Collaging on Internal Representations: An Intuitive Approach for Semantic Transfiguration
We present a novel CNN-based image editing method that allows the user to change the semantic information of an image over a user-specified region. Our method makes this possible by combining the idea of manifold projection with spatial conditional batch normalization (sCBN), a version of conditional batch normalization with user-specifiable spatial weight maps. With sCBN and manifold projection, our method lets the user perform (1) spatial class translation that changes the class of an object over an arbitrary region of user’s choice, and (2) semantic transplantation that transplants semantic information contained in an arbitrary region of the reference image to an arbitrary region in the target image. These two transformations can be used simultaneously, and can realize a complex composite image-editing task like “change the nose of a beagle to that of a bulldog, and open her mouth”. The user can also use our method with intuitive copy-paste-style manipulations. We demonstrate the power of our method on various images. Code will be available at https://github.com/pfnet-research/neural-collage.
Recent advances in deep generative models like generative adversarial networks (GANs)  and variational autoencoders (VAEs)  make possible the unsupervised learning of rich latent semantic information from images, such as compositions and poses. \knoteThese models have been \knoteextensively studied for the purposes of image editing, and many conditional models have been proposed for various \knotetasks, such as image colorization [14, 17], inpainting [30, 15, 41], domain translation [17, 39, 46, 35, 37], style transfer [12, 42], and object transfiguration [45, 23, 13, 22].
Image conditional GANs [24, 40, 17] based on encoder-decoder architectures have been \knotepopular both for their convenient implemention in end-to-end differentiable ML frameworks, and their uncanny ability to produce photo-realistic images. \knoteIn particular, object transfiguration, where an object in one image is replaced with an object from another, has drawn significant attention for the impact of recent successes. \knoteNumerous methods have been proposed for this task. For example, [39, 45] have proposed methods that do not require a paired dataset, [46, 37] have proposed probabilistic transformations that take into account the inherent ambiguity in the translation process, and [13, 21, 1] devised models that can conduct one-to-many domain mapping. \knoteAll these methods are capable of drastically changing the image while maintaining its contextual information, but may also alter the appearance of regions the user wants to remain unchanged (\eg, CycleGAN may change the background color of an image when transforming one animal into another). \snoteMost of these methods, however, are designed to transform the entire image, and little work has addressed the topic of partial image transformation. \snotePrevious work has explored attention mechanisms that automatically learn this region of interest [4, 38]. \knoteStill others have [33, 6] employed inpainting techniques [30, 15] for interactive \knotepartial semantic manipulation of portraits, \knotee.g. changing hairstyles. \snoteThese methods, however, offer limited freedom in transformation. For instance, the works based on attention mechanisms can transform the user-selected object, but are designed in such that the network automatically determines the region of transformation. \snoteInpainting methods can also perform such object transformation tasks, but current methods are only successful in restricted image domains.
However, in practical image editing tasks like those done with Photoshop, users may want to retain spatial freedom during the transformation; that is, users want fine control over the region of transformation. \snoteAlso in high demand is a tool for partial translation that exclusively translates an object in the image that is semantically independent from the rests. \snoteTasks like “translate the nose of a beagle into a nose of a bulldog” and “translate rabbit ears into cat ears” are easily described in words, but difficult to formulate in equations. \snoteIt is indeed not an ordinary task to perform arbitrary, user-selected partial transformations (like those described above) over an arbitrary region of a user’s choice.
In this paper, we present a novel CNN-based image editing method that grants the user this very freedom. With our method, the user can transform a user-chosen part of image in a copy-paste fashion–and the user can do this all the while preserving semantic consistency. \snoteMore precisely, we present a method that features two types of image transformation: (1) spatial class-translation that translates a class category of a region of interest, and (2) semantic transplantation that transplants a semantic feature of an user-selected region in an arbitrary image to a region of interest in the target image. To facilitate this editing process, we also propose an efficient optimization method to project images onto the latent space of generator.
We demonstrate on multiple datasets that our method can carry out a diverse set of photorealistic transformations with user-intuitive manipulation. \knoteOur method boasts a high object transfiguration fidelity that is on-par with other image translation methods [13, 23]. \knoteMoreover, our method does not require task-specific training of the generator; \knotewe can use our method with publicly available pre-trained models of class conditional GANs without fine-tuning. \snoteThis also means that we can “stand on the shoulders of giants” and exploit the power of large conditional GANs with high representation power, like those presented in .
2 Related Work\snote
In this section, we will briefly describe the ideas of related works on which our method is built, with a particular focus on two approaches from which we will borrow the philosophy.
Generative adversarial networks (GANs).
GAN is a deep generative framework consisting of a generator and a discriminator playing a min-max game where tries to transform a prior distribution \eg, into the dataset distribution and tries to distinguish generated (fake) data from true samples . Thanks to the development of techniques \knoteagainst the training instability such as gradient penalty  and spectral normalization , deep convolutional GANs  are becoming the de facto standard for image generation tasks. GANs also excel in representation learning, and \knotenumerous studies reports the ability of GANs to capture semantic information. \knoteOne can produce a sequence of semantically meaningful images by interpolating the latent variable [34, 3, 29, 2].
Class-conditional GAN [29, 26, 43, 2] is a framework designed to learn an invariant latent representation among various classes, and it is capable of generating diverse images from a same latent code by changing class embedding (Figure 2). \snoteThe work of [26, 2], in particular, succeeded in producing an impressive results by interpolating the parameters of conditional batch normalization layer, which was first introduced in [31, 5]. \snoteConditional batch normalization (CBN) is mechanism that learns conditional information by separately learning condition-specific scaling parameter and shifting parameter for batch normalization. \snoteOur method extends the technique used in  by restricting the region of interpolation to a region that corresponds to the region of interest in the pixel space. \snoteWe will refer to our approach spatial conditional batch normalization (sCBN). \snoteUnlike the manipulation done in style transfer , we introduce the conditional information at multiple levels in the network, depending on the style preference of the user. \snoteAs we will show, sCBN in the lower layers transforms global features, and sCBN at upper layers transforms local features.
In order to grant the user with wide freedom of semantic transformation, there has to be some mechanism to finely adjust the user-suggested transformation so that the final product becomes natural. \knoteMethods introduced in [44, 3, 19] overcame this challenge by projecting the original image to the manifold on which the generative model is supported, and prompting the user to make a change on the manifold. In algorithm, this is done by identifying the latent variable that can faithfully reproduce the original image, followed by the transformation of the image on the latent space that will, in the original space, manifest as a transformation desired by the user. \knoteA major difficulty lies in the finding of the exact transformation in the latent space that will result in a transformation of the user’s intention. \knoteThe works of [44, 3] resorts to an implicitly formulated latent space transformation based on pixel level constraints, and  specializes the model for the transformations of pre-defined attributes. \snoteWith some technicalities omitted, we will transplant a image attribute of source image into the target image by directly conducting the copy-paste operation in the feature space of the images. \snoteIn the semantic transplantation example in Figure 1, we change the expression of a canine. \snoteOur method does not require any training process for pre-defined set of attributes. \snoteWe will also propose a method of expanding the latent space that speeds up the optimization process of manifold projection. \snoteWe can in fact conduct both semantic transplantation and spatial class translation together as well (see Figure 1).
3 Featured Functionalities\knote
We first provide a brief overview for the usages of our method along with the functionalities featured within. \knoteGiven the input image of interest and the conditional generator , the process begins by prompting the user to specify the region of an image to be edited, or the region of the image containing the object that the user wants to transform. The user can then apply spatial class translation and semantic transplantation on the selected region.
3.1 Spatial Class-translation\snote
With our spatial class translation, the user can change the class of the object in the user-selected region of interest (ROI). \snoteThe user can change the class of a part of the target objects in intuitive fashion. Figure 1 shows the result of transforming the nose of a beagle into that of French bulldog. \snoteThe user will first be prompted to specify the set of subregions on which to conduct the translation. Multiple subregions (corresponding to the target objects) can be specified, and the user can assign different class labels to each subregion. See Figure 3 top, for example. \snoteIf the user selects the whole ROI as the object of interest, our method will behave like an ordinary object transfiguration. \snoteThe user can also specify the morphing strength on a continuous scale from to . \snoteSee the bottom figure in Figure 3, bottom, for example. We see that the strength of the user-selected class features are continuously increasing with the morphing strength.
3.2 Spatial Semantic Transplantation\snote
With our semantic transplantation, the user can transplant a semantic feature of the user-selected object in the reference image to an object in the target image to be transformed. \snoteOur method first prompts the user to specify the region in the target image containing the object of interest, along with the reference image of equal size. \snoteThe user will be also asked to specify the region in the reference image that contains the semantic information to be transplanted. \snoteThe method then automatically transplants the semantic information of the specified region of the reference image into the target image. As shown in Figure 4, the user does not have to prepare the reference image containing the object of interest that is precisely aligned with the object in the target image. Also, the object class of the reference image does not have to be the same as the target. \snoteFor the best performance, the user should select a reference image closely related to the target image. For example, if the user wants to open the mouth of a dog (Figure 1), using another image of a dog with an opened mouth and otherwise similar pose to the target will achieve the best result.
4 Architecture and Algorithms\snote
In this section, we will provide the mechanism behind our method. \snoteLet us first briefly present an overview of the algorithm (Figure 5), \snoteWe will then articulate the details of each functional component that appear in the procedures in the following subsections.
spatial class translation\snote
Our method functions on a trained conditional generator , paired with the discriminator with which was trained. \snoteUpon receiving the region of interest clipped from the target image and the class of the target object contained in , the algorithm begins by looking for a latent variable such that will be close to in the feature space of (Manifold Projection step). \snoteThe class can either be specified by the user or by a pre-trained classifier. \snoteSuppose that the user wants to partially translate a region in to a class , and let be the set of features in -th conditional batch normalization(CBN) layers that correspond to in the pixel space. \snoteOur method then simply substitutes the parameters governing the shift and mean parameters of with those of (Figure 6). \snoteThis will result in a modification of in which the CBN parameters of exclusively carry the style information of the class . \snoteA transformed image can be constructed by applying this modified to .
The procedure of the semantic transplantation also begins by looking for the latent variables that reconstructs with (Figure 7). \snoteOnly, the algorithm also looks for the latent variable that reconstructs the clip from the reference image. \snoteLet be the user-selected subregion of on which to conduct the transformation, and let be the region in from which the user wants to transplant the semantic information. \snoteAlgorithm expects that and are roughly aligned and are of same dimension. \snoteLet be the set of features in the intermediate layers that correspond to , and let be the outputs of the -th layer of . \snoteBy the choice of and , also corresponds to . \snoteThe algorithm then proceeds by simply conducting a location-dependent mixing of and for each with heavy weights assigned on .
For realistic image transformation, we also conducted a post processing. \snoteBecause the model is not designed to handle the background, naively pasting a generated patch on the target image \snotecan produce artifacts in the region surrounding the object of interest. \snoteIn order to brush up the final product, we applied Poisson blending  exclusively on the region that was specified by the user as the region pertaining to the object of interest. \knoteIn principle, our spatial editing method is applicable to any generative model (\eg, GAN, VAE) that is equipped with a machanism to iteratively incorporate class information during its image generation process.
We will next elaborate on the design of our sCBN and spatial semantic implantation, along with the other details omitted in the brief description above.
4.1 Spatial Conditional Batch Normalization\snote
Spatial conditional batch normalization (sCBN) is the core of our spatial class translation. \snoteAs can be inferred from our naming, sCBN is based on batch normalization (BN) , a technique developed for the purpose of reducing the internal covariance shift to accelerate the training of neural network. \snoteMore precisely, we will borrow our idea from conditional batch normalization (CBN) [8, 5], a variant of BN that incorporates the class specific semantic information in the parameters for BN. \snoteGiven a set of batches sampled each from a single class, the conditional batch normalization [8, 5] works by modulating the set of intermediate features produced from each batch of inputs so that it follow a normal distribution with mean and variance that are specific to the corresponding class. \snoteLet us fix the layer , and let represent the feature of -th layer at channel , height location , and width location . \snoteGiven a batch of s generated from class , the CBN at layer then transforms by:
where are trainable multiplication and bias parameters specific to the class . \snotesCBN modifies (1) by replacing the with given by
where is a user-selected non-negative tensor map (class-map) that partitions the unity, that is, for each position . \snoteNumber of class can be freely chosen. Spatial CBN also conducts an analogous modification on to produce .
In our implementation, we replaced CBN at each layer with sCBN, with having support on we described above. \snoteFor standard usage of our method, we expect the user to use supported on the region in the feature space produced by simply downsampling , the region in the pixel space over which the user wants to transform the image. \snoteHowever, applying the sets of that are differently constructed for each layer can produce interesting results as well (see section 5.3).
4.2 Spatial Feature Blending\snote
For spatial feature blending, let be the -th layer feature map of the -th reference image . Then, with the understanding that stands for the -th feature map of target image , we produce the transformed image by recursively replacing in from bottom to top by
where is a user-selected non-negative tensor (feature blending weights) of same dimension as such that is the tensor whose entries are all , and \snoterepresents the Hadamard product. For our implementation, we choose that has support on , the set of features in the -th layer that correspond to the region to be transplanted in the -th image in the pixel space. \snoteSee Figure 7 for the illustration of this process. \snoteIn general, feature maps in the earlier layer tend to contain abstract, semantic information that is class invariant. \snoteAs such, for better performance, the user should choose that are more concentrated on for higher (see section 5.3). We use the same sCBN class-map to generate and to reduce artifacts.
4.3 Manifold Projection
For the Manifold Projection step, we follow a similar procedure as the one used in , using a pre-trained generator and discriminator pair . \snoteGiven the clipped image of interest, the goal of the manifold projection step is to train the encoder such that is small for some dissimilarity measure . \snoteThe choice of in our method is the cosine distance on the final feature space of the discriminator . That is, if is the normalized version of the image of in the final layer of ,
After training the encoder, one can produce the reconstruction of by applying to . In the reconstructed image, however, semantically independent objects are often dis-aligned. We therefore calibrate by backpropagating the loss . After some rounds of calibration, we can use the resulting for the image transformation.
However, image features are generally entangled in the latent space of generative model, \knoteand the optimization on the complex landscape of loss \knotecan be time-consuming. \knoteTo speed up the process, we construct an auxiliary network that embeds in higher dimensional space. \knoteThe auxiliary network consists of an embedding map that converts into high dimensional , and a projection map that converts back to (Figure 8). \knoteThat is, instead of calibrating the latent variable by backpropagating through , we will calibrate by backpropagating through and . \knoteThe goal of training this auxiliary network is to find the map that grants us with a representation of the landscape of that is more suitable for optimization, together with the map that can embed in a way that is well-suited for the learning on the landscape. \knoteLet be the variable in the high dimensional latent space after rounds of calibration. The update rule of we use here is
where is the length parameter at the -th round. \knoteWe train the networks and using the following loss function:
where constants determine the importance of the -th round of the calibration. \knoteFor the first term, and are updated through the backpropagation from . \knoteThe second term makes sure that can be reconstructed from . This process can be seen as a variant of meta-learning [9, 27].
In this section, we present the results of our experiments together with the design of our network architecture and the experimental settings. For further details, please see the supplementary material.
5.1 Experimental Settings\snote
The generator used in our study is a ResNet-based generator trained as part of a conditional DCGAN. \snoteEach residual block in our generator contains the conditional batch normalization (CBN) layer. \snoteAt the time of inference, these CBN layers are replaced by the aforementioned sCBN layers that are tailored to the user’s preference. \snoteWe base our architectures on those used in previous work [25, 26], and \snoteused the pre-trained model from . \snoteOur generator is designed to receive a low dimensional signal ( in our experiments). \snoteThe generator maps the input into feature maps of dimension . \snoteThese feature maps are to be up-sampled to create new maps, doubling the resolution at every ResBlock, ultimately mapping to RGB images of user-specified dimensions (, in our experiments). \snoteFor the auxiliary maps and , we used a -layer MLP with hidden units of size at the respective layers, and treated the 10000-dimensional layer as extended latent variable . \snoteFor the input of in equation (4) for the transformation of () dimensional image, we used dimensional feature vectors of the discriminator prior to the final global pooling, which is expected to capture the semantic image features [7, 18].
We demonstrate the representation power of our algorithm using the set of DCGANs trained on different image dataset: (1) dogs+cats (subset of ImageNet, 143 classes), (2) birds (200 classes) , (3) flowers (102 classes) .
\knoteFor the dog+cat images, we reused the pre-trained model publicized in 
Figure 9 shows the example results of \knoteour spatial class-translation for various object categories. \knoteWe see that our translation is modulating both local information like color and texture together with global information in a way that renders the final image semantically consistent. \knoteWe can discern the user-selected region from the translated image if we pay attention to the texture and color. \knoteOn the other hand, the user-selection region is not too reflective of global features like the face of the Husky and the shape of the petal. \knoteIn the light of the fact that user is prompted to specify the target region in the pixel space, this observation is indicative of the fact that the global features are governed by the layers that are closer to the latent space. Figure 10 shows the examples of \knoteour semantic transplanting. \knoteHere our method succeeds in modulating the semantic features like the postures of a dog and a bird, and the sex of a lion. \knoteWe also can see that the performance is robust against the alignment of the region to be transformed.
Note the difference in results between our method and the interpolation in the latent space (Figure 12). For a pair of latent variable , our method uses as a reference and interpolates the feature map only in a specified region, while the latent-space interpolation directly changes the value of . We can see that, our method does not change the semantic context outside the specified region, and preserves the object identity. \knoteFor more editing results, see Figure 11.
5.3 More Customized Image Transformation\knote
As mentioned in the method section, we can assign different modulation parameters for each ResBlock. \snoteIn particular, the user is given freedom to independently control feature blending weights and the choice of class-map for spatial class translation. \snoteHere, we demonstrate the power of this freedom in the form of highly-customized image transformations. \knoteFigure 13 illustrates the result obtained by applying spatial class translation applied to (1) all layers, (2) the layer closest to the input (first layer), and (3) all the layers except the first layer. \knoteAs we implied in the previous section, the lower layers tend to regulate global features, and the high layers tend to regulate local features. \knoteAs we can see in the figure, the features affected by the manipulation of the lower layers (face, body shape) are somewhat independent from those \knoteof the higher layers (texture). \knoteWe can therefore choose the level of transformation at each level of locality.
Figure 14 shows the result of applying feature blending done at different layers () with different blending weights. \knoteWhen the blending is done exclusively at the layer , the transformation is globally smooth and the extent of the change is rather contained. \knoteWhen the blending is done for the higher layers, local features like fur textures are transplanted as well. \knoteHowever, when the reference image is significantly different from the target image in terms of its topology, the transplantation at higher layers tend to produce artifacts. \knoteThis tendency become stronger for the transplantation in the layers higher than .
5.4 Transfiguration Fidelity\snote
In order to provide some quantitative measurement for the quality of our translation, we evaluate the transfiguration fidelity of our method. \snoteWe conducted the translation of (1) cat big cat, (2) cat dog, and (3) dog species another dog species, and evaluated the classification accuracy by inception-v3 classifier trained on ImageNet to classify 1000 classes. \snoteFor all experiments, we selected four classes from both source and target domains, and conducted 1000 translation tasks from a randomly selected class from the source domain to a randomly selected class from the target domain. \knoteWe used UNIT  and MNUIT  for baselines. \knoteFor their evaluations, we used the models described in their publications. \knoteBecause MUNIT is not designed for class to class translation, we conducted MUNIT using the set of images in the target class as the reference images. \knoteTable 1 summarizes the result. \knoteFor each set of translation task, our method achieved better top-5 error rate than the other methods. \knoteThis result confirms the efficacy of the combination of the class conditional GANs and the manifold projection. \knoteOur method is also capable of many to many translation over a set of 100 or more classes.
|Method||cat big cat||cat dog||dog dog|
5.5 Speed-up by Latent Space Expansion
We also conducted an experiment to verify that our auxiliary network can indeed speed up the learning process. We used the DCGANs trained on dog+cat dataset, and evaluated the average loss for 1000 images randomly selected from ImageNet. The optimization was done with Adam, implemented with the best learning rate in the search-grid that achieved the fastest loss decrease. Figure 15 compares the transition of the loss function learned on -latent space against the loss transition produced on -latent space. To decrease the loss by the same amount, the learning of required only less than the number of iterations required by the learning of . Calculation overhead due to the latent space expansion was negligible (%). Indeed, the learning process depends on the initial value of the latent variable or . When compared to random initialization, the optimization process on both space and space proceeded faster when we set the initial value at and . This result is indicative of the importance of training .
6 Discussion and Conclusions
Needless to say, our method can only handle image expressions in the scope of the used generator. While it can spatially translate images of a wide variety, its capability is limited by the diversity of the dataset used to train the models. Also, our method is not necessarily suitable for the transformation of images for which the manifold projection is difficult, since some information is bound to be lost in the process. Transformation of the face of a specific person can be difficult unless the model is trained with an ample set of images for the target person, for example.
The conditional batch normalization is capable of handling not only class conditions, but other types of information like verbal statements as well. One might be able to conduct even more flexible transformation by making use of them. Also, one might be able to take advantage of the high dimensionality of the extended latent space to disentangle the semantic features. Further experimental and theoretical exploration of the properties of the space might enable even more flexible image manipulations. Finally, we might be able to improve the post-processing by introducing less heuristic machinery like an attention mechanism.
In this paper, we introduced a novel image transformation method that allows the user to translate the class of an object and transplant semantic features over a user-specified pixel region of the image. Indeed, there is still much room left for the exploration of the semantic information contained in the intermediate feature spaces of CNNs. We were, however, able to show that we can manipulate this information in a somewhat intuitive manner and produce customized photorealistic images.
We thank Jason Naradowsky and Masaki Saito for helpful discussions.
Appendix A Global Transltation\knote
In order to verify the sheer ability of our translation method, we conducted a translation task for the entire image as well (as opposed to the translation for a user-specified subregion). For each of selected images from the ImageNet, we calculated a latent encoding variable , and applied a set of spatial uniform class-condition to the layers of the decoder. The results are listed in the Figure 18. \knoteThe Figure 19 contains the translation of an image (designated as original) to all 143 object classes that were used for the training of the dog+cat model. We can see that the semantic information of the original image is naturally preserved in most of the translations.
Appendix B Experimental Details
This section provides further details for the experimental settings.
b.1 Implementation of networks
We trained the following three components of the model separately, in order. We first prepared a generator and discriminator pair following the training procedure of conditional GANs described in [25, 26]. We then trained the encoder network for the trained generator using the objective function defined based on the trained discriminator (see the main section in the article about the manifold projection). Finally, fixing the encoder and the generator we trained, we trained the auxiliary networks to enhance the manifold-embedding optimization. For the evaluation of , we used , , and .
We used the batch size of 8 for the training of all network components. For the training of the cGANs and the encoder, we used the Adam optimizer. The parameters were set at respectively for the training of both architecture. The learning rate was set at for the training of cGANs, and was set at for the training of the encoder and the auxiliary networks.
For the training of the auxiliary network, we used AdaGrad with adaptive learning rate for the gradient descent to calculate . We used AdaGrad for this procedure because other methods (\eg, Adam) causes numerical instability in double backpropagation for network parameter update. We trained each model over iterations. For the training of auxiliary networks, we applied the gradient clipping with the threshold of and the weight decay at the rate of . We trained the networks on GeForce GTX TITAN X for about a week for each component.
b.2 Manifold projection
For the optimization process about and in the manifold projection at the time of inference, we conducted backpropation from and used Adam optimizer for the iterative updates.
We conducted a grid search about the choice of . The choice of achieved the fastest learning rates for the optimization about , and the choice of achieved the fastest learning rates for the optimization about .
We also implemented naïve GD, AdaGrad, and L-BFGS-B for the optimization, but Adam performed the best among all methods we tried.
b.3 Transfiguration fidelity study
For the transfiguration fidelity experiment, we conducted a set of automatic
spatial class translations.
For each one the selected images, we
(1) used a pre-trained model to extract the region of the object to be transformed (dog/cat), (2) conducted the manifold projection to obtain the ,
(3) passed to the generator with the class map corresponding to the segmented region, and
(4) conducted a post-processing over the segmented region.
For the semantic segmentation, we used a TensorFlow implementation of DeepLab v3 Xception model trained on MS COCO dataset
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