better handling comparison with different alpha channel

[ccanepa]

While testing for cocos 0.6.10 it was spotted that

• the reference snapshots were RGBA images with alpha channel set to 255
• snapshots taken with an AMD APU had non-constant alpha channel
• pixels in the framebufer not painted by the scene will be (0, 0, 0, 255) in the reference and (0, 0, 0, 0) in the AMD ones

As a consequence the comparison report is of little use

• in the side by side of 'ref' and 'other' snapshots the unpainted pixels would look black for 'ref' and white for 'other' , because the html background color is white. This makes difficult to eyeball the differences
• the diff visualization has little value, because in general big areas will show different
• too much snapshots will be marked as different, augmenting the manual labor for the human reviever.

Taking a (win11 + AMD APU) machine and an (win7 + nvidia discrete gpu) and running do_test.py on both, at same python + pyglet + cocos, the difference holds.

Same if a test is manually run, snapshots taken by ctrl + S and inspected with irfanview.

Thinking maybe not pyglet nor cocos were setting the glClearColor, and thus a trivial difference on the driver layer was bubling up, I added after the director.init in test_acceldeccel.py the line from pyglet import gl; gl.glClearColor(255, 0, 0, 255)

This one worked nearly as expected in both systems, solid red background seen in the cocos screen and in the snapshots (alpha == 255 over al alpha in nvidia, same except the grossini contour in AMD)

Changing the color to (0, 0, 0, 0) gets transparent background on amd but still constant alpha == 255 in nvidia.

With (0, 0, 0, 255) alpha == 255 on nvidia and same on amd except the countour of grossini.

Things that could be investigated as the root of the difference

• differences in gl blend mode
• differences in glClearColor
• pyglet can bind to different functions if some extension is present
• take snapshots different behavior
• ?

On the practical side, to make comparisons better now, snapshots could be converted to RGB and then compared.

[ccanepa]

for cocos 0.6.10 I ran the comparisons on a third machine, win10 + nvidia