[Mesa-dev] [PATCH 1/6] i965: Make Gen4-5 SF_STATE use the point size calculations from Gen6+.

[Kenneth Graunke]

Apparently, Nanhai made the Gen4-5 point size calculations round to the
nearest integer in commit 8d5231a3582e4f2769ac0685cf0174e09750700e,
"according to spec".  When Eric first ported the driver to Sandybridge,
he did not implement this rounding.

In the GL 2.1 and 3.0 specs "Basic Point Rasterization" section, it does
say "If antialiasing and point sprites are disabled, the actual width is
determined by rounding the supplied width to the nearest integer, then
clamping it to the implementation-dependent maximum non-antialised point
width."

In contrast, GL 3.1 and later do not appear to contain this rounding.

It might be reasonable to round, given that we only implement GL 2.1.
Of course, if we were to do that, we should actually implement the AA
vs. non-AA distinction.  Brian added an XXX comment reminding us to fix
this 10 years ago, but it never happened.

I think a better plan is to follow the newer, unrounded behavior.  This
is what we do on Gen6+ and it passes all the relevant conformance tests.
---
 src/mesa/drivers/dri/i965/brw_sf_state.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/src/mesa/drivers/dri/i965/brw_sf_state.c b/src/mesa/drivers/dri/i965/brw_sf_state.c
index d50ceb12133..ff6b5ebf79b 100644
--- a/src/mesa/drivers/dri/i965/brw_sf_state.c
+++ b/src/mesa/drivers/dri/i965/brw_sf_state.c
@@ -254,14 +254,15 @@ static void upload_sf_unit( struct brw_context *brw )
        */
       sf->sf6.point_rast_rule = BRW_RASTRULE_LOWER_RIGHT;
    }
-   /* XXX clamp max depends on AA vs. non-AA */
 
    /* _NEW_POINT */
    sf->sf7.sprite_point = ctx->Point.PointSprite;
-   sf->sf7.point_size = CLAMP(rintf(CLAMP(ctx->Point.Size,
-                                          ctx->Point.MinSize,
-                                          ctx->Point.MaxSize)), 1.0f, 255.0f) *
-                        (1<<3);
+
+   float point_sz;
+   point_sz = CLAMP(ctx->Point.Size, ctx->Point.MinSize, ctx->Point.MaxSize);
+   point_sz = CLAMP(point_sz, 0.125f, 255.875f);
+   sf->sf7.point_size = U_FIXED(point_sz, 3);
+
    /* _NEW_PROGRAM | _NEW_POINT */
    sf->sf7.use_point_size_state = !(ctx->VertexProgram.PointSizeEnabled ||
 				    ctx->Point._Attenuated);
-- 
2.12.2