freetype: add NewAlpha{Src,Over}Painter constructor function for

consistency with existing NewFooPainter functions.

Unbreak build from ColorImage change in the core image package.

R=adg
CC=golang-dev
http://codereview.appspot.com/2450041

example/gamma/main.go

@@ -54,8 +54,7 @@ func main() {
 	draw.Draw(rgba, image.Rect(0, 0, w, h/2), image.Black, image.ZP)
 	draw.Draw(rgba, image.Rect(0, h/2, w, h), image.White, image.ZP)
 	mask := image.NewAlpha(50, 50)
-	painter := raster.NewAlphaPainter(mask)
-	painter.Op = draw.Src
+	painter := raster.NewAlphaSrcPainter(mask)
 	gammas := []float{1.0 / 10.0, 1.0 / 3.0, 1.0 / 2.0, 2.0 / 3.0, 4.0 / 5.0, 1.0, 5.0 / 4.0, 3.0 / 2.0, 2.0, 3.0, 10.0}
 	for i, g := range gammas {
 		clear(mask)

example/raster/main.go

@@ -141,13 +141,12 @@ func main() {
 	contour(r, outside)
 	contour(r, inside)
 	mask := image.NewAlpha(w, h)
-	p := raster.NewAlphaPainter(mask)
-	p.Op = draw.Src
+	p := raster.NewAlphaSrcPainter(mask)
 	r.Rasterize(p)
 
 	// Draw the mask image (in gray) onto an RGBA image.
 	rgba := image.NewRGBA(w, h)
-	gray := image.ColorImage{image.AlphaColor{0x1f}}
+	gray := image.NewColorImage(image.AlphaColor{0x1f})
 	draw.Draw(rgba, rgba.Bounds(), image.Black, image.ZP)
 	draw.DrawMask(rgba, rgba.Bounds(), gray, image.ZP, mask, image.ZP, draw.Over)
 	showNodes(rgba, outside)

freetype/freetype.go

@@ -178,7 +178,7 @@ func (c *Context) rasterize(glyph truetype.Index, fx, fy raster.Fix32) (*image.A
 		e0 = e1
 	}
 	a := image.NewAlpha(xmax-xmin, ymax-ymin)
-	c.r.Rasterize(raster.AlphaSrcPainter{a})
+	c.r.Rasterize(raster.NewAlphaSrcPainter(a))
 	return a, image.Point{xmin, ymin}, nil
 }
 

freetype/raster/paint.go

@@ -69,6 +69,11 @@ func (r AlphaOverPainter) Paint(ss []Span, done bool) {
 	}
 }
 
+// NewAlphaOverPainter creates a new AlphaOverPainter for the given image.
+func NewAlphaOverPainter(m *image.Alpha) AlphaOverPainter {
+	return AlphaOverPainter{m}
+}
+
 // An AlphaSrcPainter is a Painter that paints Spans onto an image.Alpha
 // using the Src Porter-Duff composition operator.
 type AlphaSrcPainter struct {
@@ -103,6 +108,11 @@ func (r AlphaSrcPainter) Paint(ss []Span, done bool) {
 	}
 }
 
+// NewAlphaSrcPainter creates a new AlphaSrcPainter for the given image.
+func NewAlphaSrcPainter(m *image.Alpha) AlphaSrcPainter {
+	return AlphaSrcPainter{m}
+}
+
 type RGBAPainter struct {
 	// The image to compose onto.
 	Image *image.RGBA