Freetype-Go: new freetype package to provide a convenience API to

draw text onto an image.

This is a simple API that doesn't handle line breaking, ligatures,
right-to-left or vertical scripts, and other fancy features.

R=r, rsc
CC=golang-dev
http://codereview.appspot.com/1121045

example/freetype/main.go

@@ -0,0 +1,127 @@
+package main
+
+import (
+	"bufio"
+	"exp/draw"
+	"flag"
+	"fmt"
+	"freetype-go.googlecode.com/hg/freetype"
+	"image"
+	"image/png"
+	"io/ioutil"
+	"log"
+	"os"
+)
+
+var (
+	dpi      = flag.Int("dpi", 72, "screen resolution in Dots Per Inch")
+	fontfile = flag.String("fontfile", "../../luxi-fonts/luxisr.ttf", "filename of the ttf font")
+	size     = flag.Float("size", 12, "font size in points")
+	spacing  = flag.Float("spacing", 1.5, "line spacing (e.g. 2 means double spaced)")
+	wonb     = flag.Bool("whiteonblack", false, "white text on a black background")
+)
+
+var text = []string{
+	"’Twas brillig, and the slithy toves",
+	"Did gyre and gimble in the wabe;",
+	"All mimsy were the borogoves,",
+	"And the mome raths outgrabe.",
+	"",
+	"“Beware the Jabberwock, my son!",
+	"The jaws that bite, the claws that catch!",
+	"Beware the Jubjub bird, and shun",
+	"The frumious Bandersnatch!”",
+	"",
+	"He took his vorpal sword in hand:",
+	"Long time the manxome foe he sought—",
+	"So rested he by the Tumtum tree,",
+	"And stood awhile in thought.",
+	"",
+	"And as in uffish thought he stood,",
+	"The Jabberwock, with eyes of flame,",
+	"Came whiffling through the tulgey wood,",
+	"And burbled as it came!",
+	"",
+	"One, two! One, two! and through and through",
+	"The vorpal blade went snicker-snack!",
+	"He left it dead, and with its head",
+	"He went galumphing back.",
+	"",
+	"“And hast thou slain the Jabberwock?",
+	"Come to my arms, my beamish boy!",
+	"O frabjous day! Callooh! Callay!”",
+	"He chortled in his joy.",
+	"",
+	"’Twas brillig, and the slithy toves",
+	"Did gyre and gimble in the wabe;",
+	"All mimsy were the borogoves,",
+	"And the mome raths outgrabe.",
+}
+
+func main() {
+	flag.Parse()
+
+	// Read the font data.
+	fontBytes, err := ioutil.ReadFile(*fontfile)
+	if err != nil {
+		log.Stderr(err)
+		return
+	}
+	font, err := freetype.ParseFont(fontBytes)
+	if err != nil {
+		log.Stderr(err)
+		return
+	}
+
+	// Initialize the context.
+	fg, bg := image.Black, image.White
+	ruler := image.RGBAColor{0xdd, 0xdd, 0xdd, 0xff}
+	if *wonb {
+		fg, bg = image.White, image.Black
+		ruler = image.RGBAColor{0x22, 0x22, 0x22, 0xff}
+	}
+	rgba := image.NewRGBA(640, 480)
+	draw.Draw(rgba, draw.Rect(0, 0, rgba.Width(), rgba.Height()), bg, draw.ZP)
+	c := freetype.NewRGBAContext(rgba)
+	c.SetColor(fg)
+	c.SetDPI(*dpi)
+	c.SetFont(font)
+	c.SetFontSize(*size)
+
+	// Draw the guidelines.
+	for i := 0; i < 200; i++ {
+		rgba.Set(10, 10+i, ruler)
+		rgba.Set(10+i, 10, ruler)
+	}
+
+	// Draw the text.
+	pt := freetype.Pt(10, 10)
+	for _, s := range text {
+		err = c.DrawText(pt, s)
+		if err != nil {
+			log.Stderr(err)
+			return
+		}
+		pt.Y += c.PointToFixed(*size * *spacing)
+	}
+
+	// Save that RGBA image to disk.
+	f, err := os.Open("out.png", os.O_CREAT|os.O_WRONLY, 0600)
+	if err != nil {
+		log.Stderr(err)
+		os.Exit(1)
+	}
+	defer f.Close()
+	b := bufio.NewWriter(f)
+	err = png.Encode(b, rgba)
+	if err != nil {
+		log.Stderr(err)
+		os.Exit(1)
+	}
+	err = b.Flush()
+	if err != nil {
+		log.Stderr(err)
+		os.Exit(1)
+	}
+	fmt.Println("Wrote out.png OK.")
+}

example/gamma/main.go

@@ -31,7 +31,7 @@ func clear(m *image.Alpha) {
 
 func main() {
 	// Draw a rounded corner that is one pixel wide.
-	r := raster.New(50, 50)
+	r := raster.NewRasterizer(50, 50)
 	r.Start(p(5, 5))
 	r.Add1(p(5, 25))
 	r.Add2(p(5, 45), p(25, 45))
@@ -51,11 +51,12 @@ func main() {
 	draw.Draw(rgba, draw.Rect(0, 0, w, h/2), image.Black, draw.ZP)
 	draw.Draw(rgba, draw.Rect(0, h/2, w, h), image.White, draw.ZP)
 	mask := image.NewAlpha(50, 50)
-	painter := raster.AlphaSrcPainter(mask)
+	painter := raster.NewAlphaPainter(mask)
+	painter.Op = draw.Src
 	gammas := []float64{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)
-		r.Rasterize(raster.GammaCorrectionPainter(painter, g))
+		r.Rasterize(raster.NewGammaCorrectionPainter(painter, g))
 		x, y := 50*i+25, 25
 		draw.DrawMask(rgba, draw.Rect(x, y, x+50, y+50), image.White, draw.ZP, mask, draw.ZP, draw.Over)
 		y += 100

example/raster/main.go

@@ -137,11 +137,13 @@ func main() {
 		w = 400
 		h = 400
 	)
-	r := raster.New(w, h)
+	r := raster.NewRasterizer(w, h)
 	contour(r, outside)
 	contour(r, inside)
 	mask := image.NewAlpha(w, h)
-	r.Rasterize(raster.AlphaSrcPainter(mask))
+	p := raster.NewAlphaPainter(mask)
+	p.Op = draw.Src
+	r.Rasterize(p)
 
 	// Draw the mask image (in gray) onto an RGBA image.
 	rgba := image.NewRGBA(w, h)

example/truetype/main.go

@@ -20,7 +20,7 @@ func printBounds(b truetype.Bounds) {
 	fmt.Printf("XMin:%d YMin:%d XMax:%d YMax:%d\n", b.XMin, b.YMin, b.XMax, b.YMax)
 }
 
-func printGlyph(g *truetype.Glyph) {
+func printGlyph(g *truetype.GlyphBuf) {
 	printBounds(g.B)
 	fmt.Print("Points:\n---\n")
 	e := 0
@@ -58,7 +58,7 @@ func main() {
 
 	i0 := font.Index(c0)
 	hm := font.HMetric(i0)
-	g := truetype.NewGlyph()
+	g := truetype.NewGlyphBuf()
 	err = g.Load(font, i0)
 	if err != nil {
 		log.Stderr(err)

freetype/Makefile

@@ -0,0 +1,13 @@
+# Copyright 2010 The Freetype-Go Authors. All rights reserved.
+# Use of this source code is governed by your choice of either the
+# FreeType License or the GNU General Public License version 2,
+# both of which can be found in the LICENSE file.
+
+include $(GOROOT)/src/Make.$(GOARCH)
+
+TARG=freetype-go.googlecode.com/hg/freetype
+GOFILES=\
+	freetype.go\
+
+include $(GOROOT)/src/Make.pkg
+

freetype/freetype.go

@@ -0,0 +1,248 @@
+// Copyright 2010 The Freetype-Go Authors. All rights reserved.
+// Use of this source code is governed by your choice of either the
+// FreeType License or the GNU General Public License version 2,
+// both of which can be found in the LICENSE file.
+
+// The freetype package provides a convenient API to draw text onto an image.
+// Use the freetype/raster and freetype/truetype packages for lower level
+// control over rasterization and TrueType parsing.
+package freetype
+
+import (
+	"freetype-go.googlecode.com/hg/freetype/raster"
+	"freetype-go.googlecode.com/hg/freetype/truetype"
+	"image"
+	"os"
+)
+
+// ParseFont just calls the Parse function from the freetype/truetype package.
+// It is provided here so that code that imports this package doesn't need
+// to also include the freetype/truetype package.
+func ParseFont(b []byte) (*truetype.Font, os.Error) {
+	return truetype.Parse(b)
+}
+
+// Pt converts from a co-ordinate pair measured in pixels to a raster.Point
+// co-ordinate pair measured in raster.Fixed units.
+func Pt(x, y int) raster.Point {
+	return raster.Point{raster.Fixed(x << 8), raster.Fixed(y << 8)}
+}
+
+// An RGBAContext holds the state for drawing text from a given font at a
+// given size.
+type RGBAContext struct {
+	r        *raster.Rasterizer
+	p        *raster.RGBAPainter
+	font     *truetype.Font
+	glyphBuf *truetype.GlyphBuf
+	fontSize float
+	dpi      int
+	upe      int
+	// A TrueType's glyph's nodes can have negative co-ordinates, but the
+	// rasterizer clips anything left of x=0 or above y=0. xmin and ymin
+	// are the pixel offsets, based on the font's FUnit metrics, that let
+	// a negative co-ordinate in TrueType space be non-negative in
+	// rasterizer space. xmin and ymin are typically <= 0.
+	xmin, ymin int
+	// scale is a multiplication factor to convert 256 FUnits (which is truetype's
+	// native unit) to 24.8 fixed point units (which is the rasterizer's native unit).
+	// At the default values of 72 DPI and 2048 units-per-em, one em of a 12 point
+	// font is 12 pixels, which is 3072 fixed point units, and scale is
+	// (pointSize * resolution * 256 * 256) / (unitsPerEm * 72), or
+	// (12 * 72 * 256 * 256) / (2048 * 72),
+	// which equals 384 fixed point units per 256 FUnits.
+	// To check this, 1 em * 2048 FUnits per em * 384 fixed point units per 256 FUnits
+	// equals 3072 fixed point units.
+	scale int
+}
+
+// FUnitToFixed converts the given number of FUnits into fixed point units,
+// rounding to nearest.
+func (c *RGBAContext) FUnitToFixed(x int) raster.Fixed {
+	return raster.Fixed((x*c.scale + 128) >> 8)
+}
+
+// FUnitToPixelRD converts the given number of FUnits into pixel units,
+// rounding down.
+func (c *RGBAContext) FUnitToPixelRD(x int) int {
+	return x * c.scale >> 16
+}
+
+// FUnitToPixelRU converts the given number of FUnits into pixel units,
+// rounding up.
+func (c *RGBAContext) FUnitToPixelRU(x int) int {
+	return (x*c.scale + 0xffff) >> 16
+}
+
+// PointToFixed converts the given number of points (as in ``a 12 point font'')
+// into fixed point units.
+func (c *RGBAContext) PointToFixed(x float) raster.Fixed {
+	return raster.Fixed(x * float(c.dpi) * (256.0 / 72.0))
+}
+
+// drawContour draws the given closed contour with the given offset.
+func (c *RGBAContext) drawContour(ps []truetype.Point, dx, dy raster.Fixed) {
+	if len(ps) == 0 {
+		return
+	}
+	// ps[0] is a truetype.Point measured in FUnits and positive Y going upwards.
+	// start is the same thing measured in fixed point units and positive Y
+	// going downwards, and offset by (dx, dy)
+	start := raster.Point{
+		dx + c.FUnitToFixed(int(ps[0].X)),
+		dy + c.FUnitToFixed(c.upe-int(ps[0].Y)),
+	}
+	c.r.Start(start)
+	q0, on0 := start, true
+	for _, p := range ps[1:] {
+		q := raster.Point{
+			dx + c.FUnitToFixed(int(p.X)),
+			dy + c.FUnitToFixed(c.upe-int(p.Y)),
+		}
+		on := p.Flags&0x01 != 0
+		if on {
+			if on0 {
+				c.r.Add1(q)
+			} else {
+				c.r.Add2(q0, q)
+			}
+		} else {
+			if on0 {
+				// No-op.
+			} else {
+				mid := raster.Point{
+					(q0.X + q.X) / 2,
+					(q0.Y + q.Y) / 2,
+				}
+				c.r.Add2(q0, mid)
+			}
+		}
+		q0, on0 = q, on
+	}
+	// Close the curve.
+	if on0 {
+		c.r.Add1(start)
+	} else {
+		c.r.Add2(q0, start)
+	}
+}
+
+// DrawText draws s at pt. The text is placed so that the top left of the em
+// square of the first character of s is equal to pt. The majority of the
+// affected pixels will be below and to the right of pt, but some may be above
+// or to the left. For example, drawing a string that starts with a 'J' in an
+// italic font may affect pixels to the left of pt.
+// pt is a raster.Point and can therefore represent sub-pixel positions.
+func (c *RGBAContext) DrawText(pt raster.Point, s string) (err os.Error) {
+	if c.font == nil {
+		return os.NewError("freetype: DrawText called with a nil font")
+	}
+	// pt.X, pt.Y, x, y, dx, dy and x0 are measured in raster.Fixed units,
+	// c.p.Dx, c.p.Dy, c.xmin and c.ymin are measured in pixels, and
+	// advance is measured in FUnits.
+	var x, y raster.Fixed
+	advance, x0 := 0, pt.X
+	dx := raster.Fixed(-c.xmin << 8)
+	dy := raster.Fixed(-c.ymin << 8)
+	c.p.Dy, y = c.ymin+int(pt.Y>>8), pt.Y&0xff
+	y += dy
+	prev, hasPrev := truetype.Index(0), false
+	for _, ch := range s {
+		index := c.font.Index(ch)
+		// Load the next glyph (if it was different from the previous one)
+		// and add any kerning adjustment.
+		if hasPrev {
+			advance += int(c.font.Kerning(prev, index))
+			if prev != index {
+				err = c.glyphBuf.Load(c.font, index)
+				if err != nil {
+					return
+				}
+			}
+		} else {
+			err = c.glyphBuf.Load(c.font, index)
+			if err != nil {
+				return
+			}
+		}
+		// Convert the advance from FUnits to raster.Fixed units.
+		x = x0 + c.FUnitToFixed(advance)
+		// Break the co-ordinate down into an integer pixel part and a
+		// sub-pixel part, making sure that the latter is non-negative.
+		c.p.Dx, x = c.xmin+int(x>>8), x&0xff
+		x += dx
+		// Draw the contours.
+		c.r.Clear()
+		e0 := 0
+		for _, e := range c.glyphBuf.End {
+			c.drawContour(c.glyphBuf.Point[e0:e], x, y)
+			e0 = e
+		}
+		c.r.Rasterize(c.p)
+		// Advance the cursor.
+		advance += int(c.font.HMetric(index).AdvanceWidth)
+		prev, hasPrev = index, true
+	}
+	return
+}
+
+// recalc recalculates scale and bounds values from the font size, screen
+// resolution and font metrics.
+func (c *RGBAContext) recalc() {
+	c.scale = int((c.fontSize * float(c.dpi) * 256 * 256) / (float(c.upe) * 72))
+	if c.font == nil {
+		c.xmin, c.ymin = 0, 0
+	} else {
+		b := c.font.Bounds()
+		c.xmin = c.FUnitToPixelRD(int(b.XMin))
+		c.ymin = c.FUnitToPixelRD(c.upe - int(b.YMax))
+		xmax := c.FUnitToPixelRU(int(b.XMax))
+		ymax := c.FUnitToPixelRU(c.upe - int(b.YMin))
+		c.r.SetBounds(xmax-c.xmin, ymax-c.ymin)
+	}
+}
+
+// SetColor sets the color to draw text.
+func (c *RGBAContext) SetColor(color image.Color) {
+	c.p.SetColor(color)
+}
+
+// SetDPI sets the screen resolution in dots per inch.
+func (c *RGBAContext) SetDPI(dpi int) {
+	c.dpi = dpi
+	c.recalc()
+}
+
+// SetFont sets the font used to draw text.
+func (c *RGBAContext) SetFont(font *truetype.Font) {
+	c.font = font
+	c.upe = font.UnitsPerEm()
+	if c.upe <= 0 {
+		c.upe = 1
+	}
+	c.recalc()
+}
+
+// SetFontSize sets the font size in points (as in ``a 12 point font'').
+func (c *RGBAContext) SetFontSize(fontSize float) {
+	c.fontSize = fontSize
+	c.recalc()
+}
+
+// SetRGBA sets the image that the RGBAContext draws onto.
+func (c *RGBAContext) SetRGBA(m *image.RGBA) {
+	c.p.Image = m
+}
+
+// NewRGBAContext creates a new RGBAContext.
+func NewRGBAContext(m *image.RGBA) *RGBAContext {
+	return &RGBAContext{
+		r:        raster.NewRasterizer(0, 0),
+		p:        raster.NewRGBAPainter(m),
+		glyphBuf: truetype.NewGlyphBuf(),
+		fontSize: 12,
+		dpi:      72,
+		upe:      2048,
+		scale:    (12 * 72 * 256 * 256) / (2048 * 72),
+	}
+}

freetype/raster/paint.go

@@ -6,6 +6,7 @@
 package raster
 
 import (
+	"exp/draw"
 	"image"
 	"math"
 )
@@ -33,46 +34,138 @@ type PainterFunc func(ss []Span)
 // Paint just delegates the call to f.
 func (f PainterFunc) Paint(ss []Span) { f(ss) }
 
-// AlphaOverPainter returns a Painter that paints onto the given Alpha image
-// using the "src over dst" Porter-Duff composition operator.
-func AlphaOverPainter(m *image.Alpha) Painter {
-	return PainterFunc(func(ss []Span) {
-		for _, s := range ss {
+// An AlphaPainter is a Painter that paints Spans onto an image.Alpha.
+type AlphaPainter struct {
+	// The image to compose onto.
+	Image *image.Alpha
+	// The Porter-Duff composition operator.
+	Op draw.Op
+	// An offset (in pixels) to the painted spans.
+	Dx, Dy int
+}
+
+// Paint satisfies the Painter interface by painting ss onto an image.Alpha.
+func (r *AlphaPainter) Paint(ss []Span) {
+	for _, s := range ss {
+		y := r.Dy + s.Y
+		if y < 0 {
+			continue
+		}
+		if y >= len(r.Image.Pixel) {
+			return
+		}
+		p := r.Image.Pixel[y]
+		x0, x1 := r.Dx+s.X0, r.Dx+s.X1
+		if x0 < 0 {
+			x0 = 0
+		}
+		if x1 > len(p) {
+			x1 = len(p)
+		}
+		if r.Op == draw.Over {
 			a := int(s.A >> 24)
-			p := m.Pixel[s.Y]
-			for i := s.X0; i < s.X1; i++ {
-				ai := int(p[i].A)
-				ai = (ai*255 + (255-ai)*a) / 255
-				p[i] = image.AlphaColor{uint8(ai)}
+			for x := x0; x < x1; x++ {
+				ax := int(p[x].A)
+				ax = (ax*255 + (255-ax)*a) / 255
+				p[x] = image.AlphaColor{uint8(ax)}
 			}
-		}
-	})
-}
-
-// AlphaSrcPainter returns a Painter that paints onto the given Alpha image
-// using the "src" Porter-Duff composition operator.
-func AlphaSrcPainter(m *image.Alpha) Painter {
-	return PainterFunc(func(ss []Span) {
-		for _, s := range ss {
+		} else {
 			color := image.AlphaColor{uint8(s.A >> 24)}
-			p := m.Pixel[s.Y]
-			for i := s.X0; i < s.X1; i++ {
-				p[i] = color
+			for x := x0; x < x1; x++ {
+				p[x] = color
 			}
 		}
-	})
+	}
 }
 
-// A monochromePainter has a wrapped painter and an accumulator for merging
-// adjacent opaque Spans.
-type monochromePainter struct {
-	p         Painter
+// NewAlphaPainter creates a new AlphaPainter for the given image.
+func NewAlphaPainter(m *image.Alpha) *AlphaPainter {
+	return &AlphaPainter{Image: m}
+}
+
+type RGBAPainter struct {
+	// The image to compose onto.
+	Image *image.RGBA
+	// The Porter-Duff composition operator.
+	Op draw.Op
+	// An offset (in pixels) to the painted spans.
+	Dx, Dy int
+	// The 16-bit color to paint the spans.
+	cr, cg, cb, ca uint32
+}
+
+// Paint satisfies the Painter interface by painting ss onto an image.RGBA.
+func (r *RGBAPainter) Paint(ss []Span) {
+	for _, s := range ss {
+		y := r.Dy + s.Y
+		if y < 0 {
+			continue
+		}
+		if y >= len(r.Image.Pixel) {
+			return
+		}
+		p := r.Image.Pixel[y]
+		x0, x1 := r.Dx+s.X0, r.Dx+s.X1
+		if x0 < 0 {
+			x0 = 0
+		}
+		if x1 > len(p) {
+			x1 = len(p)
+		}
+		for x := x0; x < x1; x++ {
+			// This code is duplicated from drawGlyphOver in $GOROOT/src/pkg/exp/draw/draw.go.
+			// TODO(nigeltao): Factor out common code into a utility function, once the compiler
+			// can inline such function calls.
+			ma := s.A >> 16
+			const M = 1<<16 - 1
+			if r.Op == draw.Over {
+				rgba := p[x]
+				dr := uint32(rgba.R)
+				dg := uint32(rgba.G)
+				db := uint32(rgba.B)
+				da := uint32(rgba.A)
+				a := M - (r.ca * ma / M)
+				a *= 0x101
+				dr = (dr*a + r.cr*ma) / M
+				dg = (dg*a + r.cg*ma) / M
+				db = (db*a + r.cb*ma) / M
+				da = (da*a + r.ca*ma) / M
+				p[x] = image.RGBAColor{uint8(dr >> 8), uint8(dg >> 8), uint8(db >> 8), uint8(da >> 8)}
+			} else {
+				dr := r.cr * ma / M
+				dg := r.cg * ma / M
+				db := r.cb * ma / M
+				da := r.ca * ma / M
+				p[x] = image.RGBAColor{uint8(dr >> 8), uint8(dg >> 8), uint8(db >> 8), uint8(da >> 8)}
+			}
+		}
+	}
+}
+
+// SetColor sets the color to paint the spans.
+func (r *RGBAPainter) SetColor(c image.Color) {
+	r.cr, r.cg, r.cb, r.ca = c.RGBA()
+	r.cr >>= 16
+	r.cg >>= 16
+	r.cb >>= 16
+	r.ca >>= 16
+}
+
+// NewRGBAPainter creates a new RGBAPainter for the given image.
+func NewRGBAPainter(m *image.RGBA) *RGBAPainter {
+	return &RGBAPainter{Image: m}
+}
+
+// A MonochromePainter wraps another Painter, quantizing each Span's alpha to
+// be either fully opaque or fully transparent.
+type MonochromePainter struct {
+	Painter   Painter
 	y, x0, x1 int
 }
 
 // Paint delegates to the wrapped Painter after quantizing each Span's alpha
-// values and merging adjacent fully opaque Spans.
-func (m *monochromePainter) Paint(ss []Span) {
+// value and merging adjacent fully opaque Spans.
+func (m *MonochromePainter) Paint(ss []Span) {
 	// We compact the ss slice, discarding any Spans whose alpha quantizes to zero.
 	j := 0
 	for _, s := range ss {
@@ -93,11 +186,11 @@ func (m *monochromePainter) Paint(ss []Span) {
 			if j < len(ss) {
 				ss[j] = Span{}
 				j++
-				m.p.Paint(ss[0:j])
+				m.Painter.Paint(ss[0:j])
 			} else if j == len(ss) {
-				m.p.Paint(ss)
+				m.Painter.Paint(ss)
 				ss[0] = Span{}
-				m.p.Paint(ss[0:1])
+				m.Painter.Paint(ss[0:1])
 			} else {
 				panic("unreachable")
 			}
@@ -106,25 +199,27 @@ func (m *monochromePainter) Paint(ss []Span) {
 			return
 		}
 	}
-	m.p.Paint(ss[0:j])
+	m.Painter.Paint(ss[0:j])
 }
 
-// A MonochromePainter wraps another Painter, quantizing each Span's alpha to
-// be either fully opaque or fully transparent.
-func MonochromePainter(p Painter) Painter {
-	return &monochromePainter{p: p}
+// NewMonochromePainter creates a new MonochromePainter that wraps the given
+// Painter.
+func NewMonochromePainter(p Painter) *MonochromePainter {
+	return &MonochromePainter{Painter: p}
 }
 
-// A gammaCorrectionPainter has a wrapped painter and a precomputed linear
-// interpolation of the exponential gamma-correction curve.
-type gammaCorrectionPainter struct {
-	p Painter
-	a [256]uint16 // Alpha values, with fully opaque == 1<<16-1.
+// A GammaCorrectionPainter wraps another Painter, performing gamma-correction
+// on each Span's alpha value.
+type GammaCorrectionPainter struct {
+	// The wrapped Painter.
+	Painter Painter
+	// Precomputed alpha values for linear interpolation, with fully opaque == 1<<16-1.
+	a [256]uint16
 }
 
 // Paint delegates to the wrapped Painter after performing gamma-correction
 // on each Span.
-func (g *gammaCorrectionPainter) Paint(ss []Span) {
+func (g *GammaCorrectionPainter) Paint(ss []Span) {
 	const (
 		M = 0x1010101 // 255*M == 1<<32-1
 		N = 0x8080    // N = M>>9, and N < 1<<16-1
@@ -145,17 +240,22 @@ func (g *gammaCorrectionPainter) Paint(ss []Span) {
 		}
 		ss[i].A = a
 	}
-	g.p.Paint(ss)
+	g.Painter.Paint(ss)
 }
 
-// A GammaCorrectionPainter wraps another Painter, performing gamma-correction
-// on the alpha values of each Span.
-func GammaCorrectionPainter(p Painter, gamma float64) Painter {
-	g := &gammaCorrectionPainter{p: p}
+// SetGamma sets the gamma value.
+func (g *GammaCorrectionPainter) SetGamma(gamma float64) {
 	for i := 0; i < 256; i++ {
 		a := float64(i) / 0xff
 		a = math.Pow(a, gamma)
 		g.a[i] = uint16(0xffff * a)
 	}
+}
+
+// NewGammaCorrectionPainter creates a new GammaCorrectionPainter that wraps
+// the given Painter.
+func NewGammaCorrectionPainter(p Painter, gamma float64) *GammaCorrectionPainter {
+	g := &GammaCorrectionPainter{Painter: p}
+	g.SetGamma(gamma)
 	return g
 }

freetype/raster/raster.go

@@ -52,7 +52,7 @@ type Point struct {
 	X, Y Fixed
 }
 
-// A cell is part of a linked list (for a given yi coordinate) of accumulated
+// A cell is part of a linked list (for a given yi co-ordinate) of accumulated
 // area/coverage for the pixel at (xi, yi).
 type cell struct {
 	xi          int
@@ -546,15 +546,15 @@ func (r *Rasterizer) Clear() {
 	}
 }
 
-// Creates a new Rasterizer with the given bounds.
-func New(width, height int) *Rasterizer {
+// SetBounds sets the maximum width and height of the rasterized image and
+// calls Clear. The width and height are in pixels, not Fixed units.
+func (r *Rasterizer) SetBounds(width, height int) {
 	if width < 0 {
 		width = 0
 	}
 	if height < 0 {
 		height = 0
 	}
-
 	// Use the same ssN heuristic as the C Freetype implementation.
 	// The C implementation uses the values 32, 16, but those are in
 	// 26.6 fixed point units, and we use 24.8 fixed point everywhere.
@@ -565,8 +565,6 @@ func New(width, height int) *Rasterizer {
 			ss2, ss3 = 2*ss2, 2*ss3
 		}
 	}
-
-	r := new(Rasterizer)
 	r.width = width
 	r.splitScale2 = ss2
 	r.splitScale3 = ss3
@@ -576,8 +574,12 @@ func New(width, height int) *Rasterizer {
 	} else {
 		r.cellIndex = r.cellIndexBuf[0:height]
 	}
-	for i := 0; i < len(r.cellIndex); i++ {
-		r.cellIndex[i] = -1
-	}
+	r.Clear()
+}
+
+// NewRasterizer creates a new Rasterizer with the given bounds.
+func NewRasterizer(width, height int) *Rasterizer {
+	r := new(Rasterizer)
+	r.SetBounds(width, height)
 	return r
 }

freetype/truetype/truetype.go

@@ -418,9 +418,9 @@ type Point struct {
 	Flags uint8
 }
 
-// A Glyph holds a glyph's contours. A Glyph can be re-used to load a series
-// of glyphs from a Font.
-type Glyph struct {
+// A GlyphBuf holds a glyph's contours. A GlyphBuf can be re-used to load a
+// series of glyphs from a Font.
+type GlyphBuf struct {
 	// The glyph's bounding box.
 	B Bounds
 	// Point contains all Points from all contours of the glyph.
@@ -433,7 +433,7 @@ type Glyph struct {
 
 // decodeFlags decodes a glyph's run-length encoded flags,
 // and returns the remaining data.
-func (g *Glyph) decodeFlags(d data) data {
+func (g *GlyphBuf) decodeFlags(d data) data {
 	for i := 0; i < len(g.Point); {
 		c := d.u8()
 		g.Point[i].Flags = c
@@ -450,7 +450,7 @@ func (g *Glyph) decodeFlags(d data) data {
 }
 
 // decodeCoords decodes a glyph's delta encoded co-ordinates.
-func (g *Glyph) decodeCoords(d data) {
+func (g *GlyphBuf) decodeCoords(d data) {
 	var x int16
 	for i := 0; i < len(g.Point); i++ {
 		f := g.Point[i].Flags
@@ -484,9 +484,9 @@ func (g *Glyph) decodeCoords(d data) {
 }
 
 // Load loads a glyph's contours from a Font, overwriting any previously
-// loaded contours for this Glyph.
-func (g *Glyph) Load(f *Font, i Index) os.Error {
-	// Reset the Glyph.
+// loaded contours for this GlyphBuf.
+func (g *GlyphBuf) Load(f *Font, i Index) os.Error {
+	// Reset the GlyphBuf.
 	g.B = Bounds{}
 	g.Point = g.Point[0:0]
 	g.End = g.End[0:0]
@@ -537,9 +537,9 @@ func (g *Glyph) Load(f *Font, i Index) os.Error {
 	return nil
 }
 
-// NewGlyph returns a newly allocated Glyph.
-func NewGlyph() *Glyph {
-	g := new(Glyph)
+// NewGlyphBuf returns a newly allocated GlyphBuf.
+func NewGlyphBuf() *GlyphBuf {
+	g := new(GlyphBuf)
 	g.Point = make([]Point, 0, 256)
 	g.End = make([]int, 0, 32)
 	return g