• Nanley Chery's avatar
    i965/formats: Update the three-channel DXT1 mappings · 87d16afa
    Nanley Chery authored
    The procedure for decompressing an opaque DXT1 OpenGL format is
    dependant on the comparison of two colors stored in the first 32 bits of
    the compressed block. Here's the specified OpenGL behavior 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
    
    The sampling operation performed on an opaque DXT1 Intel format essentially
    hard-codes the comparison result of the two colors as color0 > color1.
    This means that the behavior is incompatible with OpenGL. This is stated
    in the SKL PRM, Vol 5: Memory Views:
    
       Opaque Textures (DXT1_RGB)
          Texture format DXT1_RGB is identical to DXT1, with the exception that the
          One-bit Alpha encoding is removed. Color 0 and Color 1 are not compared, and
          the resulting texel color is derived strictly from the Opaque Color Encoding.
          The alpha channel defaults to 1.0.
    
          Programming Note
          Context: Opaque Textures (DXT1_RGB)
          The behavior of this format is not compliant with the OGL spec.
    
    The opaque and non-opaque DXT1 OpenGL formats are specified to be
    decoded in exactly the same way except the BLACK value must have a
    transparent alpha channel in the latter. Use the four-channel BC1 Intel
    formats with the alpha set to 1 to provide the behavior required by the
    spec. Note that the alpha is already set to 1 for RGB formats in
    brw_get_texture_swizzle().
    
    v2: Provide a more detailed commit message (Kenneth Graunke).
    v3: Ensure the alpha channel is set to 1 for DXT1 formats.
    
    Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100925
    Cc: <mesa-stable@lists.freedesktop.org>
    Acked-by: Tapani Pälli <tapani.palli@intel.com> (v1)
    Reviewed-by: Kenneth Graunke's avatarKenneth Graunke <kenneth@whitecape.org>
    Signed-off-by: Nanley Chery's avatarNanley Chery <nanley.g.chery@intel.com>
    (cherry picked from commit 688ddb85)
    [Emil Velikov: attribute for BRW to ISL format rename]
    Signed-off-by: default avatarEmil Velikov <emil.velikov@collabora.com>
    
    Conflicts:
    	src/mesa/drivers/dri/i965/brw_surface_formats.c
    87d16afa
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