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freetype
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Quantize sub-pixel glyph rendering. 

This is in anticipation of caching glyph images. Quantization means that cache
hits are more likely.

Also make NewFace take an *Options instead of an Options.
This commit is contained in:
Nigel Tao 2015-08-25 22:34:31 +10:00
parent a021a5f23e
commit 62e59645ee
3 changed files with 107 additions and 21 deletions
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2
example/drawer/main.go
View File

@ -115,7 +115,7 @@ func main() {
d := &font.Drawer{
Dst: rgba,
Src: fg,
Face: truetype.NewFace(f, truetype.Options{
Face: truetype.NewFace(f, &truetype.Options{
Size: *size,
DPI: *dpi,
Hinting: h,

124
truetype/face.go
View File

@ -29,38 +29,116 @@ type Options struct {
//
// A zero value means to use no hinting.
Hinting font.Hinting
// SubPixelsX is the number of sub-pixel locations a glyph's dot is
// quantized to, in the horizontal direction. For example, a value of 8
// means that the dot is quantized to 1/8th of a pixel. This quantization
// only affects the glyph mask image, not its bounding box or advance
// width. A higher value gives a more faithful glyph image, but reduces the
// effectiveness of the glyph cache.
//
// It must be a power of 2, and be between 1 and 64 inclusive.
//
// A zero value means to use 4 sub-pixel locations.
SubPixelsX int
// SubPixelsY is the number of sub-pixel locations a glyph's dot is
// quantized to, in the vertical direction. For example, a value of 8
// means that the dot is quantized to 1/8th of a pixel. This quantization
// only affects the glyph mask image, not its bounding box or advance
// width. A higher value gives a more faithful glyph image, but reduces the
// effectiveness of the glyph cache.
//
// It must be a power of 2, and be between 1 and 64 inclusive.
//
// A zero value means to use 1 sub-pixel location.
SubPixelsY int
}
func (o *Options) size() float64 {
if o.Size > 0 {
if o != nil && o.Size > 0 {
return o.Size
}
return 12
}
func (o *Options) dpi() float64 {
if o.DPI > 0 {
if o != nil && o.DPI > 0 {
return o.DPI
}
return 72
}
func (o *Options) hinting() font.Hinting {
switch o.Hinting {
case font.HintingVertical, font.HintingFull:
// TODO: support vertical hinting.
return font.HintingFull
if o != nil {
switch o.Hinting {
case font.HintingVertical, font.HintingFull:
// TODO: support vertical hinting.
return font.HintingFull
}
}
return font.HintingNone
}
func (o *Options) subPixelsX() (halfQuantum, mask fixed.Int26_6) {
if o != nil {
switch o.SubPixelsX {
case 1, 2, 4, 8, 16, 32, 64:
return subPixels(o.SubPixelsX)
}
}
// This default value of 4 isn't based on anything scientific, merely as
// small a number as possible that looks almost as good as no quantization,
// or returning subPixels(64).
return subPixels(4)
}
func (o *Options) subPixelsY() (halfQuantum, mask fixed.Int26_6) {
if o != nil {
switch o.SubPixelsX {
case 1, 2, 4, 8, 16, 32, 64:
return subPixels(o.SubPixelsX)
}
}
// This default value of 1 isn't based on anything scientific, merely that
// vertical sub-pixel glyph rendering is pretty rare. Baseline locations
// can usually afford to snap to the pixel grid, so the vertical direction
// doesn't have the deal with the horizontal's fractional advance widths.
return subPixels(1)
}
// subPixels returns the bias and mask that leads to q quantized sub-pixel
// locations per full pixel.
//
// For example, q == 4 leads to a bias of 8 and a mask of 0xfffffff0, or -16,
// because we want to round fractions of fixed.Int26_6 as:
// - 0 to 7 rounds to 0.
// - 8 to 23 rounds to 16.
// - 24 to 39 rounds to 32.
// - 40 to 55 rounds to 48.
// - 56 to 63 rounds to 64.
// which means to add 8 and then bitwise-and with -16, in two's complement
// representation.
//
// When q == 1, we want bias == 32 and mask == -64.
// When q == 2, we want bias == 16 and mask == -32.
// When q == 4, we want bias == 8 and mask == -16.
// ...
// When q == 64, we want bias == 0 and mask == -1. (The no-op case).
// The pattern is clear.
func subPixels(q int) (bias, mask fixed.Int26_6) {
return 32 / fixed.Int26_6(q), -64 / fixed.Int26_6(q)
}
// NewFace returns a new font.Face for the given Font.
func NewFace(f *Font, opts Options) font.Face {
func NewFace(f *Font, opts *Options) font.Face {
a := &face{
f: f,
hinting: opts.hinting(),
scale: fixed.Int26_6(0.5 + (opts.size() * opts.dpi() * 64 / 72)),
}
a.subPixelBiasX, a.subPixelMaskX = opts.subPixelsX()
a.subPixelBiasY, a.subPixelMaskY = opts.subPixelsY()
// Set the rasterizer's bounds to be big enough to handle the largest glyph.
b := f.Bounds(a.scale)
@ -78,15 +156,19 @@ func NewFace(f *Font, opts Options) font.Face {
}
type face struct {
f *Font
hinting font.Hinting
scale fixed.Int26_6
mask *image.Alpha
r raster.Rasterizer
p raster.Painter
maxw int
maxh int
glyphBuf GlyphBuf
f *Font
hinting font.Hinting
scale fixed.Int26_6
subPixelBiasX fixed.Int26_6
subPixelMaskX fixed.Int26_6
subPixelBiasY fixed.Int26_6
subPixelMaskY fixed.Int26_6
mask *image.Alpha
r raster.Rasterizer
p raster.Painter
maxw int
maxh int
glyphBuf GlyphBuf
// TODO: clip rectangle?
}
@ -109,9 +191,13 @@ func (a *face) Kern(r0, r1 rune) fixed.Int26_6 {
func (a *face) Glyph(dot fixed.Point26_6, r rune) (
newDot fixed.Point26_6, dr image.Rectangle, mask image.Image, maskp image.Point, ok bool) {
// Split p.X and p.Y into their integer and fractional parts.
ix, fx := int(dot.X>>6), dot.X&0x3f
iy, fy := int(dot.Y>>6), dot.Y&0x3f
// Quantize to the sub-pixel granularity.
dotX := (dot.X + a.subPixelBiasX) & a.subPixelMaskX
dotY := (dot.Y + a.subPixelBiasY) & a.subPixelMaskY
// Split the coordinates into their integer and fractional parts.
ix, fx := int(dotX>>6), dotX&0x3f
iy, fy := int(dotY>>6), dotY&0x3f
advanceWidth, offset, gw, gh, ok := a.rasterize(a.f.Index(r), fx, fy)
if !ok {

2
truetype/face_test.go
View File

@ -35,7 +35,7 @@ func BenchmarkDrawString(b *testing.B) {
d := &font.Drawer{
Dst: dst,
Src: image.Black,
Face: NewFace(f, Options{}),
Face: NewFace(f, nil),
}
b.ReportAllocs()
b.ResetTimer()