On Fri, May 12, 2017 at 01:33:54PM -0700, Nanley Chery wrote: > 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().
The last sentence actually turned out to be false. I plan to send out a v3 of this patch along with the Piglit test used to find this issue. _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/mesa-dev
