Fill what a rounded rectangle contains outright as spans instead of pixel by pixel

fb.go

@@ -352,29 +352,68 @@ func (fb *framebuffer) rect(x0, y0, w, h int, c rgb) {
 	}
 }
 
-// Distance to the rectangle the corner radius sweeps around, which is zero
-// across the whole flat middle and grows only near a corner. Taking coverage
-// from that rather than from a plain inside test keeps the curves smooth
-// instead of stepped.
+// The rectangle the corner radius sweeps around. Distance to it is zero across
+// the whole flat middle and grows only near a corner. Taking coverage from that
+// rather than from a plain inside test keeps the curves smooth instead of
+// stepped.
+type sweep struct {
+	ix0, iy0, ix1, iy1 float64
+	r                  int
+}
+
+func (s sweep) pixel(fb *framebuffer, x, y int, c rgb) {
+	fx, fy := float64(x), float64(y)
+	dx := math.Max(math.Max(s.ix0-fx, fx-s.ix1), 0)
+	dy := math.Max(math.Max(s.iy0-fy, fy-s.iy1), 0)
+	cov := float64(s.r) - math.Sqrt(dx*dx+dy*dy) + 0.5
+	if cov <= 0 {
+		return
+	}
+	fb.blend(x, y, c, uint8(math.Min(cov, 1)*255))
+}
+
+// Partial coverage reaches no further than the corner blocks and the one line
+// of pixels the curve runs tangent to along each flat edge. The sweep contains
+// everything else outright, which fills as spans instead of a pixel at a time.
 func (fb *framebuffer) roundRect(x0, y0, w, h, r int, c rgb) {
 	// A radius past half the shorter side has no meaning and would put the
 	// swept rectangle inside out, which matters while something is growing from
 	// nothing.
 	r = min(r, min(w, h)/2)
-	ix0, iy0 := float64(x0+r), float64(y0+r)
-	ix1, iy1 := float64(x0+w-1-r), float64(y0+h-1-r)
+	s := sweep{float64(x0 + r), float64(y0 + r),
+		float64(x0 + w - 1 - r), float64(y0 + h - 1 - r), r}
+
+	// With no radius the sweep never reaches a whole pixel, so the sliver it
+	// leaves is partly covered throughout rather than solid anywhere.
+	if r == 0 {
 		for y := y0; y < y0+h; y++ {
 			for x := x0; x < x0+w; x++ {
-			fx, fy := float64(x), float64(y)
-			dx := math.Max(math.Max(ix0-fx, fx-ix1), 0)
-			dy := math.Max(math.Max(iy0-fy, fy-iy1), 0)
-			cov := float64(r) - math.Sqrt(dx*dx+dy*dy) + 0.5
-			if cov <= 0 {
-				continue
-			}
-			fb.blend(x, y, c, uint8(math.Min(cov, 1)*255))
+				s.pixel(fb, x, y, c)
 			}
 		}
+		return
+	}
+
+	for j := 0; j < r; j++ {
+		for i := 0; i < r; i++ {
+			s.pixel(fb, x0+i, y0+j, c)
+			s.pixel(fb, x0+w-1-i, y0+j, c)
+			s.pixel(fb, x0+i, y0+h-1-j, c)
+			s.pixel(fb, x0+w-1-i, y0+h-1-j, c)
+		}
+	}
+	for x := x0 + r; x < x0+w-r; x++ {
+		s.pixel(fb, x, y0, c)
+		s.pixel(fb, x, y0+h-1, c)
+	}
+	for y := y0 + r; y < y0+h-r; y++ {
+		s.pixel(fb, x0, y, c)
+		s.pixel(fb, x0+w-1, y, c)
+	}
+
+	fb.rect(x0+r, y0+1, w-2*r, h-2, c)
+	fb.rect(x0+1, y0+r, r-1, h-2*r, c)
+	fb.rect(x0+w-r, y0+r, r-1, h-2*r, c)
 }
 
 // Blends src over the existing pixel, with cov as 0-255 coverage.