IS (Inception Score) measures: diversity and quality of generated images

The Inception Score (IS) is an algorithm used to assess the quality of images created by a generative image model such as a generative adversarial network (GAN). The score is calculated based on the output of a separate, pretrained Inception v3 image classification model applied to a sample of (typically around 30,000) images generated by the generative model. The Inception Score is maximized when the following conditions are true:

The entropy of the distribution of labels predicted by the Inceptionv3 model for the generated images is minimized. This corresponds to the desideratum of generated images being "sharp" or "distinct".

The predictions of the classification model are evenly distributed across all possible labels. This corresponds to the desideratum that the output of the generative model is "diverse".

It has been somewhat superseded by the related Fréchet inception distance (FID). While the Inception Score only evaluates the distribution of generated images, the FID compares the distribution of generated images with the distribution of a set of real images ("ground truth").