Support per-draw blend mode and alpha in WebGPU

Author: JimBobSquarePants

src/ImageSharp.Drawing.WebGPU.ShaderGen/WgslSourceGenerator.cs

@@ -102,10 +102,9 @@ private static void Execute(SourceProductionContext context, ImmutableArray<Shad
     // property exposes both the expanded WGSL text and a null-terminated UTF-8 payload.
     private static string GenerateSource(SourceProductionContext context, Dictionary<string, string> fileMap)
     {
-        IReadOnlyList<string> rootFiles = fileMap.Keys
+        IEnumerable<string> rootFiles = fileMap.Keys
             .Where(static path => !path.StartsWith("Shared/", StringComparison.Ordinal))
-            .OrderBy(static path => path, StringComparer.Ordinal)
-            .ToArray();
+            .OrderBy(static path => path, StringComparer.Ordinal);
 
         StringBuilder builder = new();
         _ = builder.AppendLine("// <auto-generated/>");

src/ImageSharp.Drawing.WebGPU/Shaders/Shared/blend.wgsl

@@ -9,6 +9,8 @@ const MIX_SCREEN = 2u;
 const MIX_OVERLAY = 3u;
 const MIX_DARKEN = 4u;
 const MIX_LIGHTEN = 5u;
+const MIX_ADD = 16u;
+const MIX_SUBTRACT = 17u;
 const MIX_COLOR_DODGE = 6u;
 const MIX_COLOR_BURN = 7u;
 const MIX_HARD_LIGHT = 8u;
@@ -162,6 +164,12 @@ fn blend_mix(cb: vec3<f32>, cs: vec3<f32>, mode: u32) -> vec3<f32> {
         case MIX_LIGHTEN: {
             b = max(cb, cs);
         }
+        case MIX_ADD: {
+            b = min(vec3(1.0), cb + cs);
+        }
+        case MIX_SUBTRACT: {
+            b = max(vec3(0.0), cb - cs);
+        }
         case MIX_COLOR_DODGE: {
             b = vec3(color_dodge(cb.x, cs.x), color_dodge(cb.y, cs.y), color_dodge(cb.z, cs.z));
         }

src/ImageSharp.Drawing.WebGPU/Shaders/Shared/drawtag.wgsl

@@ -32,6 +32,10 @@ const DRAWTAG_END_CLIP = 0x21u;
 /// draw object stream but get used after the draw objects have been reduced on the GPU.
 /// 0 represents a non-zero fill. 1 represents an even-odd fill.
 const DRAW_INFO_FLAGS_FILL_RULE_BIT = 1u;
+const DRAW_FLAGS_BLEND_MODE_SHIFT = 1u;
+const DRAW_FLAGS_BLEND_MODE_MASK = 0x3ffeu;
+const DRAW_FLAGS_BLEND_ALPHA_SHIFT = 14u;
+const DRAW_FLAGS_BLEND_ALPHA_MASK = 0x3fffc000u;
 
 fn draw_monoid_identity() -> DrawMonoid {
     return DrawMonoid();

src/ImageSharp.Drawing.WebGPU/Shaders/Shared/ptcl.wgsl

@@ -46,12 +46,14 @@ struct CmdJump {
 
 struct CmdColor {
     rgba_color: u32,
+    draw_flags: u32,
 }
 
 struct CmdRecolor {
     source_color: u32,
     target_color: u32,
     threshold: f32,
+    draw_flags: u32,
 }
 
 struct CmdBlurRect {

src/ImageSharp.Drawing.WebGPU/Shaders/coarse.wgsl

@@ -110,19 +110,21 @@ fn write_path(tile: Tile, tile_ix: u32, draw_flags: u32) {
 }
 
 fn write_color(color: CmdColor) {
-    alloc_cmd(2u);
+    alloc_cmd(3u);
     ptcl[cmd_offset] = CMD_COLOR;
     ptcl[cmd_offset + 1u] = color.rgba_color;
-    cmd_offset += 2u;
+    ptcl[cmd_offset + 2u] = color.draw_flags;
+    cmd_offset += 3u;
 }
 
-fn write_recolor(source_color: u32, target_color: u32, threshold: u32) {
-    alloc_cmd(4u);
+fn write_recolor(source_color: u32, target_color: u32, threshold: u32, draw_flags: u32) {
+    alloc_cmd(5u);
     ptcl[cmd_offset] = CMD_RECOLOR;
     ptcl[cmd_offset + 1u] = source_color;
     ptcl[cmd_offset + 2u] = target_color;
     ptcl[cmd_offset + 3u] = threshold;
-    cmd_offset += 4u;
+    ptcl[cmd_offset + 4u] = draw_flags;
+    cmd_offset += 5u;
 }
 
 fn write_grad(ty: u32, index: u32, info_offset: u32) {
@@ -388,17 +390,17 @@ fn main(
                     case DRAWTAG_FILL_COLOR: {
                         write_path(tile, tile_ix, draw_flags);
                         let rgba_color = scene[dd];
-                        write_color(CmdColor(rgba_color));
+                        write_color(CmdColor(rgba_color, draw_flags));
                     }
                     case DRAWTAG_FILL_RECOLOR: {
                         write_path(tile, tile_ix, draw_flags);
-                        write_recolor(scene[dd], scene[dd + 1u], scene[dd + 2u]);
+                        write_recolor(scene[dd], scene[dd + 1u], scene[dd + 2u], draw_flags);
                     }
                     case DRAWTAG_BLURRED_ROUNDED_RECT: {
                         write_path(tile, tile_ix, draw_flags);
                         let rgba_color = scene[dd];
                         let info_offset = di + 1u;
-                        write_blurred_rounded_rect(CmdColor(rgba_color), info_offset);
+                        write_blurred_rounded_rect(CmdColor(rgba_color, draw_flags), info_offset);
                     }
                     case DRAWTAG_FILL_LIN_GRADIENT: {
                         write_path(tile, tile_ix, draw_flags);