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, 5 insertions, 0 deletions

diff --git a/src/video_core/renderer_opengl/gl_device.h b/src/video_core/renderer_opengl/gl_device.h
index 3ff8cad83..e8104c4de 100644
--- a/src/video_core/renderer_opengl/gl_device.h
+++ b/src/video_core/renderer_opengl/gl_device.h
@@ -160,6 +160,10 @@ public:
         return need_fastmath_off;
     }
 
+    bool NeedsGatherSubpixelOffset() const {
+        return need_gather_subpixel_offset;
+    }
+
     bool HasCbufFtouBug() const {
         return has_cbuf_ftou_bug;
     }
@@ -225,6 +229,7 @@ private:
     bool has_draw_texture{};
     bool warp_size_potentially_larger_than_guest{};
     bool need_fastmath_off{};
+    bool need_gather_subpixel_offset{};
     bool has_cbuf_ftou_bug{};
     bool has_bool_ref_bug{};
     bool can_report_memory{};