Blending issue with multiple ImageGLs

[astrofrog]

The following notebook demonstrates a bug in the blending of multiple ImageGL marks:

https://gist.github.com/astrofrog/08735bc30c2f8980edeaae0fd0842352

In short, I set up two arrays that look like:

image1 = np.array([[[255, 0, 0, 255], [0, 0, 255, 255]], [[255, 0, 0, 0], [0, 0, 255, 0]]]).astype(np.float32)
image2 = np.array([[[0, 0, 255, 0], [255, 0, 0, 0]], [[0, 0, 255, 255], [255, 0, 0, 255]]]).astype(np.float32)

In both cases the top and bottom row of pixels is the same within the image except that the top row (in the first case) and the bottom row (in the second case) are transparent. If I show the arrays individually I get sensible results:

But if I show the two at the same time the result looks incorrect:

It seems perhaps the alpha compositing is not correctly taking the opacity layer into account. I haven't had a chance to dig into the shader code yet.

[pllim]

Does this affect Imviz?

[astrofrog]

Not for now