[Bug 100925] [HSW/BSW/BDW/SKL] Google Earth is not resolving all the details in the map correctly

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=100925

--- Comment #16 from Nanley Chery <nanleychery at gmail.com> ---
I found some text from the SKL PRM, Vol 5: Memory Views, that explains the
errors we're seeing:

   Example of Opaque Color Encoding
   [...]
   00 ? color_0
   01 ? color_1
   10 ? 2/3 color_0 + 1/3 color_1
   11 ? 1/3 color_0 + 2/3 color_1

   [...]
   Programming Note
   Context: Opaque Textures (DXT1_RGB)
   The behavior of this format is not compliant with the OGL spec

Here's the OGL spec behaviour for reference:

   The RGB color for a texel at location (x,y) in the block is given by:

      RGB0,              if color0 > color1 and code(x,y) == 0
      RGB1,              if color0 > color1 and code(x,y) == 1
      (2*RGB0+RGB1)/3,   if color0 > color1 and code(x,y) == 2
      (RGB0+2*RGB1)/3,   if color0 > color1 and code(x,y) == 3

      RGB0,              if color0 <= color1 and code(x,y) == 0
      RGB1,              if color0 <= color1 and code(x,y) == 1
      (RGB0+RGB1)/2,     if color0 <= color1 and code(x,y) == 2
      BLACK,             if color0 <= color1 and code(x,y) == 3

   Arithmetic operations are done per component, and BLACK refers to an
   RGB color where red, green, and blue are all zero.

http://developer.download.nvidia.com/opengl/specs/GL_EXT_texture_compression_s3tc.txt

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