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Title: On the Evaluation of Conditional GANs

Paper content:
Conditional Generative Adversarial Networks (cGANs) are finding increasingly widespread use in many application domains.
Despite outstanding progress, quantitative evaluation of such models often involves multiple distinct metrics to assess different desirable properties, such as image quality, conditional consistency, and intra-conditioning diversity.
In this setting, model benchmarking becomes a challenge, as each metric may indicate a different "best" model.
In this paper, we propose the Frechet Joint Distance (FJD), which is defined as the Frechet distance between joint distributions of images and conditioning, allowing it to implicitly capture the aforementioned properties in a single metric.
We conduct proof-of-concept experiments on a controllable synthetic dataset, which consistently highlight the benefits of FJD when compared to currently established metrics.
Moreover, we use the newly introduced metric to compare existing cGAN-based models for a variety of conditioning modalities (e.g. class labels, object masks, bounding boxes, images, and text captions).
We show that FJD can be used as a promising single metric for model benchmarking.
The use of generative models is growing across many domains (van den Oord et al., 2016c; Vondrick et al., 2016; Serban et al., 2017; Karras et al., 2018; Brock et al., 2019) .
Among the most promising approaches, Variational Auto-Encoders (VAEs) (Kingma & Welling, 2014) , auto-regressive models (van den Oord et al., 2016a; b) , and Generative Adversarial Networks (GANs) (Goodfellow et al., 2014) have been driving significant progress, with the latter at the forefront of a wide-range of applications (Mirza & Osindero, 2014; Reed et al., 2016; Zhang et al., 2018a; Vondrick et al., 2016; Almahairi et al., 2018; Subramanian et al., 2018; Salvador et al., 2019) .
In particular, significant research has emerged from practical applications, which require generation to be based on existing context.
For example, tasks such as image inpainting, super-resolution, or text-to-image synthesis have been successfully addressed within the framework of conditional generation, with conditional GANs (cGANs) among the most competitive approaches.
Despite these outstanding advances, quantitative evaluation of GANs remains a challenge (Theis et al., 2016; Borji, 2018) .
In the last few years, a significant number of evaluation metrics for GANs have been introduced in the literature (Salimans et al., 2016; Heusel et al., 2017; Bińkowski et al., 2018; Shmelkov et al., 2018; Zhou et al., 2019; Kynkäänniemi et al., 2019; Ravuri & Vinyals, 2019) .
Although there is no clear consensus on which quantitative metric is most appropriate to benchmark GAN-based models, Inception Score (IS) (Salimans et al., 2016) and Fréchet Inception Distance (FID) (Heusel et al., 2017) have been extensively used.
However, both IS and FID were introduced in the context of unconditional image generation and, hence, focus on capturing certain desirable properties such as visual quality and sample diversity, which do not fully encapsulate all the different phenomena that arise during conditional image generation.
In conditional generation, we care about visual quality, conditional consistency -i.e., verifying that the generation respects its conditioning, and intra-conditioning diversity -i.e., sample diversity per conditioning.
Although visual quality is captured by both metrics, IS is agnostic to intra-conditioning diversity and FID only captures it indirectly.
1 Moreover, neither of them can capture conditional con-sistency.
In order to overcome these shortcomings, researchers have resorted to reporting conditional consistency and diversity metrics in conjunction with FID Park et al., 2019) .
Consistency metrics often use some form of concept detector to ensure that the requested conditioning appears in the generated image as expected.
Although intuitive to use, these metrics require pretrained models that cover the same target concepts in the same format as the conditioning (i.e., classifiers for image-level class conditioning, semantic segmentation for mask conditioning, etc.), which may or may not be available off-the-shelf.
Moreover, using different metrics to evaluate different desirable properties may hinder the process of model selection, as there may not be a single model that surpasses the rest in all measures.
In fact, it has recently been demonstrated that there is a natural trade-off between image quality and sample diversity (Yang et al., 2019) , which calls into question how we might select the correct balance of these properties.
In this paper we introduce a new metric called Fréchet Joint Distance (FJD), which is able to implicitly assess image quality, conditional consistency, and intra-conditioning diversity.
FJD computes the Fréchet distance on an embedding of the joint image-conditioning distribution, and introduces only small computational overhead over FID compared to alternative methods.
We evaluate the properties of FJD on a variant of the synthetic dSprite dataset (Matthey et al., 2017) and verify that it successfully captures the desired properties.
We provide an analysis on the behavior of both FID and FJD under different types of conditioning such as class labels, bounding boxes, and object masks, and evaluate a variety of existing cGAN models for real-world datasets with the newly introduced metric.
Our experiments show that (1) FJD captures the three highlighted properties of conditional generation; (2) it can be applied to any kind of conditioning (e.g., class, bounding box, mask, image, text, etc.); and (3) when applied to existing cGAN-based models, FJD demonstrates its potential to be used as a promising unified metric for hyper-parameter selection and cGAN benchmarking.
To our knowledge, there are no existing metrics for conditional generation that capture all of these key properties.
In this paper we introduce Fréchet Joint Distance (FJD), which is able to assess image quality, conditional consistency, and intra-conditioning diversity within a single metric.
We compare FJD to FID on the synthetic dSprite-textures dataset, validating its ability to capture the three properties of interest across different types of conditioning, and highlighting its potential to be adopted as a unified cGAN benchmarking metric.
We also demonstrate how FJD can be used to address the potentially ambiguous trade-off between image quality and sample diversity when performing model selection.
Looking forward, FJD could serve as valuable metric to ground future research, as it has the potential to help elucidate the most promising contributions within the scope of conditional generation.
In this section, we illustrate the claim made in Section 1 that FID cannot capture intra-conditioning diversity when the joint distribution of two variables changes but the marginal distribution of one of them is not altered.
Consider two multivariate Gaussian distributions, (X 1 , Y 1 ) ∼ N (0, Σ 1 ) and (X 2 , Y 2 ) ∼ N (0, Σ 2 ), where If we let X i take the role of the embedding of the conditioning variables (e.g., position) and Y i take the role of the embedding of the generated variables (i.e., images), then computing FID in this example would correspond to computing the FD between f Y1 and f Y2 , which is zero.
On the other hand, computing FJD would correspond to the FD between f X1,Y1 and f X2,Y2 , which equals 0.678.
But note that Dist1 and Dist2 have different degrees of intra-conditioning diversity, as illustrated by Figure 5 (right), where two histograms of f Yi|Xi∈(0.9,1.1) are displayed, showing marked differences to each other (similar plots can be constructed for other values of X i ).
Therefore, this example illustrates a situation in which FID is unable to capture changes in intra-conditioning diversity, while FJD is able to do so.

assistant: text: We propose a new metric for evaluating conditional GANs that captures image quality, conditional consistency, and intra-conditioning diversity in a single measure.