The DIFF function allows to set full reference metrics between pairs of images, using the MSE (Mean Square Error), RMSE (Root Mean Square Error), PSNR (Peak Signal Noise Ratio) and SSIM (Structural Similarity Index) metrics.
This requires opening two images simultaneously in ImageQA, no ROI is required as the images are compared in full.
The image shown is an SSIM map where the blue zones represent the most similar areas and the red zones the most different ones.
In the SSIM difference map you can determine areas affected by lack of focus in the absence of depth of field, compression artifacts and areas that have lost sharpness due to it, noise, etc.
These metrics are mostly used to estimate quality deterioration as a function of a reference image when using lossy compression algorithms, but, as seen above, it has more functions.