In draw.Draw, separate the source-point and mask-point.

This lets you draw text (i.e. with mask = a font image) with
sources that aren't uniform colors.

R=r, rsc
CC=golang-dev
https://golang.org/cl/193067

2 files changed, 66 insertions, 56 deletions

diff --git a/src/pkg/exp/4s/xs.go b/src/pkg/exp/4s/xs.go
index 94a6f8597b..e1d28c8850 100644
--- a/src/pkg/exp/4s/xs.go
+++ b/src/pkg/exp/4s/xs.go
@@ -226,8 +226,8 @@ func collider(pt, pmax draw.Point) bool {
 }
 
 func setpiece(p *Piece) {
-	draw.Draw(bb, bbr, draw.White, nil, draw.ZP)
-	draw.Draw(bbmask, bbr, draw.Transparent, nil, draw.ZP)
+	draw.Draw(bb, bbr, draw.White, draw.ZP)
+	draw.Draw(bbmask, bbr, draw.Transparent, draw.ZP)
 	br = draw.Rect(0, 0, 0, 0)
 	br2 = br
 	piece = p
@@ -243,13 +243,13 @@ func setpiece(p *Piece) {
 		r.Max.X = r.Min.X + pcsz
 		r.Max.Y = r.Min.Y + pcsz
 		if i == 0 {
-			draw.Draw(bb, r, draw.Black, nil, draw.ZP)
-			draw.Draw(bb, r.Inset(1), txpix[piece.tx], nil, draw.ZP)
-			draw.Draw(bbmask, r, draw.Opaque, nil, draw.ZP)
+			draw.Draw(bb, r, draw.Black, draw.ZP)
+			draw.Draw(bb, r.Inset(1), txpix[piece.tx], draw.ZP)
+			draw.Draw(bbmask, r, draw.Opaque, draw.ZP)
 			op = r.Min
 		} else {
-			draw.Draw(bb, r, bb, nil, op)
-			draw.Draw(bbmask, r, bbmask, nil, op)
+			draw.Draw(bb, r, bb, op)
+			draw.Draw(bbmask, r, bbmask, op)
 		}
 		if br.Max.X < r.Max.X {
 			br.Max.X = r.Max.X
@@ -263,17 +263,17 @@ func setpiece(p *Piece) {
 	br2.Max = br.Max.Add(delta)
 	r = br.Add(bb2r.Min)
 	r2 := br2.Add(bb2r.Min)
-	draw.Draw(bb2, r2, draw.White, nil, draw.ZP)
-	draw.Draw(bb2, r.Add(delta), bb, nil, bbr.Min)
-	draw.Draw(bb2mask, r2, draw.Transparent, nil, draw.ZP)
-	draw.Draw(bb2mask, r, draw.Opaque, bbmask, bbr.Min)
-	draw.Draw(bb2mask, r.Add(delta), draw.Opaque, bbmask, bbr.Min)
+	draw.Draw(bb2, r2, draw.White, draw.ZP)
+	draw.Draw(bb2, r.Add(delta), bb, bbr.Min)
+	draw.Draw(bb2mask, r2, draw.Transparent, draw.ZP)
+	draw.DrawMask(bb2mask, r, draw.Opaque, bbr.Min, bbmask, draw.ZP, draw.SoverD)
+	draw.DrawMask(bb2mask, r.Add(delta), draw.Opaque, bbr.Min, bbmask, draw.ZP, draw.SoverD)
 }
 
 func drawpiece() {
-	draw.Draw(screen, br.Add(pos), bb, bbmask, bbr.Min)
+	draw.DrawMask(screen, br.Add(pos), bb, bbr.Min, bbmask, draw.ZP, draw.SoverD)
 	if suspended {
-		draw.Draw(screen, br.Add(pos), draw.White, whitemask, draw.ZP)
+		draw.DrawMask(screen, br.Add(pos), draw.White, draw.ZP, whitemask, draw.ZP, draw.SoverD)
 	}
 }
 
@@ -282,7 +282,7 @@ func undrawpiece() {
 	if collider(pos, br.Max) {
 		mask = bbmask
 	}
-	draw.Draw(screen, br.Add(pos), draw.White, mask, bbr.Min)
+	draw.DrawMask(screen, br.Add(pos), draw.White, bbr.Min, mask, bbr.Min, draw.SoverD)
 }
 
 func rest() {
@@ -323,7 +323,7 @@ func canfit(p *Piece) bool {
 func score(p int) {
 	points += p
 	//	snprint(buf, sizeof(buf), "%.6ld", points);
-	//	draw.Draw(screen, draw.Rpt(pscore, pscore.Add(scoresz)), draw.White, nil, draw.ZP);
+	//	draw.Draw(screen, draw.Rpt(pscore, pscore.Add(scoresz)), draw.White, draw.ZP);
 	//	string(screen, pscore, draw.Black, draw.ZP, font, buf);
 }
 
@@ -332,14 +332,14 @@ func drawsq(b draw.Image, p draw.Point, ptx int) {
 	r.Min = p
 	r.Max.X = r.Min.X + pcsz
 	r.Max.Y = r.Min.Y + pcsz
-	draw.Draw(b, r, draw.Black, nil, draw.ZP)
-	draw.Draw(b, r.Inset(1), txpix[ptx], nil, draw.ZP)
+	draw.Draw(b, r, draw.Black, draw.ZP)
+	draw.Draw(b, r.Inset(1), txpix[ptx], draw.ZP)
 }
 
 func drawboard() {
 	draw.Border(screen, rboard.Inset(-2), 2, draw.Black, draw.ZP)
 	draw.Draw(screen, draw.Rect(rboard.Min.X, rboard.Min.Y-2, rboard.Max.X, rboard.Min.Y),
-		draw.White, nil, draw.ZP)
+		draw.White, draw.ZP)
 	for i := 0; i < NY; i++ {
 		for j := 0; j < NX; j++ {
 			if board[i][j] != 0 {
@@ -349,7 +349,7 @@ func drawboard() {
 	}
 	score(0)
 	if suspended {
-		draw.Draw(screen, screenr, draw.White, whitemask, draw.ZP)
+		draw.DrawMask(screen, screenr, draw.White, draw.ZP, whitemask, draw.ZP, draw.SoverD)
 	}
 }
 
@@ -375,7 +375,7 @@ func movepiece() bool {
 	if collider(pos, br2.Max) {
 		mask = bb2mask
 	}
-	draw.Draw(screen, br2.Add(pos), bb2, mask, bb2r.Min)
+	draw.DrawMask(screen, br2.Add(pos), bb2, bb2r.Min, mask, bb2r.Min, draw.SoverD)
 	pos.Y += DY
 	display.FlushImage()
 	return true
@@ -444,7 +444,7 @@ func horiz() bool {
 	for j := 0; j < h; j++ {
 		r.Min.Y = rboard.Min.Y + lev[j]*pcsz
 		r.Max.Y = r.Min.Y + pcsz
-		draw.Draw(screen, r, draw.White, whitemask, draw.ZP)
+		draw.DrawMask(screen, r, draw.White, draw.ZP, whitemask, draw.ZP, draw.SoverD)
 		display.FlushImage()
 	}
 	PlaySound(whoosh)
@@ -457,7 +457,7 @@ func horiz() bool {
 		for j := 0; j < h; j++ {
 			r.Min.Y = rboard.Min.Y + lev[j]*pcsz
 			r.Max.Y = r.Min.Y + pcsz
-			draw.Draw(screen, r, draw.White, whitemask, draw.ZP)
+			draw.DrawMask(screen, r, draw.White, draw.ZP, whitemask, draw.ZP, draw.SoverD)
 		}
 		display.FlushImage()
 	}
@@ -467,9 +467,9 @@ func horiz() bool {
 		score(250 + 10*i*i)
 		r.Min.Y = rboard.Min.Y
 		r.Max.Y = rboard.Min.Y + lev[j]*pcsz
-		draw.Draw(screen, r.Add(draw.Pt(0, pcsz)), screen, nil, r.Min)
+		draw.Draw(screen, r.Add(draw.Pt(0, pcsz)), screen, r.Min)
 		r.Max.Y = rboard.Min.Y + pcsz
-		draw.Draw(screen, r, draw.White, nil, draw.ZP)
+		draw.Draw(screen, r, draw.White, draw.ZP)
 		for k := lev[j] - 1; k >= 0; k-- {
 			board[k+1] = board[k]
 		}
@@ -703,7 +703,7 @@ func redraw(new bool) {
 	bb2r = draw.Rect(0, 0, N*pcsz, N*pcsz+DY)
 	bb2 = image.NewRGBA(bb2r.Dx(), bb2r.Dy())
 	bb2mask = image.NewRGBA(bb2r.Dx(), bb2r.Dy()) // actually just a bitmap
-	draw.Draw(screen, screenr, draw.White, nil, draw.ZP)
+	draw.Draw(screen, screenr, draw.White, draw.ZP)
 	drawboard()
 	setpiece(piece)
 	if piece != nil {
diff --git a/src/pkg/exp/draw/draw.go b/src/pkg/exp/draw/draw.go
index 3455eacb9e..d7722acfc7 100644
--- a/src/pkg/exp/draw/draw.go
+++ b/src/pkg/exp/draw/draw.go
@@ -12,32 +12,37 @@ package draw
 
 import "image"
 
+// A Porter-Duff compositing operator.
+type Op int
+
+const SoverD Op = 0
+
 // A draw.Image is an image.Image with a Set method to change a single pixel.
 type Image interface {
 	image.Image
 	Set(x, y int, c image.Color)
 }
 
-// Draw aligns r.Min in dst with pt in src and mask
-// and then replaces the rectangle r in dst with the
-// result of the Porter-Duff compositing operation
-// ``(src in mask) over dst.''  If mask is nil, the operation
-// simplifies to ``src over dst.''
-// The implementation is simple and slow.
-func Draw(dst Image, r Rectangle, src, mask image.Image, pt Point) {
-	// Plenty of room for optimizations here.
+// Draw calls DrawMask with a nil mask and an SoverD op.
+func Draw(dst Image, r Rectangle, src image.Image, sp Point) {
+	DrawMask(dst, r, src, sp, nil, ZP, SoverD)
+}
 
-	dx, dy := src.Width(), src.Height()
+// DrawMask aligns r.Min in dst with sp in src and mp in mask and then replaces the rectangle r
+// in dst with the result of a Porter-Duff composition. For the SoverD operator, the result
+// is ``(src in mask) over dst''. If mask is nil, this simplifies to ``src over dst''.
+// The implementation is simple and slow.
+// TODO(nigeltao): Optimize this.
+func DrawMask(dst Image, r Rectangle, src image.Image, sp Point, mask image.Image, mp Point, op Op) {
+	dx, dy := src.Width()-sp.X, src.Height()-sp.Y
 	if mask != nil {
-		if dx > mask.Width() {
-			dx = mask.Width()
+		if dx > mask.Width()-mp.X {
+			dx = mask.Width() - mp.X
 		}
-		if dy > mask.Height() {
-			dy = mask.Height()
+		if dy > mask.Height()-mp.Y {
+			dy = mask.Height() - mp.Y
 		}
 	}
-	dx -= pt.X
-	dy -= pt.Y
 	if r.Dx() > dx {
 		r.Max.X = r.Min.X + dx
 	}
@@ -45,26 +50,31 @@ func Draw(dst Image, r Rectangle, src, mask image.Image, pt Point) {
 		r.Max.Y = r.Min.Y + dy
 	}
 
+	// TODO(nigeltao): Clip r to dst's bounding box, and handle the case when sp or mp has negative X or Y.
+
 	x0, x1, dx := r.Min.X, r.Max.X, 1
 	y0, y1, dy := r.Min.Y, r.Max.Y, 1
-	if image.Image(dst) == src && r.Overlaps(r.Add(pt.Sub(r.Min))) {
+	if image.Image(dst) == src && r.Overlaps(r.Add(sp.Sub(r.Min))) {
 		// Rectangles overlap: process backward?
-		if pt.Y < r.Min.Y || pt.Y == r.Min.Y && pt.X < r.Min.X {
+		if sp.Y < r.Min.Y || sp.Y == r.Min.Y && sp.X < r.Min.X {
 			x0, x1, dx = x1-1, x0-1, -1
 			y0, y1, dy = y1-1, y0-1, -1
 		}
 	}
 
 	var out *image.RGBA64Color
-	for y := y0; y != y1; y += dy {
-		for x := x0; x != x1; x += dx {
-			sx := pt.X + x - r.Min.X
-			sy := pt.Y + y - r.Min.Y
+	sy := sp.Y + y0 - r.Min.Y
+	my := mp.Y + y0 - r.Min.Y
+	for y := y0; y != y1; y, sy, my = y+dy, sy+dy, my+dy {
+		sx := sp.X + x0 - r.Min.X
+		mx := mp.X + x0 - r.Min.X
+		for x := x0; x != x1; x, sx, mx = x+dx, sx+dx, mx+dx {
+			// TODO(nigeltao): Check that op == SoverD.
 			if mask == nil {
 				dst.Set(x, y, src.At(sx, sy))
 				continue
 			}
-			_, _, _, ma := mask.At(sx, sy).RGBA()
+			_, _, _, ma := mask.At(mx, my).RGBA()
 			switch ma {
 			case 0:
 				continue
@@ -109,17 +119,17 @@ func Border(dst Image, r Rectangle, w int, src image.Image, sp Point) {
 	i := w
 	if i > 0 {
 		// inside r
-		Draw(dst, Rect(r.Min.X, r.Min.Y, r.Max.X, r.Min.Y+i), src, nil, sp)                          // top
-		Draw(dst, Rect(r.Min.X, r.Min.Y+i, r.Min.X+i, r.Max.Y-i), src, nil, sp.Add(Pt(0, i)))        // left
-		Draw(dst, Rect(r.Max.X-i, r.Min.Y+i, r.Max.X, r.Max.Y-i), src, nil, sp.Add(Pt(r.Dx()-i, i))) // right
-		Draw(dst, Rect(r.Min.X, r.Max.Y-i, r.Max.X, r.Max.Y), src, nil, sp.Add(Pt(0, r.Dy()-i)))     // bottom
+		Draw(dst, Rect(r.Min.X, r.Min.Y, r.Max.X, r.Min.Y+i), src, sp)                          // top
+		Draw(dst, Rect(r.Min.X, r.Min.Y+i, r.Min.X+i, r.Max.Y-i), src, sp.Add(Pt(0, i)))        // left
+		Draw(dst, Rect(r.Max.X-i, r.Min.Y+i, r.Max.X, r.Max.Y-i), src, sp.Add(Pt(r.Dx()-i, i))) // right
+		Draw(dst, Rect(r.Min.X, r.Max.Y-i, r.Max.X, r.Max.Y), src, sp.Add(Pt(0, r.Dy()-i)))     // bottom
 		return
 	}
 
 	// outside r;
 	i = -i
-	Draw(dst, Rect(r.Min.X-i, r.Min.Y-i, r.Max.X+i, r.Min.Y), src, nil, sp.Add(Pt(-i, -i))) // top
-	Draw(dst, Rect(r.Min.X-i, r.Min.Y, r.Min.X, r.Max.Y), src, nil, sp.Add(Pt(-i, 0)))      // left
-	Draw(dst, Rect(r.Max.X, r.Min.Y, r.Max.X+i, r.Max.Y), src, nil, sp.Add(Pt(r.Dx(), 0)))  // right
-	Draw(dst, Rect(r.Min.X-i, r.Max.Y, r.Max.X+i, r.Max.Y+i), src, nil, sp.Add(Pt(-i, 0)))  // bottom
+	Draw(dst, Rect(r.Min.X-i, r.Min.Y-i, r.Max.X+i, r.Min.Y), src, sp.Add(Pt(-i, -i))) // top
+	Draw(dst, Rect(r.Min.X-i, r.Min.Y, r.Min.X, r.Max.Y), src, sp.Add(Pt(-i, 0)))      // left
+	Draw(dst, Rect(r.Max.X, r.Min.Y, r.Max.X+i, r.Max.Y), src, sp.Add(Pt(r.Dx(), 0)))  // right
+	Draw(dst, Rect(r.Min.X-i, r.Max.Y, r.Max.X+i, r.Max.Y+i), src, sp.Add(Pt(-i, 0)))  // bottom
 }