Comment # 8
on bug 89156
from Stefan Dösinger
I ran some more tests, it seems that the format is operating at 3 bits precision. I can produce 8 different output colors. Otherwise it seems to follow the spec, so I don't think we're accidentally feeding the data into an R3G3B2 texture. On Windows the format operates at the expected precision - I can get any output data from 0x00 to 0xff. I skimmed the GPU docs for clues what may cause this behavior but could not find anything. The things I checked were enabling / disabling filtering, make sure texture address handling follows conditional NP2 texture rules, disabling alpha blending. For the sake of testing I also tried disabling FBOs and all our sRGB code. I'm also quite sure that all 8 bits of red0 and red1 input arrive on the GPU. I tested that by setting the code of each texel to 7 and then testing red0=1, red1=0 and red0=0 and red1=1. In the former case this gives the result 0 (interpolation between red0 and red1), in the latter case this gives 0xfc (MAXRED). The same works for the input values 0x80 and 0x7f. I tested interpolation codes (e.g. red0=0x2, red1=0xa2, code 2 for each texel, then try to reduce red0 or red1 by 1), and it seems that the input into the interpolation is OK, but either the interpolation happens at a lower precision or the output is clamped afterwards.
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