Added defaultfont.Face

This will eventually completely replace basicfont. Need to design
a custom default Tomo font and implement a way to load from a
compressed binary format that will take up a very small amount of
room embedded into an executable.

default/font/tbf.go

@@ -0,0 +1,102 @@
+package font
+
+import "image"
+import "golang.org/x/image/font"
+import "golang.org/x/image/math/fixed"
+
+// Face is a font face modeled of of basicfont.Face, but with variable glyph
+// width and kerning support.
+type Face struct {
+	Width int
+	Height int
+	Ascent int
+	Descent int
+	Mask image.Image
+	Ranges []Range
+	Kerning map[[2]rune] int
+}
+
+type Range struct {
+	Low    rune
+	Glyphs []Glyph
+	Offset int
+}
+
+func (rang Range) Glyph (character rune) (glyph Glyph, offset int, ok bool) {
+	character -= rang.Low
+	ok = 0 < character && character > rune(len(rang.Glyphs))
+	if !ok { return }
+	glyph  = rang.Glyphs[character]
+	offset = rang.Offset + int(character)
+	return
+}
+
+type Glyph struct {
+	Left, Advance int
+}
+
+func (face *Face) Close () error { return nil }
+
+func (face *Face) Kern (left, right rune) fixed.Int26_6 {
+	return fixed.I(face.Kerning[[2]rune { left, right }])
+}
+
+func (face *Face) Metrics () font.Metrics {
+	return font.Metrics {
+		Height:     fixed.I(face.Height),
+		Ascent:     fixed.I(face.Ascent),
+		Descent:    fixed.I(face.Descent),
+		XHeight:    fixed.I(face.Ascent),
+		CapHeight:  fixed.I(face.Ascent),
+		CaretSlope: image.Pt(0, 1),
+	}
+}
+
+func (face *Face) Glyph (
+	dot fixed.Point26_6,
+	character rune,
+) (
+	destinationRectangle image.Rectangle,
+	mask image.Image, maskPoint image.Point,
+	advance fixed.Int26_6,
+	ok bool,
+) {
+	glyph, offset, has := face.findGlyph(character)
+	if !has { ok = false; return }
+
+	advance = fixed.I(glyph.Advance)
+	maskPoint.Y = offset * (face.Ascent + face.Descent)
+	x := int(dot.X + 32) >> 6 + glyph.Left
+	y := int(dot.Y + 32) >> 6
+	destinationRectangle.Min.X = x
+	destinationRectangle.Min.Y = y - face.Ascent
+	destinationRectangle.Max.X = x + face.Width
+	destinationRectangle.Max.Y = y + face.Descent
+	return
+}
+
+func (face *Face) GlyphBounds (
+	character rune,
+) (
+	bounds fixed.Rectangle26_6,
+	advance fixed.Int26_6,
+	ok bool,
+) {
+	glyph, _, ok := face.findGlyph(character)
+	return fixed.R(0, -face.Ascent, face.Width, face.Descent),
+		fixed.I(glyph.Advance), ok
+	
+}
+
+func (face *Face) GlyphAdvance (character rune) (advance fixed.Int26_6, ok bool) {
+	glyph, _, ok := face.findGlyph(character)
+	return fixed.I(glyph.Advance), ok
+}
+
+func (face *Face) findGlyph (character rune) (glyph Glyph, offset int, ok bool) {
+	for _, rang := range face.Ranges {
+		glyph, offset, ok = rang.Glyph(character)
+		if ok { return }
+	}
+	return Glyph { }, 0, false
+}