Clay custom dispatch improvements & DPI scaling fixes (#26)

Co-authored-by: Zachary Levy <zachary@sunforge.is>
Reviewed-on: #26

draw/tess/tess.odin

@@ -21,8 +21,8 @@ auto_segments :: proc(radius: f32, arc_degrees: f32) -> int {
 
 // ----- Internal helpers -----
 
-// Color is premultiplied: the tessellated fragment shader passes it through directly
-// and the blend state is ONE, ONE_MINUS_SRC_ALPHA.
+// Premultiplies the color before storing it on the vertex (see draw package doc's
+// "Color and blending" section for why).
 //INTERNAL
 solid_vertex :: proc(position: draw.Vec2, color: draw.Color) -> draw.Vertex_2D {
 	return draw.Vertex_2D{position = position, color = draw.premultiply_color(color)}
@@ -108,16 +108,23 @@ triangle :: proc(
 	draw.prepare_shape(layer, vertices[:])
 }
 
-// Draw an anti-aliased triangle via extruded edge quads.
+// Draw an anti-aliased triangle via extruded edge quads plus corner fan caps.
 // Interior vertices get the full premultiplied color; outer fringe vertices get BLANK (0,0,0,0).
-// The rasterizer linearly interpolates between them, producing a smooth 1-pixel AA band.
-// `aa_px` controls the extrusion width in logical pixels (default 1.0).
-// This proc emits 21 vertices (3 interior + 6 edge quads × 3 verts each).
+// The rasterizer linearly interpolates between them, producing a smooth ~1-physical-pixel AA band.
+// `aa_ppx` controls the extrusion width in *physical* pixels (default 1.0). The CPU divides by
+// `dpi_scaling` here so the vertex stream stays in logical px; the mode-0 vertex shader scales
+// back to physical at draw time. Net AA band is ~aa_ppx physical pixels regardless of DPI.
+//
+// Topology: 3 interior verts + 6 edge-quad triangles (×3 verts) + 3 corner-fan triangles (×3 verts)
+// = 30 verts total. The corner fans plug the wedge gaps that would otherwise appear between
+// adjacent edge fringes at each triangle vertex; without them, sharp corners show a small
+// background-colored crescent. Apex vertex is full color, both fringe verts are BLANK, so the
+// fan rasterizes as an alpha-falloff triangle that blends visually into the adjacent edge bands.
 triangle_aa :: proc(
 	layer: ^draw.Layer,
 	v1, v2, v3: draw.Vec2,
 	color: draw.Color,
-	aa_px: f32 = draw.DFT_FEATHER_PX,
+	aa_ppx: f32 = draw.DFT_FEATHER_PPX,
 	origin: draw.Vec2 = {},
 	rotation: f32 = 0,
 ) {
@@ -164,7 +171,9 @@ triangle_aa :: proc(
 	normal_12 := edge_normal(p1, p2, centroid_x, centroid_y)
 	normal_20 := edge_normal(p2, p0, centroid_x, centroid_y)
 
-	extrude_distance := aa_px * draw.GLOB.dpi_scaling
+	// aa_ppx is in physical pixels; divide by dpi_scaling so the extrusion lives in logical-pixel
+	// space (the mode-0 vertex shader will scale back to physical at draw time).
+	extrude_distance := aa_ppx / draw.GLOB.dpi_scaling
 
 	// Outer fringe vertices: each edge vertex extruded outward
 	outer_0_01 := p0 + normal_01 * extrude_distance
@@ -178,8 +187,8 @@ triangle_aa :: proc(
 	// Outer fringe is BLANK = {0,0,0,0} which is already premul.
 	transparent := draw.BLANK
 
-	// 3 interior + 6 × 3 edge-quad = 21 vertices
-	vertices: [21]draw.Vertex_2D
+	// 3 interior + 6 edge-quad tris (×3 verts) + 3 corner-fan tris (×3 verts) = 30 vertices
+	vertices: [30]draw.Vertex_2D
 
 	// Interior triangle
 	vertices[0] = solid_vertex(p0, color)
@@ -210,6 +219,27 @@ triangle_aa :: proc(
 	vertices[19] = solid_vertex(outer_0_20, transparent)
 	vertices[20] = solid_vertex(outer_2_20, transparent)
 
+	// Corner fan caps: each fills the wedge gap between the two edge fringes meeting at a
+	// triangle vertex. Apex is full color; both fringe verts are BLANK, so the rasterizer
+	// produces a smooth alpha falloff across the wedge (matches the adjacent edge-band
+	// gradients at the shared edges, so the seams are invisible). Vertex order per fan:
+	// [apex, fringe-from-incoming-edge, fringe-from-outgoing-edge].
+
+	// Cap at p0 (between incoming edge p2→p0 and outgoing edge p0→p1)
+	vertices[21] = solid_vertex(p0, color)
+	vertices[22] = solid_vertex(outer_0_20, transparent)
+	vertices[23] = solid_vertex(outer_0_01, transparent)
+
+	// Cap at p1 (between incoming edge p0→p1 and outgoing edge p1→p2)
+	vertices[24] = solid_vertex(p1, color)
+	vertices[25] = solid_vertex(outer_1_01, transparent)
+	vertices[26] = solid_vertex(outer_1_12, transparent)
+
+	// Cap at p2 (between incoming edge p1→p2 and outgoing edge p2→p0)
+	vertices[27] = solid_vertex(p2, color)
+	vertices[28] = solid_vertex(outer_2_12, transparent)
+	vertices[29] = solid_vertex(outer_2_20, transparent)
+
 	draw.prepare_shape(layer, vertices[:])
 }