op/clip: don't accept open Paths for Outline

Outline represents a clipping operations that clips all drawing outside
a closed path. Before this change, paths not closed we're patched up by
adding an implicit line from the endpoint to the beginning.

These fixups are inefficient for a rare case, but acceptable because the
old renderer post-processes all paths anyway. However, the new compute
renderer don't need post-processing in most cases, making fixups too
expensive.

Given that clipping to an open path is fundamentally undefined and that
implicit fixup with a closing line segment is merely a way to force the
clip to be well-defined, this change adds a panic to Outline for Paths
that are not closed.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gpu/gpu.go

@@ -1385,37 +1385,15 @@ func (d *drawOps) buildVerts(aux []byte, tr f32.Affine2D, outline bool, stroke c
 
 	case outline:
 		// Outline path.
-		first := true
-		var firstPt, lastPt f32.Point
 		for len(aux) >= ops.QuadSize+4 {
 			d.qs.contour = bo.Uint32(aux)
 			quad := ops.DecodeQuad(aux[4:])
 			quad = quad.Transform(tr)
 
-			if first {
-				first = false
-				firstPt = quad.From
-				lastPt = quad.From
-			}
-			if quad.From != lastPt {
-				if lastPt != firstPt {
-					// Close outline before starting a new.
-					mid := firstPt.Add(lastPt).Mul(.5)
-					d.qs.splitAndEncode(ops.Quad{From: lastPt, To: firstPt, Ctrl: mid})
-				}
-				firstPt = quad.From
-			}
-			lastPt = quad.To
-
 			d.qs.splitAndEncode(quad)
 
 			aux = aux[ops.QuadSize+4:]
 		}
-		// Close last outline if necessary.
-		if !first && lastPt != firstPt {
-			mid := firstPt.Add(lastPt).Mul(.5)
-			d.qs.splitAndEncode(ops.Quad{From: lastPt, To: firstPt, Ctrl: mid})
-		}
 	}
 
 	fillMaxY(d.vertCache[startLength:])

gpu/internal/rendertest/clip_test.go

@@ -91,6 +91,7 @@ func TestPaintArc(t *testing.T) {
 		p.Arc(f32.Pt(-10, -20), f32.Pt(10, -5), math.Pi)
 		p.Line(f32.Pt(0, -10))
 		p.Line(f32.Pt(-50, 0))
+		p.Close()
 		clip.Outline{
 			Path: p.End(),
 		}.Op().Add(o)
@@ -112,6 +113,7 @@ func TestPaintAbsolute(t *testing.T) {
 		p.MoveTo(f32.Pt(20, 20))
 		p.LineTo(f32.Pt(80, 20))
 		p.QuadTo(f32.Pt(80, 80), f32.Pt(20, 80))
+		p.Close()
 		clip.Outline{
 			Path: p.End(),
 		}.Op().Add(o)