On Thu, Aug 11, 2016 at 10:41 AM, Anuj Phogat <anuj.pho...@gmail.com> wrote:
> There are no standard sample positions defined in OpenGL and OpenGL > ES specs. Implementations have the freedom to pick the positions > which give plausible results. But the Vulkan 1.0 spec does define > standard sample positions for different sample counts. Defined > positions in Vulkan for all the sample counts except 8X match with > the positions we set in i965. We have an upcoming plan to share the > blorp code between OpenGL and Vulkan driver in near future. Keeping > the 8X sample positions same on both the drivers will help us move > in that direction. > > Here is an argument by Neil Roberts (from commit 20250e85) against > any advantage of current 8X sample positions over the new ones: > > "The comment above for the 8x sample positions says that the hardware > implements centroid interpolation by picking the centre-most sample > that is inside the primitive. That implies that it might be worthwhile > to pick a pattern that includes 0.5,0.5. However by experimentation > this doesn't seem to actually be the case. With the sample positions > in this patch, if I modify the piglit test below so that it instead > reports the centroid position, it reports 0.492188,0.421875 which > doesn't match any of the positions. If I modify the sample positions > so that they include one at exactly 0.5,0.5 it doesn't help and it > reports another position which is even further from the center for > some reason. > > arb_gpu_shader5-interpolateAtSample-different > > Kenneth Graunke experimented with some other patterns that have a > higher standard deviation but I think after some discussion it was > decided that it would be better to pick the same pattern as the other > graphics API in case there are games that rely on this pattern." > > Observed no regressions in jenkins testing. > Did you try the scaled blit tests? I bet we have to rework 8x scaled blit for the new positions. Thanks for working on this! It's going to make the blorp transition way easier. > Signed-off-by: Anuj Phogat <anuj.pho...@gmail.com> > Cc: Jason Ekstrand <ja...@jlekstrand.net> > --- > src/mesa/drivers/dri/i965/brw_multisample_state.h | 33 > ++++++++--------------- > 1 file changed, 11 insertions(+), 22 deletions(-) > > diff --git a/src/mesa/drivers/dri/i965/brw_multisample_state.h > b/src/mesa/drivers/dri/i965/brw_multisample_state.h > index 42a7fd3..78fb03a 100644 > --- a/src/mesa/drivers/dri/i965/brw_multisample_state.h > +++ b/src/mesa/drivers/dri/i965/brw_multisample_state.h > @@ -46,20 +46,9 @@ static const uint32_t > brw_multisample_positions_4x = 0xae2ae662; > > /** > - * Sample positions are based on a solution to the "8 queens" puzzle. > - * Rationale: in a solution to the 8 queens puzzle, no two queens share > - * a row, column, or diagonal. This is a desirable property for samples > - * in a multisampling pattern, because it ensures that the samples are > - * relatively uniformly distributed through the pixel. > - * > - * There are several solutions to the 8 queens puzzle (see > - * http://en.wikipedia.org/wiki/Eight_queens_puzzle). This solution was > - * chosen because it has a queen close to the center; this should > - * improve the accuracy of centroid interpolation, since the hardware > - * implements centroid interpolation by choosing the centermost sample > - * that overlaps with the primitive being drawn. > + * Sample positions: > Might be nice to reference the Vulkan spec here. > * > - * Note: from the Ivy Bridge PRM, Vol2 Part1 p304 (3DSTATE_MULTISAMPLE: > + * From the Ivy Bridge PRM, Vol2 Part1 p304 (3DSTATE_MULTISAMPLE: > * Programming Notes): > * > * "When programming the sample offsets (for NUMSAMPLES_4 or _8 and > @@ -70,17 +59,17 @@ brw_multisample_positions_4x = 0xae2ae662; > * > * Sample positions: > * 1 3 5 7 9 b d f > - * 1 5 > - * 3 2 > - * 5 6 > - * 7 4 > - * 9 0 > - * b 3 > - * d 1 > - * f 7 > + * 1 7 > + * 3 3 > + * 5 0 > + * 7 5 > + * 9 2 > + * b 1 > + * d 4 > + * f 6 > */ > static const uint32_t > -brw_multisample_positions_8x[] = { 0xdbb39d79, 0x3ff55117 }; > +brw_multisample_positions_8x[] = { 0x53d97b95, 0xf1bf173d }; > > /** > * Sample positions: > -- > 2.5.5 > >
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