The AMD_compressed_ATC_texture format is documented here:
It was brought to my attention that this compressed texture format is a good candidate for exposing as a WebGL extension. And that it is only missing one piece of information: the buffer size computation. Below is that information that has been verified both with experts in the format and empirically.
Compressed texture images stored using the ATC compressed image formats are represented as a collection of 4x4 texel blocks.
The size in bytes of a compressed ATC_RGB_AMD image is:
((width_in_texels+3)/4) * ((height_in_texels+3)/4) * 8
The size in bytes of a compressed ATC_RGBA_EXPLICIT_ALPHA_AMD or ATC_RGBA_INTERPOLATED_ALPHA_AMD image is:
((width_in_texels+3)/4) * ((height_in_texels+3)/4) * 16
I’m currently working to include this in public documentation as well.