It is only tangentially related to this plugin but I will mention this here because it may relevant to those interested in validating EPUBs for Kindle publication.
The latest version of the Kindle Previewer, 3.25, has some code to perform an image quality check and includes the
OpenCV library for analyzing images. It appears to be a work in progress and I haven't found a way to actually enable this feature. Some new messages contained in the program include:
"File Validation failed: For details, please see Quality Report."
"Cover Image does not meet the image quality bar of 300 PPI."
"Cover image occupies 100% of the screen, therefore the image either needs to be greater than 1200 PX width or 1800 PX height."
"High Definition Visual Warnings (HDV) - These warnings are for images which does not meet high definition image bar of 300 PPI. We highly recommend fixing these issues to improve the quality of the book for the readers."