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// Copyright 2015 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package font defines an interface for font faces, for drawing text on an
// image.
//
// Other packages provide font face implementations. For example, a truetype
// package would provide one based on .ttf font files.
package font
import (
"image"
"image/draw"
"io"
"golang.org/x/image/math/fixed"
)
// TODO: who is responsible for caches (glyph images, glyph indices, kerns)?
// The Drawer or the Face?
// Face is a font face. Its glyphs are often derived from a font file, such as
// "Comic_Sans_MS.ttf", but a face has a specific size, style, weight and
// hinting. For example, the 12pt and 18pt versions of Comic Sans are two
// different faces, even if derived from the same font file.
//
// A Face is not safe for concurrent use by multiple goroutines, as its methods
// may re-use implementation-specific caches and mask image buffers.
//
// To create a Face, look to other packages that implement specific font file
// formats.
type Face interface {
io.Closer
// Glyph returns the draw.DrawMask parameters (dr, mask, maskp) to draw r's
// glyph at the sub-pixel destination location dot, and that glyph's
// advance width.
//
// It returns !ok if the face does not contain a glyph for r.
//
// The contents of the mask image returned by one Glyph call may change
// after the next Glyph call. Callers that want to cache the mask must make
// a copy.
Glyph(dot fixed.Point26_6, r rune) (
dr image.Rectangle, mask image.Image, maskp image.Point, advance fixed.Int26_6, ok bool)
// GlyphBounds returns the bounding box of r's glyph, drawn at a dot equal
// to the origin, and that glyph's advance width.
//
// It returns !ok if the face does not contain a glyph for r.
//
// The glyph's ascent and descent equal -bounds.Min.Y and +bounds.Max.Y. A
// visual depiction of what these metrics are is at
// https://developer.apple.com/library/mac/documentation/TextFonts/Conceptual/CocoaTextArchitecture/Art/glyph_metrics_2x.png
GlyphBounds(r rune) (bounds fixed.Rectangle26_6, advance fixed.Int26_6, ok bool)
// GlyphAdvance returns the advance width of r's glyph.
//
// It returns !ok if the face does not contain a glyph for r.
GlyphAdvance(r rune) (advance fixed.Int26_6, ok bool)
// Kern returns the horizontal adjustment for the kerning pair (r0, r1). A
// positive kern means to move the glyphs further apart.
Kern(r0, r1 rune) fixed.Int26_6
// TODO: per-font Metrics.
// TODO: ColoredGlyph for various emoji?
// TODO: Ligatures? Shaping?
}
// TODO: Drawer.Layout or Drawer.Measure methods to measure text without
// drawing?
// Drawer draws text on a destination image.
//
// A Drawer is not safe for concurrent use by multiple goroutines, since its
// Face is not.
type Drawer struct {
// Dst is the destination image.
Dst draw.Image
// Src is the source image.
Src image.Image
// Face provides the glyph mask images.
Face Face
// Dot is the baseline location to draw the next glyph. The majority of the
// affected pixels will be above and to the right of the dot, but some may
// be below or to the left. For example, drawing a 'j' in an italic face
// may affect pixels below and to the left of the dot.
Dot fixed.Point26_6
// TODO: Clip image.Image?
// TODO: SrcP image.Point for Src images other than *image.Uniform? How
// does it get updated during DrawString?
}
// TODO: should DrawString return the last rune drawn, so the next DrawString
// call can kern beforehand? Or should that be the responsibility of the caller
// if they really want to do that, since they have to explicitly shift d.Dot
// anyway?
//
// In general, we'd have a DrawBytes([]byte) and DrawRuneReader(io.RuneReader)
// and the last case can't assume that you can rewind the stream.
//
// TODO: how does this work with line breaking: drawing text up until a
// vertical line? Should DrawString return the number of runes drawn?
// DrawString draws s at the dot and advances the dot's location.
func (d *Drawer) DrawString(s string) {
var prevC rune
for i, c := range s {
if i != 0 {
d.Dot.X += d.Face.Kern(prevC, c)
}
dr, mask, maskp, advance, ok := d.Face.Glyph(d.Dot, c)
if !ok {
// TODO: is falling back on the U+FFFD glyph the responsibility of
// the Drawer or the Face?
// TODO: set prevC = '\ufffd'?
continue
}
draw.DrawMask(d.Dst, dr, d.Src, image.Point{}, mask, maskp, draw.Over)
d.Dot.X += advance
prevC = c
}
}
// MeasureString returns how far dot would advance by drawing s.
func (d *Drawer) MeasureString(s string) (advance fixed.Int26_6) {
var prevC rune
for i, c := range s {
if i != 0 {
advance += d.Face.Kern(prevC, c)
}
a, ok := d.Face.GlyphAdvance(c)
if !ok {
// TODO: is falling back on the U+FFFD glyph the responsibility of
// the Drawer or the Face?
// TODO: set prevC = '\ufffd'?
continue
}
advance += a
prevC = c
}
return advance
}
// Hinting selects how to quantize a vector font's glyph nodes.
//
// Not all fonts support hinting.
type Hinting int
const (
HintingNone Hinting = iota
HintingVertical
HintingFull
)
// Stretch selects a normal, condensed, or expanded face.
//
// Not all fonts support stretches.
type Stretch int
const (
StretchUltraCondensed Stretch = -4
StretchExtraCondensed Stretch = -3
StretchCondensed Stretch = -2
StretchSemiCondensed Stretch = -1
StretchNormal Stretch = +0
StretchSemiExpanded Stretch = +1
StretchExpanded Stretch = +2
StretchExtraExpanded Stretch = +3
StretchUltraExpanded Stretch = +4
)
// Style selects a normal, italic, or oblique face.
//
// Not all fonts support styles.
type Style int
const (
StyleNormal Style = iota
StyleItalic
StyleOblique
)
// Weight selects a normal, light or bold face.
//
// Not all fonts support weights.
type Weight int
const (
WeightThin Weight = 100
WeightExtraLight Weight = 200
WeightLight Weight = 300
WeightNormal Weight = 400
WeightMedium Weight = 500
WeightSemiBold Weight = 600
WeightBold Weight = 700
WeightExtraBold Weight = 800
WeightBlack Weight = 900
)
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