional RNN network for adversarial generation of talking face. Chen et al. design a hierarchical structure that first predicts facial landmarks from the audio and then generates faces conditioned on the landmarks. However, the head pose is almost fixed in the talking faces generated by these approaches. To improve the perceptual realism, recent approaches [75, 78, 47, 79] take head pose into consideration when generating talking face.
2.4 Other Modality Guidance
Image generation from textual descriptions usually work on simple scenes (e.g., CUB-200 Birds ) but struggle in complex scenarios with multiple contextual objects. Therefore, Johnson et al. propose to generate images from scene graphs which define the explicit relationship among objects. Specifically, the guided scene graph is encoded through a graph convolution, yielding a scene layout by predicting bounding boxes for objects. Then, realistic images can be generated through adversarial training against a pair of discriminators.
Some work aims to synthesize images conditioned on certain specific parameters. For example, Liu et al. explore to generate one-hot images conditioned on point coordinates in (x, y) Cartesian space. EMLight [82, 83] and NeedleLight  aim to synthesize High Dynamic Range (HDR) scene illumination conditioned to a set of lighting parameters.
We broadly categorize multimodal image synthesis and editing methods into four categories: the GAN-based methods (Sec. 3.1), the GAN inversion methods (Sec. 3.2), the Transformer-based methods (Sec. 3.3), and other methods (Sec. 3.4). We first discuss the GAN-based methods and GAN inversion methods, which generally rely on generative adversarial networks. We then discuss the prevailing Transformer-based frameworks comprehensively. Finally, we present several different methods in multimodal image synthesis and editing.
3.1 GAN-based Methods
Different GAN-based networks have been designed for multimodal image synthesis and editing. We will introduce them according to their adopted guidance modality in the following subsections.
3.1.1 Visual Guidance
Visual guidance in multimodal image synthesis and editing includes paired visual guidance and unpaired visual guidance.
Paired Visual Guidance. Paired visual guidance means the provided guidance is accompanied with corresponding ground truth images to provide certain direct supervision. Except for adversarial loss, image synthesis with paired visual guidance is usually trained with certain supervised loss between the generated image and the ground truth. Isola et al.
first investigate conditional GAN as a general framework named Pix2Pix for various image translation tasks (e.g., edge-to-image, day-to-night, and semantic-to-image). To mitigate the constraint in high-resolution image synthesis in Pix2Pix , Wang et al. propose Pix2PixHD that enables to synthesize images of 20481024. However, Pix2Pix  as well as its variant  cannot encode complex scene structural relationships between the guidance and real images when there exist very different views or severe deformations. Therefore, Tang et al. proposed an attention selection module to align the cross-view guidance with the target view. On the other hand, previous methods directly encode the visual guidance with deep networks for further generation which is suboptimal as part of the guidance information tends to be lost in normalizatin layers. SPADE  is designed to inject the guided feature effectively through a spatially-adaptive de-normalization as shown in Fig. 3. SEAN  introduces a semantic region-adaptive normalization layer to achieve region-wise style injection. Claiming that traditional image translation networks [2, 85, 3] suffer from high computational cost while handling high-resolution images, Shaham et al.  propose ASAPNet which is a lightweight yet efficient network for the translation of high-resolution images. Recently, Zhang et al. and Zhan et al. introduce exemplar-based image translation frameworks which build dense correspondence between exemplar and condition input to provide accurate guidance. However, building the dense correspondence incurs quadratic memory cost. Thus, Zhou et al. proposed to leverage PatchMatch  with GRU assistance to build high-resolution correspondence efficiently. In addition, Zhan et al. introduce a bi-level alignment scheme to reduce memory cost while building dense correspondence.
At the other end, as the mapping between visual guidance and real images are naturally non-deterministic, several studies [52, 93] focus on learning to map the same guidance to different images which leads to diverse generation outcome. For example, BicycleGAN  combines cVAE-GAN [94, 95, 39] and cLR-GAN [96, 97, 98] to generate diverse and realistic outputs. Besides, image translation models can also be combined with disentangled representation methods [96, 99, 100, 101] to achieve diverse outputs by randomly sampling disentangled features (e.g., style) from a Gaussian distribution. For instance, Gonzalez-Garcia et al. propose to disentangle the domain feature representation into shared part across domains, and two exclusive parts for specific domain, enabling diverse generation by sampling from the disentangled domains.
Unpaired Visual Guidance. Unpaired image synthesis utilizes unpaired training images to convert images from one domain to another. The generation of realistic images mainly relies on adversarial learning  with certain constraint losses. Specially, Zhu et al. design a cycle-consistency loss to preserve the image content by ensuring the input image can be recovered from the translation result. However, cycle-consistency loss is too restrictive for image translation as it assumes a bi-jectional relationship between the two domains. Several studies [103, 104, 105] thus aim to explore one-way translation and bypass the bijection constraint of cycle-consistency. With the emergence of contrastive learning, CUT 
proposes to maximize the mutual information of positive pairs via noise contrastive estimation for the preservation of contents in unpaired image translation. Andonian et al.
introduce contrastive learning to measure the inter-image similarity in paired image translation. However, there exist mappings between the two domains where an individual image in one domain may not share any characteristics with its representation in the other domain after mapping. Therefore, TravelGAN proposes to preserve the intra-domain vector transformations in a latent space learned by a siamese network, which enables to learn mappings between more complex domains that are very different from each other.
3.1.2 Text Guidance
Reed et al. are the first who extend conditional GANs  for image synthesis conditioned on textual descriptions. Empowered by the advance of GANs for image synthesis, text guided image synthesis has made significant progress with the employment of stacked architecture, attention mechanism, siamese architecture, cycle consistency, and adapting unconditional models.
Stacked Architectures Targeting to synthesize high-resolution images, stacked architectures are widely adopted in text-to-image synthesis. Specially, StackGAN  generates a coarse image of at the first stage, followed by a second generator to further output an image of at the second stage. StackGAN++  further improves StackGAN  by jointly training three generators and discriminators. Instead of using multiple generators, HDGAN  propose to employ hierarchically-nested discriminators at multi-scale layers to generate high-resolution images. PPAN  proposes to employ an auxiliary classification loss and a perceptual loss  based on a pre-trained VGG  network.
Attention Mechanisms By allowing the model to focus on specific part of an input, attention mechanisms have proven to be beneficial to language and vision models [112, 31]. In terms of text guided image synthesis, AttnGAN  incorporates attention mechanisms in a multi-stage manner to synthesize fine-grained details based on both relevant words and global sentence. Huang et al. introduce an attention mechanism between text words and object regions obtained from bounding boxes. SEGAN  introduces an attention regularization term  that only preserve the weights for keywords with zero weight for other words. As the spatial attention in  mainly focuses on color information, ControlGAN  proposes a word-level spatial attention which allows to correlate the words with the corresponding semantic region.
To ensure cycle consistency for the text prompt or encoded text feature, some works explore to pass the generated images through an image captioning[117, 18] or image encoder network  as shown in Fig. 4. Specifically, PPGN  employs an image captioning model to iteratively retrieve a latent code which maximizes a feature activation of the corresponding image according to a feedback network. Inspired by CycleGAN , cycle-consistent re-description architectures [18, 119] allows to learn a consistent feature embedding between images and the corresponding text description. Specially, MirrorGAN  aims to re-describe the generated images via a semantic text regeneration and alignment module. Inspired by adversarial inference methods , Lao et al. proposed to disentangle style and content in the latent space, with a cycle consistency loss to learn consistent encoder and decoder.
Adapting Unconditional Models Grounded in the progress of large-scale GAN models [20, 19], several studies explore to leverage the architecture of large-scale models for text-to-image generation. Specially, textStyleGAN  extends StyleGAN  to achieve high-resolution image synthesis from text guidance. Similar to , Bridge-GAN  employs a progressive generator and discriminator with cross-modal projection matching and cross-modal projection classification losses  to align generated images with text description. Built on BigGAN  which achieves SOTA performance on conditional image synthesis, Souza et al.
propose to generate interpolated sentence embeddings leveraging the available captions corresponding to a particular image. Similarly, TVBi-GAN employs the architecture of BiGAN  with a latent space defined in ALI  to project sentence features.
3.1.3 Audio Guidance
The task of audio-driven talking face generation aims to synthesize talking faces that say the given audio clips , which has wide applications in digital face animation, film production, visual dubbing, etc. One fundamental challenge in audio-driven talking face generation is how to accurately convert audio contents into visual information. Leveraging generative adversarial models , researchers develop different techniques to address this challenge. For instance, Chung et al. learn the joint embedding of raw audio and video data and project it to image plane with a decoder to generate talking faces. Following , Zhou et al. propose DAVS that learns a disentangled audio-visual representation which helps improve the quality of the synthesized talking faces. Chen et al. design a hierarchical structure that first maps the audio clip into facial landmarks and further generates talking faces based on the landmarks. Zhou et al. introduce MakeItTalk that predicts speaker-aware facial landmarks from the speech content for better preserving the characteristic of the speaker. Yi et al. propose to map audio content to 3DMM parameters  for guiding the generation process of talking faces. Zhou et al. present PC-AVS that achieves pose-controllable talking face generation by learning disentangled feature spaces of pose, identity, and speech content.
3.2 GAN Inversion Methods
GANs [1, 20] have achieved remarkable progress in high-resolution and realistic image synthesis. To bridge real and fake image domains, a series of studies aim to invert a given image back into the latent space of a pre-trained GAN model, which is termed as GAN inversion. We first define the problem of GAN inversion under a unified mathematical formulation. The generator of an unconditional GAN learns the mapping , where and denote the spaces of latent codes and real images. When are close in the space, the corresponding images are visually similar. GAN inversion maps data back to latent code that be fed into pre-trained generator to reconstruct . Formally, denoting the signal to be inverted as , the well-trained generator as , and the latent code as , GAN inversion can be formulated as below:
where is a distance metric in the image or feature space. Typically, can be based on , , perceptual  or LPIPS  metrics. Some other constraints on latent codes  or face identity  could also be included in practice. With the obtained latent , we can faithfully reconstruct the original image and conduct image manipulation in the latent space.
In terms of multimodal image synthesis and editing, the key lies in how to edit or generate latent code according to the corresponding cross-modal guidance.
|Photo Raphael Painting|
|Dog The Joker|
|Dog Nicolas Cage|
|Church The Shire|
3.2.2 Cross-modal Matching in Latent Space
TediGAN  proposes to achieve multimodal image synthesis & editing by matching the embeddings of images and cross-modal input (e.g., semantic map, text) in a common embedding space as shown in Fig. 6. Specifically, a cross-modal encoder is trained to learn the embeddings with a visual-linguistic similarity loss and a pairwise ranking loss [131, 132]. To preserve the identity after editing, an instance-level optimization module is employed in the objective which enables to modify the target attributes according to text description. As texts are encoded into the StyleGAN latent space, TediGAN inherently allows image generation with given multimodal inputs. To perform text guided image manipulation, TediGAN encodes both the image and the text into a shared latent space, and then image manipulation can be performed through style-mixing.
3.2.3 Image Code Optimization in Latent Space
Instead of mapping the text into the latent space, a popular line of research aims to optimize the latent code of the original image directly, guided by certain loss that measures cross-modal consistency.
Particularly, Jiang et al. propose to optimize the image latent code through a pre-trained fine-grained attribute predictor which pushes the output latent code to change in a direction consistent with the text description. The attribute predictor also helps to keep the other irrelevant attributes unchanged through a cross-entropy score indicating whether the predicted attribute is consistent with its ground-truth label. However, this attribute predictor is specially designed for face editing with fine-grained attribute annotations, which make it hard to be generalized for other scenes.
Rather than employing specific attribute predictor, several concurrent projects use Contrastive Language-Image Pre-training (CLIP) 
to guide text-to-image generation through optimization. Aiming for text-guided image inpainting, Bauet al. define a semantic consistency loss based on CLIP to optimize latent code inside the inpainting region to achieve semantic consistency with the given text. StyleClip  proposes to use pre-trained CLIP as the loss supervision to make the manipulated results match the text condition as illustrated in Fig. 7 Besides, StyleCLIP also introduces a latent residual mapper trained for a specific text prompt. Given a starting point in latent space (the input image to be manipulated), the mapper yields a local step in latent space. Finally, StyleCLIP introduce a method for mapping a text prompt into an input-agnostic (global) direction in StyleGAN’s style space, providing control over the manipulation strength as well as the degree of disentanglement.
3.2.4 Domain Generalization
However, StyleCLIP requires to train a separate mapper for each specific text description which is not flexible in real applications. HairCLIP  thus introduces a hair editing framework that supports different texts by exploring the potential of CLIP to go beyond measuring image text similarity. Specially, HairCLIP introduce a shared condition embedding strategy which unifies the text and image conditions into the same domain. With the strategy of shared condition embedding, StyleCLIP possesses certain extrapolation capabilities after training with only a limited number of hair-editing descriptions, which allows to produce reasonable editing for texts that never appear in the training descriptions.
Instead of generalization on text description, StyleGAN-NADA  presents a text-guided image editing method that allows to shift a generative model to new domains, without having to collect even a single image from those domains as illustrated in Fig. 8. The domain shift is achieved by modifying the generator’s weights towards images aligned with the driving text, along certain textually-prescribed paths in CLIP’s embedding space. Through natural language prompts and a few minutes of training, StyleGAN-NADA can adapt a generator across a multitude of domains as illustrated in Fig. 9
3.3 Transformer-based Methods
3.3.1 Transformer Preliminary
Leveraging their powerful attention mechanisms, Transformer [31, 33, 35, 136] models have emerged as a paradigm in sequence-dependent modeling. Inspired by the success of GPT model  in natural language modeling, image GPT (iGPT)  employs Transformer for auto-regressive image generation, by treating the flattened image sequences as discrete tokens. The plausibility of generated images demonstrate that the Transformer model is able to model the spatial relationships between pixels and high-level attributes (texture, semantic, and scale).
As Transformer models inherently support multimodal inputs, a series of studies have been proposed to explore multimodal image synthesis based on Transformer [36, 35]. Overall, the pipeline for Transformer-based image synthesis consists of a vector quantization process to achieve discrete representation and compress the data dimensionality, and an auto-regressive modeling process which establishes the dependency between discrete tokens in a raster-scan order.
3.3.2 Discrete Vector Representation
Directly treating all image pixels as a sequence for auto-regressive modeling is expensive in terms of memory consumption as the self-attention mechanism in Transformer incurs quadratic memory cost. Chen et al.
adopt a color palette to reduce the dimensionality to 512 while faithfully preserving the main structure of original images, which is generated by k-means clustering of RGB pixel values with k=512 from ImageNet dataset. However, k-means cluster only reduces the size of codebook dimensionality and the sequence length is still unchanged. Thus, the Transformer model still cannot be scaled to higher resolutions, due to the quadratically increasing cost in sequence length. To this end, Vector Quantised VAE  is introduced with a series of improvements in terms of Gumbel Softmax, extra loss, and Transformer Architecture.
VQ-VAE. Instead of conducting quantization in pixel space, Oord et al.propose VQ-VAE  to quantize image patches into discrete tokens with a learnt vector codebook as shown in Fig. 10. Consider an image , VQ-VAE allows to achieve a discrete representation , where comes from a visual codebook (vocabulary) consisting of -dimensional code. For simplicity, we have , where with indicates the spatial location of the feature. Specifically, VQ-VAE consists of an encoder, a feature quantizer, and a decoder. The image is fed into the encoder to learn a continues representation , where . Then the continues feature is quantized by the feature quantizer, by assigning it to the nearest codebook entry:
where quantization module maps the feature to an index of the codebook, and quantization module reconstructs the feature from the index. Then the decoder aims to reconstruct the original image from the quantized feature , yielding a reconstruction result as denoted by
. To make the quantization process -differentiable, the gradient is approximated with the straight-through estimator by copying the gradient from decoder to encoder . Thus, the full learning objective of VQ-VAE can be formulated as:
Gumbel Softmax. However, the vanilla VQ-VAE with argmin operation suffers from severe codebook collapse, e.g., only few codebook entries is effectively utilized for quantization. As shown in Fig. 11 (images are from 111https://github.com/CompVis/taming-transformers/issues/67), most codes in the original VQ-VAE are invalid or not utilized for feature encoding. Recently, vq-wav2vec  introduces Gumbel Softmax  to replace argmin for quantization. The Gumbel-Softmax allows to sample discrete representation in a differentiable way through straight-through gradient estimator . At training stage, with an outputs logits for the Gumbel-Softmax, the predicted distribution for the -th discrete representation is
are samples from uniform distribution. Then the gradient of the Gumbel-Softmax outputs is can be used for back-propagation. At inference stage, largest index in are picked.
Extra Loss. To achieve good perceptual quality for the reconstructed image, Esser et al. propose VQGAN which incorporates an adversarial loss with a patch-based discriminator and a perceptual loss [37, 38, 39]
for image reconstruction in VQVAE. Instead of using VGG features pre-trained on ImageNet, Donget al. leverage self-supervised trained network [143, 64] for the learning of deep visual features to enforce perceptual similarity during the dVAE training. With the extra adversarial loss and perceptual loss, the image quality is clearly improved compared with the original pixel loss for image reconstruction as shown Fig. 12.
In above approaches, a convolution neural network (CNN) is learned to quantize and generate images. Instead, Yuet al. propose ViT-VQGAN which replaces the CNN encoder and decoder with Vision Transformer (ViT) . Given sufficient data (for which unlabeled image data is plentiful), ViT-VQGAN is shown to be less constrained by the inductive priors imposed by convolutions. Furthermore, ViT-VQGAN yields better computational efficiency on accelerators, and produces higher quality reconstructions. Besides, ViT-VQGAN also introduce two improvements that can significantly encourage the codebook usage even with a larger codebook size, including 1). factorized codes that introduce a linear projection from the output of the encoder to a low dimensional latent variable space for code index lookup which boost the codebook usage substantially. 2). normalization on the encoded latent variables and codebook latent variables which improves training stability and reconstruction quality.
3.3.3 Auto-regressive Modeling
Autoregresssive model has been widely explored for building sequence dependency. Previously auto-regressive models such as PixelCNN  struggle in modeling long term relationships within an image thanks to the limited receptive field. With the prevailing of Transformer , Parmar et al. develop an Transformer-based generation model with enhanced receptive field that allows to sequentially predict each pixel conditioned on previous prediction results.
Auto-regressive (AR) modeling is representative objective to accommodate sequence dependencies, complying with the chain rule of probability. The probability of each token in the sequence is conditioned on all previously prediction, yielding a joint distribution of sequences as the product of conditional distributions:. During inference, each token is predicted auto-regressively in a raster-scan order. A top- sampling strategy is adopted to randomly sample from the most likely next tokens, which naturally enables diverse sampling results. The predicted tokens are then concatenated with the previous sequence as conditions for the prediction of next token. This process repeats iteratively until all the tokens are sampled.
Sliding Window Sampling To speed up auto-regressive image generation, Esser et al. employ a sliding-window strategy to conduct sampling from the trained Transformer model as illustrated in Fig. 13. Instead of estimating current results leveraging all previous predictions, sliding window strategy only utilizes the predictions within a local window which reduces the inference time significantly. As long as that the spatial conditioning information is available or the dataset statistics are approximately spatially invariant, the local context in the sliding window is sufficient for the faithful modeling of images sequences. Actually, this is not a constraint as we can simply model the sequence condition on image coordinates when it is violated (i.e., unconditional image synthesis on aligned data).
Bidirectional Context. On the other hand, previous method incorporates image context in a raster-scan order by attending only to previously generation results. This strategy is unidirectional and suffers from sequential bias as it disregards much context information until autoregression is nearly complete. It also ignores much contextual information in different scales as it only processes the image on a single scale. Grounded in above observation, ImageBART 
proposes to address the unidirectional bias of autoregressive modelling and the corresponding exposure bias a coarse-to-fine approach in a unified framework. Specifically, it handles the learning of data density in a coarse-to-fine approach. The compressed contextual information of the image obtained in the coarse stage is provided for the autoregressive modeling on the next finer stage. A diffusion process is applied to successively eliminate information, yielding a hierarchy of representations which is further compressed via a multinomial diffusion process[148, 149]. By training a Markov chain, above diffusion process is can be inverted to recover the data from the hierarchy representation. By modeling the the Markovian transition autoregressively with attending to the preceding hierarchical state, crucial global context can be leveraged for each individual autoregressive step.
3D Nearby Self-Attention. With obtained discrete tokens, early work leverage PixelCNN or PixelRNN to build the sequence dependency. To cover language, image, and video at the same time for different scenarios, a 3D transformer encoder-decoder framework is designed, which can not only deal with videos as 3D data but also adapt to texts and images as 1D and 2D data, respectively. A unified 3D Nearby Self-Attention (3DNA) module which supports both self-attention and cross-attention can be defined as below:
where both and are 3D representations. If , 3DNA denotes the self-attention on target and if , 3DNA is cross-attention on target conditioned on . denotes learnable weights.
With a coordinate under , the corresponding coordinate under can be denoted by after a linear projection. Then, the local neighborhood around with a width, height and temporal extent is defined in Eq. (7),
is a sub-tensor of conditionand consists of the corresponding nearby information that needs to attend. With three learnable weights , the output tensor for the position is denoted in Eq. (8)(11):
where the position queries and collects corresponding nearby information in . This also handles , then just queries the nearby position of itself. 3NDA not only reduces the complexity of full attention from to , but also shows superior performance.
3.4 Other Methods
With the development of generative models and neural rendering, other up-to-date models are also explored for multimodal image synthesis and editing.
Neural Radiance Fields. Neural radiance fields (NeRF) 
achieves impressive performance for novel views synthesis by using neural network to define an implicit scene representation. Specially, a fully-connected neural network is adopted in NeRF, by taking a spatial location (x, y, z) with the corresponding viewing direction (, )) as input, and the volume density with the corresponding emitted radiance as ground truth. Specially, AD-NeRF  proposes to achieve high-fidelity talking-head synthesis based on the framework of neural radiance fields. Different from previous methods which bridges audio input and video output based on the intermediate representations, AD-NeRF directly feed the audio feature into a implicit function to yield a dynamic neural radiance field, which is further leveraged to synthesize high-fidelity talking-head video accompanied with the audio via volume rendering as shown in Fig. 17. CLIP-NeRF  introduces a multimodal manipulation method for neural radiance fields (NeRF). By leveraging the Contrastive Language-Image Pre-Training (CLIP) model, CLIP-NeRF allows to manipulate NeRF according to a short text prompt or an exemplar image. To bridge generative latent space and the CLIP embedding space, two code mappers are designed to optimize the latent codes towards the targeted manipulation driven by a CLIP-based matching loss.
Diffusion Models. Recently, diffusion models such as denoising diffusion probabilistic models (DDPM) [43, 149] and score-based generative models [44, 153] have achieved great successes in image generation tasks [43, 154, 153, 155]. The latest works [153, 156]
have demonstrated even higher quality of image synthesis performance compared to variational autoencoders (VAEs), flows [158, 159], auto-regressive models [160, 161] and generative adversarial networks (GANs) [1, 20]. Furthermore, a recent denoising diffusion implicit models (DDIM)  further accelerates sampling procedure and enables nearly perfect inversion . In terms of multimodal image synthesis & editing, Avrahami et al. are the first to performing region-based editing in generic natural images, by leveraging the text description along with the region of interest. Kim et al. propose a DiffusionCLIP which performs text-driven image manipulation with diffusion models by using CLIP loss to steer the edit towards the given text prompt as shown in Fig. 18. Besides, Gu et al. present a vector quantized diffusion (VQ-Diffusion) model for text-to-image generation. Specially, VQ-Diffusion propose to model latent space of a vector quantized variational autoencoder 
by learning a parametric model using a conditional variant of the Denoising Diffusion Probabilistic Model (DDPM)[43, 149]. GLIDE 
compares CLIP guidance and classifier-free guidance in diffusion models for the text-guided image synthesis, and concludes that a diffusion model of 3.5 billion parameters with classifier-free guidance outperforms DALL-E in terms of human evaluation.
Style Transfer CLVA  proposes to manipulate the style of a content image through text guidance, by comparing the contrastive pairs of content image and style instruction to achieve the mutual relativeness. CLIPstyler  propose to achieve text guided style transfer by training a lightweight network which transform a content image to follow the text condition by matching the similarity between the CLIP model output.
4 Experimental Evaluation
Datasets are at the core of image synthesis and editing task. We will introduce the widely adopted datasets for multimodal image synthesis and editing in the ensuing subsections.
4.1.1 Visual Guidance Datasets
Semantic Segmentation: ADE20K
dataset is annotated with a 150-class semantic segmentation. Image generation can be conducted by using its semantic segmentation as conditional inputs. Besides, COCO-Stuff
, and Cityscapes also serve as the benchmark datasets for semantic image synthesis.
Scene Layout: Datasets with bounding box annotation can be applied to layout-to-image generation. Specially, Cityscapes , ADE20K , and COCO-Stuff  are commonly adopted for benchmarking.
Edge Map: CelebA-HQ  consists of 30,000 high quality face images. By connecting the face landmarks as face edges, Canny edge detector can be applied to detect the edges in the background for guided image synthesis. Besides, several edge-to-photo datasets are introduced in .
4.1.2 Text Guidance Datasets
Widely adopted datasets for text-to-image synthesis research are Oxford-120 Flowers , CUB-200 Birds , and COCO . Both Oxford-102 Flowers  and CUB-200 Birds  are relatively small datasets in which each image only contains a single object associated with 10 captions. In contrast, COCO  are much larger dataset with around 123k images and contains multiple objects in complex scenes as shown in 19.
On the other hand, lots of works for text guided image synthesis and editing rely on pre-trained text and image representation models. These models are mainly trained on MS-COCO , Visual Genome , and YFCC100M . However, MS-COCO and Visual Genome are relatively small datasets with roughly 100,000 images for training. Although YFCC100M contains 100 million images, the metadata for each image presents varying quality and only 15 million images are associated with natural text descriptions. CLIP  constructs a new dataset named WebImageText which consists of 400 million (image, text) pairs collected from the Internet.
4.1.3 Audio Guidance Datasets
VoxCeleb2  contains 6,112 celebrities with more than 1 million utterances. The videos present varying qualities, e.g., large head pose movements, low-light conditions, and different extents of blurry.
Lip Reading in the Wild (LRW)  contains over 1000 utterances of 500 different words. The videos in this dataset are associated with high-quality and near-frontal faces.
4.1.4 Other Guidance Datasets
Image synthesis conditioned on scene graph can be conducted Visual Genome  which provides annotation of the scene graphs, and COCO-Stuff  where synthetic scene graphs are constructed from ground-truth object positions.
|Souza et al. ||4.23||11.17||-|
|Wang et al. ||5.06||12.34||86.50|
denotes the result obtained by using the corresponding open-source code.
|Lao et al. ||-||37.94|
|Souza et al. ||3.71||16.47|
|Real Images ||34.88||6.09||68.58|
|Hong et al. ||11.46||-||-|
|Wang et al. ||29.03||16.28||82.70|
|Rombach et al. ||34.7||30.63||-|
|Pavllo et al. ||-||19.65||-|
|LRW ||VoxCeleb2 |
|Rhythmic Head ||-||-||-||-||0.779||0.802||14.76||3.8|
|PC-AVS  (Fix Pose)||0.815||0.180||6.14||6.3||0.820||0.084||7.68||5.8|
4.2 Evaluation Metrics
Precise Evaluation metrics are of great importance to drive progress in the fields. However, the evaluation of synthesized images is a challenging tasks as multiple attributes account for a fine generation result and the notion of image evaluation is often subjective. Normally, synthesized images are usually assessed from two aspects including image quality and alignment with the guidance.
4.2.1 Image Quality Metrics
A number of metrics have been introduced for the evaluation of generated image quality.
Inception Score (IS). IS  is computed through a conditional distribution which is obtained through a pre-trained Inception-v3 network . The IS roughly measures the categorical distinction and overall variation between images, by computing the Kullback-Leibler (KL) divergence between and as shown below:
On the other hand, IS is restrictive evaluation metric as it struggles to detect overfitting results (the model memorizes the training set) and measure intra-domain variation (the model only produces one good sample to reach a high IS).
Fréchet Inception Distance (FID)
With visual features extracted by a pre-trained Inception-v3 model, FID  measures the distance between the real image distribution and generated image distribution. Compared with IS, FID is a more consistent evaluation metric as it captures various kinds of disturbances . Specially, FID assumes that the activations of last pooling layer in the pre-trained Inception-v3 model follow a multidimensional Gaussian distribution. Denoting the mean and covariance of real and synthesized data as and , respectively, the FID between real and synthesized data can be formulated as:
On the other hand, FID share the same problem with IS such as struggling to detect overfitting results.
Except for above common image quality metrics, some evaluation metrics are specially designed for certain generation tasks. For image synthesis conditioned on semantic map, the image quality can be assessed by leveraging pre-trained segmentation model to compute the mean average precision (mAP) and pixel accuracy (Acc). In terms of image editing, some works [207, 92] also explore to construct paired evaluation set, where other metrics such as LPIPS , PSNR and SSIM can be applied to conduct accurate evaluation.
4.2.2 Guidance Alignment Metrics
Except for the visual realism, the synthesized images are also expected to match the corresponding guidance (e.g., semantic map, a text description, reference audio). Thus, some evaluation metrics has been introduced to assess the alignment between the guidance and synthesized images, including R-precision  and Visual-Semantic similarity (VS)  for semantic image synthesis tasks, and Semantic Object Accuracy (SOA)  for text-guided image synthesis.
R-precision. R-precision  measures the semantic similarity between the text guidance and synthesized images. With the ground truth caption and several random captions sampled from the dataset, the similarity between image features and the text embedding is calculated through the Cosine distance, followed by a ranking in decreasing similarity. It will be regarded as a successful matching if the ground truth caption is ranked as the top (normally, is set to 1, the number of randomly sampled captions is 99).
Visual-Semantic (VS) Similarity. VS similarity  is designed to measure the alignment between text description and synthesized images through a trained cross-modal embedding model which maps texts and images into a common representation space. Denoting text encoder and image encoder as and , respectively, the similarity is computed as below:
On the other hand, the standard deviation of VS score is very high, which hinders it from evaluating the model performance precisely.
Captioning Metrics. Hong et al. propose to evaluate the relevance between the generated image and the corresponding text by generating captions for the generated images with image caption generator . The generated captions are expected to be similar to the guided captions for image generation, which can be measured by natural language metrics such as BLEU , METEOR , and CIDEr 
. However, many natural language metrics is computed based on n-gram overlap which may not coincides with human judgement.
Semantic Object Accuracy (SOA).
Hinz et al.propose to evaluate the semantic alignment of individual objects specifically with a pre-trained object detector . Specifically, two metrics SOA-C and SOA-I are designed to report the recalls as class average and image average, respectively. On the other hand, SOA may not be suitable for the evaluation of interaction and relationship between objects as it assumes the text description is roughly a word-list of the visual objects.
4.2.3 User Studies
Different from above heuristic metrics, some works directly perform evaluation by conducting user studies. Normally, users are presented with several sets of generated images and are asked to pick or rank images according to certain criterion. The evaluation method of user study is flexible as it can be designed according to the specific tasks and evaluation targets. We take the evaluation of semantic alignment in text-guided generation task as an example. With a number of images generated with randomly sampled captions, the participating users are presented with guided text and images generated by compared model to pick or rank the images according to the alignment with the caption.
On the other hand, the setting of user studies (e.g., number of users) are usually not consistent in different experiments, which makes the comparisons between the user studies in different papers unreliable. Besides, different person may possess distinctive opinion towards the same concept, which also makes the results less reliable. Employing large number of users could improve the reliability, while large scale studies are costly and time consuming.
4.3 Experimental Results
We quantitatively compare the image synthesis performance of different models in terms of visual guidance, text guidance, and audio guidance.
4.3.1 Visual Guidance
For visual guidance, we mainly conduct comparison on image synthesis conditioned on semantic map as there are numbers of works for benchmarking. The experimental comparison is conducted on three challenging datasets: ADE20K , ADE20K-outdoors , COCO-stuff  and Cityscapes , following the setting of . The evaluation is performed with Fréchet Inception Distance (FID)  and mean Intersection-over-Union (mIoU). Specially, the mIoU aims to assess the alignment between the generated image and the ground truth segmentation via a pre-trained semantic segmentation network. Pre-trained UperNet101 , multi-scale DRN-D-105 , and DeepLabV2  are adopted for Cityscapes, ADE20K, and COCO-Stuff, respectively.
4.3.2 Text Guidance
4.3.3 Audio Guidance
5 Future Challenges &Directions
Though multimodal image synthesis and editing have made notable progress and achieved superior performance in recent years, there exist several challenges for their practical applicability. In this section, we overview the typical challenges, share some recent efforts for addressing them, and highlight the future research directions.
5.1 Towards Integrating All Modalities
As current datasets mainly provide annotations in a single or a few modalities, most existing methods focus on image synthesis and editing conditioned on guidance from a single modality. However, humans possess the capability of creating visual contents with guidance of multiple modalities concurrently. Targeting to mimic the human intelligence, the generation models are expected to be able to handle guidance from multiple modalities concurrently. To achieve that, a comprehensive dataset which is equipped with annotations from all modalities (e.g., semantic segmentation, text description, audios) needs to be created.
5.2 Evaluation Metrics
The evaluation of multimodal image synthesis and editing is still an open problem. Leveraging pre-trained models (e.g., FID) to conduct evaluations is constraint to the pre-trained datasets, which tends to pose discrepancy with the target datasets. User study recruits human subjects to assess the synthesized images directly, which is however too subjective. Designing accurate yet faithful evaluation metrics is thus very meaningful and critical to development of multimodal image synthesis and editing.
5.3 Model Architecture
With inherent support for multimodal input, Transformer model has been established as a new paradigm for multimodal image synthesis and editing. However, Transformer model suffers from slow inference speed, which is more severe in high-resolution image synthesis. How to design an architecture with natural support for multimodal inputs and fast inference speed remains a grand challenge to explore.
This review has covered main approaches for multimodal image synthesis and editing. We provide an overview of different guidance modalities including visual guidance, text guidance, audio guidance, and other modal guidance (e.g., scene graph). We then provided a detailed introduction of the main image synthesis & editing paradigms: GAN-based method, GAN inversion method, Transformer-based method, and other methods (e.g., NeRF, Diffusion model). The corresponding strengths and weaknesses were comprehensively discussed to inspire new paradigm that takes advantage of the strengths of existing frameworks. After the introduction of methods, we conduct a comprehensive survey of datasets and evaluation metrics for image synthesis conditioned on different guidance modalities. Then, we tabularize and compare the generation performance of existing approaches in different multimodal synthesis & editing tasks. Last but not least, we provided our perspective on the current challenges and future directions related to integrating all modalities, comprehensive datasets, evaluation metrics, and model architecture.
This study is supported under the RIE2020 Industry Alignment Fund – Industry Collaboration Projects (IAF-ICP) Funding Initiative, as well as cash and in-kind contribution from the industry partner(s).
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