[Piglit] [PATCH] glsl-1.50-geometry-end-primitive: ensure position data is reasonably aligned

[Roland Scheidegger]

ping?

Am 09.01.2016 um 05:40 schrieb sroland at vmware.com:
> From: Roland Scheidegger <sroland at vmware.com>
> 
> The rect halves comparison is in fact not valid in general. The reason is that
> the same geometry does not have the same precision even if it it just shifted
> by some fixed amount in one direction.
> As an example, some calculated x value near 7 will be near 263 if drawn with
> a viewport x value of 256. The value near 7 has 5 bits more precision -
> when calculating the fixed-point values for rasterization (with subpixel
> accuracy), the lower value thus may be rounded in a different direction
> than the higher one, even with correct rounding (not even entirely sure what
> "correct" rounding here means, nearest-floor or nearest-even or whatever, the
> problem is independent from that). This can then cause different pixels to be
> covered by the primitive.
> This causes a failure with this test in llvmpipe when the rounding mode is
> changed slightly (from "mostly" nearest-away-from-zero to nearest-even). I was
> not able to reproduce this with this test on nvidia or amd hardware, but they
> are definitely affected in theory by the same issue (proven by some quick and
> dirty test).
> So, do floor(tmp*2048) / 2048 to ensure everything is reasonably aligned. This
> still retains some subpixel bits (but only very few).
> 
> This may be a problem with more tests, albeit most tend to use vertex data
> which is already sufficiently aligned (e.g. drawing simple aligned rects).
> 
> v2: use mul/floor/div sequence suggested by Ian Romanick instead of add/sub -
> trying to beat the compiler may not be reliable (as it could reorder
> operations).
> ---
>  tests/spec/glsl-1.50/execution/geometry/end-primitive.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/tests/spec/glsl-1.50/execution/geometry/end-primitive.c b/tests/spec/glsl-1.50/execution/geometry/end-primitive.c
> index 6df3a89..97d8375 100644
> --- a/tests/spec/glsl-1.50/execution/geometry/end-primitive.c
> +++ b/tests/spec/glsl-1.50/execution/geometry/end-primitive.c
> @@ -105,7 +105,9 @@ static const char *spiral_text =
>  	"  if (vertex_id % 2 == 1) r += 1.0;\n"
>  	"  float max_r = b*sqrt(a*float(num_vertices)) + 1.0;\n"
>  	"  r /= max_r;\n"
> -	"  return r*vec2(cos(theta), sin(theta));\n"
> +	"  vec2 tmp = r*vec2(cos(theta), sin(theta));\n"
> +	"  // ensure reasonably aligned vertices\n"
> +	"  return floor(tmp * 2048.0f) / 2048.0f;\n"
>  	"}\n";
>  
>  /**
>