gpu: alpha-to-coverage support

include/SDL3/SDL_gpu.h

@@ -1767,7 +1767,7 @@ typedef struct SDL_GPUMultisampleState
     SDL_GPUSampleCount sample_count;  /**< The number of samples to be used in rasterization. */
     Uint32 sample_mask;               /**< Reserved for future use. Must be set to 0. */
     bool enable_mask;             /**< Reserved for future use. Must be set to false. */
-    Uint8 padding1;
+    bool enable_alpha_to_coverage;    /**< true enables the alpha-to-coverage feature. */
     Uint8 padding2;
     Uint8 padding3;
 } SDL_GPUMultisampleState;

src/gpu/SDL_gpu.c

@@ -827,6 +827,12 @@ SDL_GPUGraphicsPipeline *SDL_CreateGPUGraphicsPipeline(
                 SDL_assert_release(!"Format is not supported for color targets on this device!");
                 return NULL;
             }
+            if (graphicsPipelineCreateInfo->multisample_state.enable_alpha_to_coverage &&
+                (IsIntegerFormat(graphicsPipelineCreateInfo->target_info.color_target_descriptions[i].format)
+                    || IsCompressedFormat(graphicsPipelineCreateInfo->target_info.color_target_descriptions[i].format))) {
+                SDL_assert_release(!"Format is not compatible with alpha-to-coverage!");
+                return NULL;
+            }
             if (graphicsPipelineCreateInfo->target_info.color_target_descriptions[i].blend_state.enable_blend) {
                 const SDL_GPUColorTargetBlendState *blend_state = &graphicsPipelineCreateInfo->target_info.color_target_descriptions[i].blend_state;
                 CHECK_BLENDFACTOR_ENUM_INVALID(blend_state->src_color_blendfactor, NULL)

src/gpu/SDL_sysgpu.h

@@ -373,6 +373,71 @@ static inline bool IsIntegerFormat(
     }
 }
 
+static inline bool IsCompressedFormat(
+    SDL_GPUTextureFormat format)
+{
+    switch (format) {
+    case SDL_GPU_TEXTUREFORMAT_BC1_RGBA_UNORM:
+    case SDL_GPU_TEXTUREFORMAT_BC1_RGBA_UNORM_SRGB:
+    case SDL_GPU_TEXTUREFORMAT_BC2_RGBA_UNORM:
+    case SDL_GPU_TEXTUREFORMAT_BC2_RGBA_UNORM_SRGB:
+    case SDL_GPU_TEXTUREFORMAT_BC3_RGBA_UNORM:
+    case SDL_GPU_TEXTUREFORMAT_BC3_RGBA_UNORM_SRGB:
+    case SDL_GPU_TEXTUREFORMAT_BC4_R_UNORM:
+    case SDL_GPU_TEXTUREFORMAT_BC5_RG_UNORM:
+    case SDL_GPU_TEXTUREFORMAT_BC6H_RGB_FLOAT:
+    case SDL_GPU_TEXTUREFORMAT_BC6H_RGB_UFLOAT:
+    case SDL_GPU_TEXTUREFORMAT_BC7_RGBA_UNORM:
+    case SDL_GPU_TEXTUREFORMAT_BC7_RGBA_UNORM_SRGB:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_4x4_UNORM:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_5x4_UNORM:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_5x5_UNORM:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_6x5_UNORM:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_6x6_UNORM:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_8x5_UNORM:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_8x6_UNORM:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_8x8_UNORM:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_10x5_UNORM:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_10x6_UNORM:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_10x8_UNORM:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_10x10_UNORM:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_12x10_UNORM:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_12x12_UNORM:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_4x4_UNORM_SRGB:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_5x4_UNORM_SRGB:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_5x5_UNORM_SRGB:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_6x5_UNORM_SRGB:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_6x6_UNORM_SRGB:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_8x5_UNORM_SRGB:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_8x6_UNORM_SRGB:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_8x8_UNORM_SRGB:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_10x5_UNORM_SRGB:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_10x6_UNORM_SRGB:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_10x8_UNORM_SRGB:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_10x10_UNORM_SRGB:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_12x10_UNORM_SRGB:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_12x12_UNORM_SRGB:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_4x4_FLOAT:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_5x4_FLOAT:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_5x5_FLOAT:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_6x5_FLOAT:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_6x6_FLOAT:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_8x5_FLOAT:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_8x6_FLOAT:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_8x8_FLOAT:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_10x5_FLOAT:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_10x6_FLOAT:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_10x8_FLOAT:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_10x10_FLOAT:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_12x10_FLOAT:
+    case SDL_GPU_TEXTUREFORMAT_ASTC_12x12_FLOAT:
+        return true;
+
+    default:
+        return false;
+    }
+}
+
 static inline Uint32 IndexSize(SDL_GPUIndexElementSize size)
 {
     return (size == SDL_GPU_INDEXELEMENTSIZE_16BIT) ? 2 : 4;

src/gpu/d3d12/SDL_gpu_d3d12.c

@@ -2819,7 +2819,7 @@ static bool D3D12_INTERNAL_ConvertBlendState(
     }
 
     SDL_zerop(blendDesc);
-    blendDesc->AlphaToCoverageEnable = FALSE;
+    blendDesc->AlphaToCoverageEnable = pipelineInfo->multisample_state.enable_alpha_to_coverage;
     blendDesc->IndependentBlendEnable = FALSE;
 
     for (UINT i = 0; i < MAX_COLOR_TARGET_BINDINGS; i += 1) {

src/gpu/metal/SDL_gpu_metal.m

@@ -1135,6 +1135,7 @@ static SDL_GPUGraphicsPipeline *METAL_CreateGraphicsPipeline(
         // Multisample
 
         pipelineDescriptor.rasterSampleCount = SDLToMetal_SampleCount[createinfo->multisample_state.sample_count];
+        pipelineDescriptor.alphaToCoverageEnabled = createinfo->multisample_state.enable_alpha_to_coverage;
 
         // Depth Stencil
 

src/gpu/vulkan/SDL_gpu_vulkan.c

@@ -6355,7 +6355,7 @@ static SDL_GPUGraphicsPipeline *VULKAN_CreateGraphicsPipeline(
     multisampleStateCreateInfo.sampleShadingEnable = VK_FALSE;
     multisampleStateCreateInfo.minSampleShading = 1.0f;
     multisampleStateCreateInfo.pSampleMask = &sampleMask;
-    multisampleStateCreateInfo.alphaToCoverageEnable = VK_FALSE;
+    multisampleStateCreateInfo.alphaToCoverageEnable = createinfo->multisample_state.enable_alpha_to_coverage;
     multisampleStateCreateInfo.alphaToOneEnable = VK_FALSE;
 
     // Depth Stencil State