mirror of
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281 lines
8.8 KiB
Go
281 lines
8.8 KiB
Go
// Copyright 2015 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Package font defines an interface for font faces, for drawing text on an
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// image.
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//
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// Other packages provide font face implementations. For example, a truetype
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// package would provide one based on .ttf font files.
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package font
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import (
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"image"
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"image/draw"
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"io"
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"unicode/utf8"
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"golang.org/x/image/math/fixed"
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)
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// TODO: who is responsible for caches (glyph images, glyph indices, kerns)?
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// The Drawer or the Face?
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// Face is a font face. Its glyphs are often derived from a font file, such as
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// "Comic_Sans_MS.ttf", but a face has a specific size, style, weight and
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// hinting. For example, the 12pt and 18pt versions of Comic Sans are two
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// different faces, even if derived from the same font file.
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//
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// A Face is not safe for concurrent use by multiple goroutines, as its methods
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// may re-use implementation-specific caches and mask image buffers.
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//
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// To create a Face, look to other packages that implement specific font file
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// formats.
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type Face interface {
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io.Closer
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// Glyph returns the draw.DrawMask parameters (dr, mask, maskp) to draw r's
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// glyph at the sub-pixel destination location dot, and that glyph's
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// advance width.
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//
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// It returns !ok if the face does not contain a glyph for r.
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//
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// The contents of the mask image returned by one Glyph call may change
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// after the next Glyph call. Callers that want to cache the mask must make
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// a copy.
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Glyph(dot fixed.Point26_6, r rune) (
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dr image.Rectangle, mask image.Image, maskp image.Point, advance fixed.Int26_6, ok bool)
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// GlyphBounds returns the bounding box of r's glyph, drawn at a dot equal
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// to the origin, and that glyph's advance width.
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//
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// It returns !ok if the face does not contain a glyph for r.
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//
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// The glyph's ascent and descent equal -bounds.Min.Y and +bounds.Max.Y. A
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// visual depiction of what these metrics are is at
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// https://developer.apple.com/library/mac/documentation/TextFonts/Conceptual/CocoaTextArchitecture/Art/glyph_metrics_2x.png
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GlyphBounds(r rune) (bounds fixed.Rectangle26_6, advance fixed.Int26_6, ok bool)
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// GlyphAdvance returns the advance width of r's glyph.
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//
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// It returns !ok if the face does not contain a glyph for r.
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GlyphAdvance(r rune) (advance fixed.Int26_6, ok bool)
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// Kern returns the horizontal adjustment for the kerning pair (r0, r1). A
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// positive kern means to move the glyphs further apart.
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Kern(r0, r1 rune) fixed.Int26_6
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// Metrics returns the metrics for this Face.
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Metrics() Metrics
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// TODO: ColoredGlyph for various emoji?
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// TODO: Ligatures? Shaping?
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}
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// Metrics holds the metrics for a Face. A visual depiction is at
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// https://developer.apple.com/library/mac/documentation/TextFonts/Conceptual/CocoaTextArchitecture/Art/glyph_metrics_2x.png
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type Metrics struct {
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// Height is the recommended amount of vertical space between two lines of
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// text.
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Height fixed.Int26_6
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// Ascent is the distance from the top of a line to its baseline.
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Ascent fixed.Int26_6
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// Descent is the distance from the bottom of a line to its baseline. The
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// value is typically positive, even though a descender goes below the
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// baseline.
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Descent fixed.Int26_6
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}
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// Drawer draws text on a destination image.
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//
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// A Drawer is not safe for concurrent use by multiple goroutines, since its
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// Face is not.
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type Drawer struct {
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// Dst is the destination image.
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Dst draw.Image
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// Src is the source image.
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Src image.Image
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// Face provides the glyph mask images.
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Face Face
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// Dot is the baseline location to draw the next glyph. The majority of the
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// affected pixels will be above and to the right of the dot, but some may
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// be below or to the left. For example, drawing a 'j' in an italic face
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// may affect pixels below and to the left of the dot.
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Dot fixed.Point26_6
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// TODO: Clip image.Image?
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// TODO: SrcP image.Point for Src images other than *image.Uniform? How
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// does it get updated during DrawString?
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}
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// TODO: should DrawString return the last rune drawn, so the next DrawString
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// call can kern beforehand? Or should that be the responsibility of the caller
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// if they really want to do that, since they have to explicitly shift d.Dot
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// anyway? What if ligatures span more than two runes? What if grapheme
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// clusters span multiple runes?
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//
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// TODO: do we assume that the input is in any particular Unicode Normalization
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// Form?
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//
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// TODO: have DrawRunes(s []rune)? DrawRuneReader(io.RuneReader)?? If we take
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// io.RuneReader, we can't assume that we can rewind the stream.
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//
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// TODO: how does this work with line breaking: drawing text up until a
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// vertical line? Should DrawString return the number of runes drawn?
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// DrawBytes draws s at the dot and advances the dot's location.
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func (d *Drawer) DrawBytes(s []byte) {
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prevC := rune(-1)
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for len(s) > 0 {
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c, size := utf8.DecodeRune(s)
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s = s[size:]
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if prevC >= 0 {
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d.Dot.X += d.Face.Kern(prevC, c)
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}
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dr, mask, maskp, advance, ok := d.Face.Glyph(d.Dot, c)
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if !ok {
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// TODO: is falling back on the U+FFFD glyph the responsibility of
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// the Drawer or the Face?
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// TODO: set prevC = '\ufffd'?
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continue
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}
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draw.DrawMask(d.Dst, dr, d.Src, image.Point{}, mask, maskp, draw.Over)
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d.Dot.X += advance
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prevC = c
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}
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}
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// DrawString draws s at the dot and advances the dot's location.
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func (d *Drawer) DrawString(s string) {
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prevC := rune(-1)
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for _, c := range s {
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if prevC >= 0 {
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d.Dot.X += d.Face.Kern(prevC, c)
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}
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dr, mask, maskp, advance, ok := d.Face.Glyph(d.Dot, c)
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if !ok {
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// TODO: is falling back on the U+FFFD glyph the responsibility of
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// the Drawer or the Face?
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// TODO: set prevC = '\ufffd'?
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continue
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}
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draw.DrawMask(d.Dst, dr, d.Src, image.Point{}, mask, maskp, draw.Over)
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d.Dot.X += advance
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prevC = c
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}
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}
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// MeasureBytes returns how far dot would advance by drawing s.
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func (d *Drawer) MeasureBytes(s []byte) (advance fixed.Int26_6) {
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return MeasureBytes(d.Face, s)
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}
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// MeasureString returns how far dot would advance by drawing s.
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func (d *Drawer) MeasureString(s string) (advance fixed.Int26_6) {
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return MeasureString(d.Face, s)
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}
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// MeasureBytes returns how far dot would advance by drawing s with f.
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func MeasureBytes(f Face, s []byte) (advance fixed.Int26_6) {
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prevC := rune(-1)
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for len(s) > 0 {
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c, size := utf8.DecodeRune(s)
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s = s[size:]
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if prevC >= 0 {
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advance += f.Kern(prevC, c)
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}
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a, ok := f.GlyphAdvance(c)
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if !ok {
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// TODO: is falling back on the U+FFFD glyph the responsibility of
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// the Drawer or the Face?
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// TODO: set prevC = '\ufffd'?
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continue
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}
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advance += a
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prevC = c
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}
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return advance
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}
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// MeasureString returns how far dot would advance by drawing s with f.
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func MeasureString(f Face, s string) (advance fixed.Int26_6) {
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prevC := rune(-1)
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for _, c := range s {
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if prevC >= 0 {
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advance += f.Kern(prevC, c)
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}
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a, ok := f.GlyphAdvance(c)
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if !ok {
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// TODO: is falling back on the U+FFFD glyph the responsibility of
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// the Drawer or the Face?
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// TODO: set prevC = '\ufffd'?
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continue
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}
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advance += a
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prevC = c
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}
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return advance
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}
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// Hinting selects how to quantize a vector font's glyph nodes.
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//
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// Not all fonts support hinting.
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type Hinting int
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const (
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HintingNone Hinting = iota
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HintingVertical
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HintingFull
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)
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// Stretch selects a normal, condensed, or expanded face.
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//
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// Not all fonts support stretches.
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type Stretch int
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const (
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StretchUltraCondensed Stretch = -4
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StretchExtraCondensed Stretch = -3
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StretchCondensed Stretch = -2
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StretchSemiCondensed Stretch = -1
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StretchNormal Stretch = +0
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StretchSemiExpanded Stretch = +1
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StretchExpanded Stretch = +2
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StretchExtraExpanded Stretch = +3
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StretchUltraExpanded Stretch = +4
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)
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// Style selects a normal, italic, or oblique face.
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//
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// Not all fonts support styles.
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type Style int
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const (
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StyleNormal Style = iota
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StyleItalic
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StyleOblique
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)
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// Weight selects a normal, light or bold face.
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//
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// Not all fonts support weights.
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//
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// The named Weight constants (e.g. WeightBold) correspond to CSS' common
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// weight names (e.g. "Bold"), but the numerical values differ, so that in Go,
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// the zero value means to use a normal weight. For the CSS names and values,
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// see https://developer.mozilla.org/en/docs/Web/CSS/font-weight
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type Weight int
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const (
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WeightThin Weight = -3 // CSS font-weight value 100.
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WeightExtraLight Weight = -2 // CSS font-weight value 200.
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WeightLight Weight = -1 // CSS font-weight value 300.
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WeightNormal Weight = +0 // CSS font-weight value 400.
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WeightMedium Weight = +1 // CSS font-weight value 500.
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WeightSemiBold Weight = +2 // CSS font-weight value 600.
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WeightBold Weight = +3 // CSS font-weight value 700.
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WeightExtraBold Weight = +4 // CSS font-weight value 800.
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WeightBlack Weight = +5 // CSS font-weight value 900.
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)
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