not sure

src/pipelines/eraser/eraser-texture.wgsl

@@ -1,8 +1,12 @@
 struct Settings {
   eraserRadiusSquared: f32,
+  lineDistanceEpsilon: f32,
+  clearRed: f32,
+  clearGreen: f32,
+  clearBlue: f32,
+  clearAlpha: f32,
   padding0: f32,
   padding1: f32,
-  padding2: f32,
 };
 
 @group(1) @binding(0) var<uniform> settings: Settings;
@@ -10,8 +14,8 @@ struct Settings {
 struct VertexOutput {
   @builtin(position) position: vec4<f32>,
   @location(0) screenPosition: vec2<f32>,
-  @location(1) start: vec2<f32>,
-  @location(2) end: vec2<f32>
+  @location(1) @interpolate(flat) start: vec2<f32>,
+  @location(2) @interpolate(flat) end: vec2<f32>
 }
 
 struct EraserTextureTargets {
@@ -20,12 +24,20 @@ struct EraserTextureTargets {
   @location(2) trail: vec4<f32>,
 }
 
+struct EraserCombinedTargets {
+  @location(0) mask: vec4<f32>,
+  @location(1) source: vec4<f32>,
+  @location(2) influence: vec4<f32>,
+  @location(3) trail: vec4<f32>,
+}
+
 @vertex
 fn vertex(
-  @location(0) screenPosition: vec2<f32>,
-  @location(1) @interpolate(flat) start: vec2<f32>,
-  @location(2) @interpolate(flat) end: vec2<f32>
+  @builtin(vertex_index) vertexIndex: u32,
+  @location(0) start: vec2<f32>,
+  @location(1) end: vec2<f32>
 ) -> VertexOutput {
+  let screenPosition = segment_vertex_position(vertexIndex, start, end, sqrt(settings.eraserRadiusSquared));
   let uv = screenPosition / state.size;
   let position = vec2(uv.x * 2.0 - 1.0, 1.0 - uv.y * 2.0);
   return VertexOutput(vec4(position, 0.0, 1.0), screenPosition, start, end);
@@ -34,30 +46,53 @@ fn vertex(
 @fragment
 fn fragment(
   @location(0) screenPosition: vec2<f32>,
-  @location(1) start: vec2<f32>,
-  @location(2) end: vec2<f32>
+  @location(1) @interpolate(flat) start: vec2<f32>,
+  @location(2) @interpolate(flat) end: vec2<f32>
 ) -> @location(0) vec4<f32> {
   if shouldDiscardEraserFragment(screenPosition, start, end) {
     discard;
   }
 
-  return vec4<f32>(0.0, 0.0, 0.0, 0.0);
+  return getEraserClearValue();
 }
 
 @fragment
 fn fragmentMrt(
   @location(0) screenPosition: vec2<f32>,
-  @location(1) start: vec2<f32>,
-  @location(2) end: vec2<f32>
+  @location(1) @interpolate(flat) start: vec2<f32>,
+  @location(2) @interpolate(flat) end: vec2<f32>
 ) -> EraserTextureTargets {
   if shouldDiscardEraserFragment(screenPosition, start, end) {
     discard;
   }
 
-  let cleared = vec4<f32>(0.0, 0.0, 0.0, 0.0);
+  let cleared = getEraserClearValue();
   return EraserTextureTargets(cleared, cleared, cleared);
 }
 
+@fragment
+fn fragmentCombined(
+  @location(0) screenPosition: vec2<f32>,
+  @location(1) @interpolate(flat) start: vec2<f32>,
+  @location(2) @interpolate(flat) end: vec2<f32>
+) -> EraserCombinedTargets {
+  if shouldDiscardEraserFragment(screenPosition, start, end) {
+    discard;
+  }
+
+  let cleared = getEraserClearValue();
+  return EraserCombinedTargets(cleared, cleared, cleared, cleared);
+}
+
+fn getEraserClearValue() -> vec4<f32> {
+  return vec4<f32>(
+    settings.clearRed,
+    settings.clearGreen,
+    settings.clearBlue,
+    settings.clearAlpha
+  );
+}
+
 fn shouldDiscardEraserFragment(
   screenPosition: vec2<f32>,
   start: vec2<f32>,
@@ -71,7 +106,7 @@ fn distanceSquaredFromLine(position: vec2<f32>, start: vec2<f32>, end: vec2<f32>
   let direction = end - start;
   let denominator = dot(direction, direction);
 
-  if denominator <= 0.0001 {
+  if denominator <= settings.lineDistanceEpsilon {
     return dot(pa, pa);
   }
 
@@ -79,3 +114,34 @@ fn distanceSquaredFromLine(position: vec2<f32>, start: vec2<f32>, end: vec2<f32>
   let nearestOffset = pa - direction * q;
   return dot(nearestOffset, nearestOffset);
 }
+
+fn segment_vertex_position(
+  vertexIndex: u32,
+  start: vec2<f32>,
+  end: vec2<f32>,
+  radius: f32
+) -> vec2<f32> {
+  let directionVector = end - start;
+  let segmentLength = length(directionVector);
+  var direction = vec2<f32>(1.0, 0.0);
+  if segmentLength > 0.0 {
+    direction = directionVector / segmentLength;
+  }
+  let perpendicular = vec2<f32>(direction.y, -direction.x);
+  let corner = segment_vertex_corner(vertexIndex % 6u);
+  let center = mix(start, end, (corner.x + 1.0) * 0.5);
+  return center + direction * corner.x * radius + perpendicular * corner.y * radius;
+}
+
+fn segment_vertex_corner(index: u32) -> vec2<f32> {
+  if index == 0u {
+    return vec2<f32>(-1.0, 1.0);
+  }
+  if index == 1u || index == 3u {
+    return vec2<f32>(-1.0, -1.0);
+  }
+  if index == 2u || index == 4u {
+    return vec2<f32>(1.0, 1.0);
+  }
+  return vec2<f32>(1.0, -1.0);
+}