[Mesa-dev] [PATCH] intel/blorp: Increase the presision of coordinate transform calculations

[Jason Ekstrand]

On Sat, Sep 3, 2016 at 11:44 AM, Kenneth Graunke <kenneth at whitecape.org>
wrote:

> On Saturday, September 3, 2016 10:42:01 AM PDT Jason Ekstrand wrote:
> > The result of this calculation goes into an fma() in the shader and we
> > would like it to be as precise as possible.  The division in particular,
> > was a source of inprecision whenever dst1 - dst0 was not a power of two.
> > This fixes a bunch of the new Vulkan CTS tests for blitting using a
> > filtering of NEAREST.
> >
> > Signed-off-by: Jason Ekstrand <jason at jlekstrand.net>
> > ---
> >  src/intel/blorp/blorp_blit.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/src/intel/blorp/blorp_blit.c b/src/intel/blorp/blorp_blit.c
> > index f4d81e6..0cb8b1a 100644
> > --- a/src/intel/blorp/blorp_blit.c
> > +++ b/src/intel/blorp/blorp_blit.c
> > @@ -1256,7 +1256,7 @@ brw_blorp_setup_coord_transform(struct
> brw_blorp_coord_transform *xform,
> >                                  GLfloat dst0, GLfloat dst1,
> >                                  bool mirror)
> >  {
> > -   float scale = (src1 - src0) / (dst1 - dst0);
> > +   double scale = (double)(src1 - src0) / (double)(dst1 - dst0);
> >     if (!mirror) {
> >        /* When not mirroring a coordinate (say, X), we need:
> >         *   src_x - src_x0 = (dst_x - dst_x0 + 0.5) * scale
> > @@ -1269,7 +1269,7 @@ brw_blorp_setup_coord_transform(struct
> brw_blorp_coord_transform *xform,
> >         * so 0.5 provides the necessary correction.
> >         */
> >        xform->multiplier = scale;
> > -      xform->offset = src0 + (-dst0 + 0.5f) * scale;
> > +      xform->offset = src0 + (-(double)dst0 + 0.5) * scale;
> >     } else {
> >        /* When mirroring X we need:
> >         *   src_x - src_x0 = dst_x1 - dst_x - 0.5
> > @@ -1277,7 +1277,7 @@ brw_blorp_setup_coord_transform(struct
> brw_blorp_coord_transform *xform,
> >         *   src_x = src_x0 + (dst_x1 -dst_x - 0.5) * scale
> >         */
> >        xform->multiplier = -scale;
> > -      xform->offset = src0 + (dst1 - 0.5f) * scale;
> > +      xform->offset = src0 + ((double)dst1 - 0.5) * scale;
> >     }
> >  }
>
> Sounds reasonable.
>
> Reviewed-by: Kenneth Graunke <kenneth at whitecape.org>
>
> (really more of an ack, but...)
>

Thanks!
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