pvrtc alpha decode bug

[attilaz]

I have a problem when using pvrtc decompression in bgfx. When I am using pvrtc textures with gles2 extension the texture alpha looks fine, but when it is decompressed with decodeBlockPtc14A it is wrong.
RGB channels are ok.
See attached image:

The right picture shows my attempt to fix it with modifying
https://github.com/bkaradzic/bgfx/blob/master/src/image.cpp#L2250
and https://github.com/bkaradzic/bgfx/blob/master/src/image.cpp#L2268 to
*_a += 255 * _factor;
The results seems better but far from perfect...

Here is our pvr with alpha. ( the images above only shows a circle/star part of the whole decal)
decals.zip

[bkaradzic]

Are you using PVRTexTool?

Also can you try with this texture:
https://github.com/bkaradzic/bgfx/blob/master/examples/assets/textures/alphatest.png

[attilaz]

I am using PVRTexTool for compression.

Thanks for the fix. It is much better, but there are still slight differences.
I have checked the rgb channels more thoroughly to see if this is some kind of precision issue, but I haven't found any difference. So rgb looks exactly the same.

[attilaz]

I think I found the solution. It seems the punchtrough value was incorrect.
This https://github.com/bkaradzic/bgfx/blob/master/src/image.cpp#L2298
should be this: const bool punchthrough = !!(bc[4] & 1);

And the change (ea67fa4)

at this line (https://github.com/bkaradzic/bgfx/blob/master/src/image.cpp#L2115) should be reverted.
The original 0,0 for alpha was good. It caused problem because of the invalid punchtrough values.

I compared multiple image with this fix. I have only found slight intensity differences. In rgb it is hardly noticable, In alpha it is more visible. It seems like the rounding or something like this is done differently? I have attached the result of conversion from ptc1a -> rgba8 with pvrtextool and bgfx.

decomp.ZIP