Mesa (main): llvmpipe: clamp z to 0..1 range when using polygon offset

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Module: Mesa
Branch: main
Commit: 856533366928300bbad8949675b1c165ccb39605
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=856533366928300bbad8949675b1c165ccb39605

Author: Erik Faye-Lund <erik.faye-lund at collabora.com>
Date:   Wed Aug 18 11:00:33 2021 +0200

llvmpipe: clamp z to 0..1 range when using polygon offset

The OpenGL 4.6 compatibility spec, section 14.6.5 (Depth Offset) says
the following:

> For fixed-point depth buffers, fragment depth values are always
> limited to the range [0,1] by clamping after offset addition is
> performed. Fragment depth values are clamped even when the depth
> buffer uses a floating-point representation.

So we need to properly clamp the result here.

This fixes the following dEQP failures, that the CI has missed:

- dEQP-GLES3.functional.polygon_offset.default_result_depth_clamp
- dEQP-GLES3.functional.polygon_offset.default_factor_1_slope
- dEQP-GLES3.functional.polygon_offset.fixed24_result_depth_clamp
- dEQP-GLES3.functional.polygon_offset.fixed24_factor_1_slope

Reviewed-by: Roland Scheidegger <sroland at vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12442>

---

 src/gallium/drivers/llvmpipe/lp_bld_interp.c | 18 ++++++------------
 1 file changed, 6 insertions(+), 12 deletions(-)

diff --git a/src/gallium/drivers/llvmpipe/lp_bld_interp.c b/src/gallium/drivers/llvmpipe/lp_bld_interp.c
index 96bf8d4ea59..8b02ac9622c 100644
--- a/src/gallium/drivers/llvmpipe/lp_bld_interp.c
+++ b/src/gallium/drivers/llvmpipe/lp_bld_interp.c
@@ -414,19 +414,13 @@ attribs_update_simple(struct lp_build_interp_soa_context *bld,
             }
 
             if ((attrib == 0) && (chan == 2) && !bld->depth_clamp){
-               /* FIXME: Depth values can exceed 1.0, due to the fact that
-                * setup interpolation coefficients refer to (0,0) which causes
-                * precision loss. So we must clamp to 1.0 here to avoid artifacts.
-                * Note though values outside [0,1] are perfectly valid with
-                * depth clip disabled.
-                * XXX: If depth clip is disabled but we force depth clamp
-                * we may get values larger than 1.0 in the fs (but not in
-                * depth test). Not sure if that's an issue...
-                * Also, on a similar note, it is not obvious if the depth values
-                * appearing in fs (with depth clip disabled) should be clamped
-                * to [0,1], clamped to near/far or not be clamped at all...
+               /* OpenGL requires clamping z to 0..1 range after polgon offset
+                * is applied if depth-clamping isn't enabled.
+                *
+                * This also fixes the problem that depth values can exceed 1.0,
+                * due to imprecision in the calculations.
                 */
-               a = lp_build_min(coeff_bld, a, coeff_bld->one);
+               a = lp_build_clamp(coeff_bld, a, coeff_bld->zero, coeff_bld->one);
             }
             bld->attribs[attrib][chan] = a;
          }