Mesa (main): draw/llvmpipe: correct exponent calculation for negative z

[GitLab Mirror]

Module: Mesa
Branch: main
Commit: e26fd607cab20715bf03d5ec221165171e0d0d1e
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=e26fd607cab20715bf03d5ec221165171e0d0d1e

Author: Erik Faye-Lund <erik.faye-lund at collabora.com>
Date:   Wed Aug 18 11:40:51 2021 +0200

draw/llvmpipe: correct exponent calculation for negative z

If the z components here contain negative values, we'll end up with the
wrong maximum value.

This updated equation is taken from the D3D11 functional spec (section
15.10 Depth Bias), which is a bit more clear than the OpenGL spec.

Reviewed-by: Roland Scheidegger <sroland at vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12443>

---

 src/gallium/auxiliary/draw/draw_pipe_offset.c |  2 +-
 src/gallium/drivers/llvmpipe/lp_state_setup.c | 13 ++++++++-----
 2 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/src/gallium/auxiliary/draw/draw_pipe_offset.c b/src/gallium/auxiliary/draw/draw_pipe_offset.c
index 769e7ca372d..87db9cddac4 100644
--- a/src/gallium/auxiliary/draw/draw_pipe_offset.c
+++ b/src/gallium/auxiliary/draw/draw_pipe_offset.c
@@ -97,7 +97,7 @@ static void do_offset_tri( struct draw_stage *stage,
    if (stage->draw->floating_point_depth) {
       float bias;
       union fi maxz;
-      maxz.f = MAX3(v0[2], v1[2], v2[2]);
+      maxz.f = MAX3(fabs(v0[2]), fabs(v1[2]), fabs(v2[2]));
       /* just do the math directly on shifted number */
       maxz.ui &= 0xff << 23;
       maxz.i -= 23 << 23;
diff --git a/src/gallium/drivers/llvmpipe/lp_state_setup.c b/src/gallium/drivers/llvmpipe/lp_state_setup.c
index 87216c04d9e..53ec2c1c38b 100644
--- a/src/gallium/drivers/llvmpipe/lp_state_setup.c
+++ b/src/gallium/drivers/llvmpipe/lp_state_setup.c
@@ -266,8 +266,8 @@ lp_do_offset_tri(struct gallivm_state *gallivm,
 
    if (key->floating_point_depth) {
       /*
-       * bias = pgon_offset_units * 2^(exponent(max(z0, z1, z2)) - mantissa_bits) +
-       *           MAX2(dzdx, dzdy) * pgon_offset_scale
+       * bias = pgon_offset_units * 2^(exponent(max(abs(z0), abs(z1), abs(z2))) -
+       *           mantissa_bits) + MAX2(dzdx, dzdy) * pgon_offset_scale
        *
        * NOTE: Assumes IEEE float32.
        */
@@ -280,11 +280,14 @@ lp_do_offset_tri(struct gallivm_state *gallivm,
       exp_mask = lp_build_const_int32(gallivm, 0xff << 23);
 
       maxz0z1_value = lp_build_max(&flt_scalar_bld,
-                         LLVMBuildExtractElement(b, attribv[0], twoi, ""),
-                         LLVMBuildExtractElement(b, attribv[1], twoi, ""));
+                         lp_build_abs(&flt_scalar_bld,
+                            LLVMBuildExtractElement(b, attribv[0], twoi, "")),
+                         lp_build_abs(&flt_scalar_bld,
+                            LLVMBuildExtractElement(b, attribv[1], twoi, "")));
 
       maxz_value = lp_build_max(&flt_scalar_bld,
-                      LLVMBuildExtractElement(b, attribv[2], twoi, ""),
+                      lp_build_abs(&flt_scalar_bld,
+                         LLVMBuildExtractElement(b, attribv[2], twoi, "")),
                       maxz0z1_value);
 
       exp = LLVMBuildBitCast(b, maxz_value, int_scalar_bld.vec_type, "");