Use image comparison metric that considers alpha channel about graphicsfuzz HOT 4 CLOSED

A candidate is SSIM: https://ece.uwaterloo.ca/~z70wang/research/ssim/
The link lists various implementations.

This implementation has alpha channel support, but its license might be an issue:
https://github.com/kornelski/dssim

from graphicsfuzz.

In general the image histogram metric that we are using is pretty limited even despite lack of alpha channel support. For example, I have found that it gives a distance of 0 between an image filled with vec4(0.0, 0.0, 0.0, 1.0) pixels and an image filled with vec4(1.0, 1.0, 1.0, 1.0) pixels.

from graphicsfuzz.

An idea is to switch from OpenCV to ImageMagick, which offers the following comparison metrics:
https://imagemagick.org/script/command-line-options.php#metric

Imagemagick has Java wrappers and is available via Maven:
http://www.jmagick.org/jmagick-doc/
https://mvnrepository.com/artifact/org.imagemagick

As I think OpenCV is used only for image comparison, we could then drop the OpenCV dependency.

from graphicsfuzz.

Added histogram alpha support in #262

Added fuzzy difference algorithm in #271 Still need to integrate this algorithm but this issue is technically fixed.

from graphicsfuzz.