3 files changed, 63 insertions, 0 deletions

diff --git a/src/shader_recompiler/backend/glsl/emit_glsl_image.cpp b/src/shader_recompiler/backend/glsl/emit_glsl_image.cpp
index f335c8af0..418505475 100644
--- a/src/shader_recompiler/backend/glsl/emit_glsl_image.cpp
+++ b/src/shader_recompiler/backend/glsl/emit_glsl_image.cpp
@@ -143,6 +143,21 @@ IR::Inst* PrepareSparse(IR::Inst& inst) {
     }
     return sparse_inst;
 }
+
+std::string ImageGatherSubpixelOffset(const IR::TextureInstInfo& info, std::string_view texture,
+                                      std::string_view coords) {
+    switch (info.type) {
+    case TextureType::Color2D:
+    case TextureType::Color2DRect:
+        return fmt::format("{}+vec2(0.001953125)/vec2(textureSize({}, 0))", coords, texture);
+    case TextureType::ColorArray2D:
+    case TextureType::ColorCube:
+        return fmt::format("vec3({0}.xy+vec2(0.001953125)/vec2(textureSize({1}, 0)),{0}.z)", coords,
+                           texture);
+    default:
+        return std::string{coords};
+    }
+}
 } // Anonymous namespace
 
 void EmitImageSampleImplicitLod(EmitContext& ctx, IR::Inst& inst, const IR::Value& index,
@@ -340,6 +355,13 @@ void EmitImageGather(EmitContext& ctx, IR::Inst& inst, const IR::Value& index,
         LOG_WARNING(Shader_GLSL, "Device does not support sparse texture queries. STUBBING");
         ctx.AddU1("{}=true;", *sparse_inst);
     }
+    std::string coords_with_subpixel_offset;
+    if (ctx.profile.need_gather_subpixel_offset) {
+        // Apply a subpixel offset of 1/512 the texel size of the texture to ensure same rounding on
+        // AMD hardware as on Maxwell or other Nvidia architectures.
+        coords_with_subpixel_offset = ImageGatherSubpixelOffset(info, texture, coords);
+        coords = coords_with_subpixel_offset;
+    }
     if (!sparse_inst || !supports_sparse) {
         if (offset.IsEmpty()) {
             ctx.Add("{}=textureGather({},{},int({}));", texel, texture, coords,
@@ -387,6 +409,13 @@ void EmitImageGatherDref(EmitContext& ctx, IR::Inst& inst, const IR::Value& inde
         LOG_WARNING(Shader_GLSL, "Device does not support sparse texture queries. STUBBING");
         ctx.AddU1("{}=true;", *sparse_inst);
     }
+    std::string coords_with_subpixel_offset;
+    if (ctx.profile.need_gather_subpixel_offset) {
+        // Apply a subpixel offset of 1/512 the texel size of the texture to ensure same rounding on
+        // AMD hardware as on Maxwell or other Nvidia architectures.
+        coords_with_subpixel_offset = ImageGatherSubpixelOffset(info, texture, coords);
+        coords = coords_with_subpixel_offset;
+    }
     if (!sparse_inst || !supports_sparse) {
         if (offset.IsEmpty()) {
             ctx.Add("{}=textureGather({},{},{});", texel, texture, coords, dref);
diff --git a/src/shader_recompiler/backend/spirv/emit_spirv_image.cpp b/src/shader_recompiler/backend/spirv/emit_spirv_image.cpp
index 02073c420..7d901c04b 100644
--- a/src/shader_recompiler/backend/spirv/emit_spirv_image.cpp
+++ b/src/shader_recompiler/backend/spirv/emit_spirv_image.cpp
@@ -261,6 +261,30 @@ Id BitTest(EmitContext& ctx, Id mask, Id bit) {
     const Id bit_value{ctx.OpBitwiseAnd(ctx.U32[1], shifted, ctx.Const(1u))};
     return ctx.OpINotEqual(ctx.U1, bit_value, ctx.u32_zero_value);
 }
+
+Id ImageGatherSubpixelOffset(EmitContext& ctx, const IR::TextureInstInfo& info, Id texture,
+                             Id coords) {
+    // Apply a subpixel offset of 1/512 the texel size of the texture to ensure same rounding on
+    // AMD hardware as on Maxwell or other Nvidia architectures.
+    const auto calculate_coords{[&](size_t dim) {
+        const Id nudge{ctx.Const(0x1p-9f)};
+        const Id image_size{ctx.OpImageQuerySizeLod(ctx.U32[dim], texture, ctx.u32_zero_value)};
+        Id offset{dim == 2 ? ctx.ConstantComposite(ctx.F32[dim], nudge, nudge)
+                           : ctx.ConstantComposite(ctx.F32[dim], nudge, nudge, ctx.f32_zero_value)};
+        offset = ctx.OpFDiv(ctx.F32[dim], offset, ctx.OpConvertUToF(ctx.F32[dim], image_size));
+        return ctx.OpFAdd(ctx.F32[dim], coords, offset);
+    }};
+    switch (info.type) {
+    case TextureType::Color2D:
+    case TextureType::Color2DRect:
+        return calculate_coords(2);
+    case TextureType::ColorArray2D:
+    case TextureType::ColorCube:
+        return calculate_coords(3);
+    default:
+        return coords;
+    }
+}
 } // Anonymous namespace
 
 Id EmitBindlessImageSampleImplicitLod(EmitContext&) {
@@ -423,6 +447,9 @@ Id EmitImageGather(EmitContext& ctx, IR::Inst* inst, const IR::Value& index, Id
                    const IR::Value& offset, const IR::Value& offset2) {
     const auto info{inst->Flags<IR::TextureInstInfo>()};
     const ImageOperands operands(ctx, offset, offset2);
+    if (ctx.profile.need_gather_subpixel_offset) {
+        coords = ImageGatherSubpixelOffset(ctx, info, TextureImage(ctx, info, index), coords);
+    }
     return Emit(&EmitContext::OpImageSparseGather, &EmitContext::OpImageGather, ctx, inst,
                 ctx.F32[4], Texture(ctx, info, index), coords, ctx.Const(info.gather_component),
                 operands.MaskOptional(), operands.Span());
@@ -432,6 +459,9 @@ Id EmitImageGatherDref(EmitContext& ctx, IR::Inst* inst, const IR::Value& index,
                        const IR::Value& offset, const IR::Value& offset2, Id dref) {
     const auto info{inst->Flags<IR::TextureInstInfo>()};
     const ImageOperands operands(ctx, offset, offset2);
+    if (ctx.profile.need_gather_subpixel_offset) {
+        coords = ImageGatherSubpixelOffset(ctx, info, TextureImage(ctx, info, index), coords);
+    }
     return Emit(&EmitContext::OpImageSparseDrefGather, &EmitContext::OpImageDrefGather, ctx, inst,
                 ctx.F32[4], Texture(ctx, info, index), coords, dref, operands.MaskOptional(),
                 operands.Span());
diff --git a/src/shader_recompiler/profile.h b/src/shader_recompiler/profile.h
index 253e0d0bd..9f88fb440 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 different rounding precision when calculating texture pixel
+    /// coordinates with the 16.8 format in the ImageGather instruction than the Maxwell
+    /// architecture. 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{};