shader_recompiler: Add subpixel offset for correct rounding at `ImageGather`

On AMD a subpixel offset of 1/512 of the texel size is applied to the texture coordinates at a ImageGather call to ensure the rounding at the texel centers is done the same way as in Maxwell or other Nvidia architectures.
See https://www.reedbeta.com/blog/texture-gathers-and-coordinate-precision/ for more details why this might be necessary.

This should fix shadow artifacts at object edges in Zelda: Breath of the Wild (#9957, #6956).

1 files changed, 4 insertions, 0 deletions

diff --git a/src/shader_recompiler/profile.h b/src/shader_recompiler/profile.h
index 253e0d0bd..31390e869 100644
--- a/src/shader_recompiler/profile.h
+++ b/src/shader_recompiler/profile.h
@@ -52,6 +52,10 @@ struct Profile {
     bool need_declared_frag_colors{};
     /// Prevents fast math optimizations that may cause inaccuracies
     bool need_fastmath_off{};
+    /// Some GPU vendors use a lower fixed point format of 16.8 when calculating pixel coordinates
+    /// in the ImageGather instruction than the Maxwell architecture does. Applying an offset does
+    /// fix this mismatching rounding behaviour.
+    bool need_gather_subpixel_offset{};
 
     /// OpFClamp is broken and OpFMax + OpFMin should be used instead
     bool has_broken_spirv_clamp{};