label-tool: respect font ascent and descent

Also try to load the values from the BDF font file.

bdf/bdf.go

@@ -49,6 +49,8 @@ func (g *glyph) At(x, y int) color.Color {
 // Font represents a particular bitmap font.
 type Font struct {
 	Name     string
+	Ascent   int // needn't be present in the font
+	Descent  int // needn't be present in the font
 	glyphs   map[rune]glyph
 	fallback glyph
 }
@@ -192,17 +194,23 @@ func (p *bdfParser) readLine() bool {
 	return true
 }
 
-func (p *bdfParser) readCharEncoding() int {
+func (p *bdfParser) readIntegerArgument() int {
 	if len(p.tokens) < 2 {
 		panic("insufficient arguments")
 	}
 	if i, err := strconv.Atoi(p.tokens[1]); err != nil {
 		panic(err)
 	} else {
-		return i // Some fonts even use -1 for things outside the encoding.
+		return i
 	}
 }
 
+// Some fonts even use -1 for things outside the encoding.
+func (p *bdfParser) readCharEncoding() int { return p.readIntegerArgument() }
+
+// XXX: Ignoring vertical advance since we only expect purely horizontal fonts.
+func (p *bdfParser) readDwidth() int { return p.readIntegerArgument() }
+
 func (p *bdfParser) parseProperties() {
 	// The wording in the specification suggests that the argument
 	// with the number of properties to follow isn't reliable.
@@ -210,22 +218,14 @@ func (p *bdfParser) parseProperties() {
 		switch p.tokens[0] {
 		case "DEFAULT_CHAR":
 			p.defaultChar = p.readCharEncoding()
+		case "FONT_ASCENT":
+			p.font.Ascent = p.readIntegerArgument()
+		case "FONT_DESCENT":
+			p.font.Descent = p.readIntegerArgument()
 		}
 	}
 }
 
-// XXX: Ignoring vertical advance since we only expect purely horizontal fonts.
-func (p *bdfParser) readDwidth() int {
-	if len(p.tokens) < 2 {
-		panic("insufficient arguments")
-	}
-	if i, err := strconv.Atoi(p.tokens[1]); err != nil {
-		panic(err)
-	} else {
-		return i
-	}
-}
-
 func (p *bdfParser) readBBX() image.Rectangle {
 	if len(p.tokens) < 5 {
 		panic("insufficient arguments")

label/label.go

@@ -51,6 +51,13 @@ func GenLabelForHeight(font *bdf.Font,
 	return combinedImg
 }
 
+func max(a, b int) int {
+	if a > b {
+		return a
+	}
+	return b
+}
+
 func GenLabelForWidth(font *bdf.Font,
 	text string, width, scale int) image.Image {
 	var lines []string
@@ -58,30 +65,43 @@ func GenLabelForWidth(font *bdf.Font,
 		lines = append(lines, strings.TrimSuffix(line, "\r"))
 	}
 
-	height := 0
+	// Respect font ascent and descent so that there are gaps between lines.
-	var rects []image.Rectangle
+	rects := make([]image.Rectangle, len(lines))
-	for _, line := range lines {
+	jumps := make([]int, len(lines))
+	for i, line := range lines {
 		r, _ := font.BoundString(line)
-		rects = append(rects, r)
+		rects[i] = r
-		height += r.Dy() * scale
+
+		if i > 0 {
+			deficitD := font.Descent - rects[i-1].Max.Y
+			jumps[i] += max(0, deficitD)
+			deficitA := font.Ascent - (-r.Min.Y)
+			jumps[i] += max(0, deficitA)
+		}
 	}
 
-	imgRect := image.Rect(0, 0, width, height)
+	height := 0
+	for i := range lines {
+		height += jumps[i] + rects[i].Dy()
+	}
+
+	imgRect := image.Rect(0, 0, width, height*scale)
 	img := image.NewRGBA(imgRect)
 	draw.Draw(img, imgRect, image.White, image.ZP, draw.Src)
 
 	y := 0
-	for i := 0; i < len(lines); i++ {
+	for i, line := range lines {
 		textImg := image.NewRGBA(rects[i])
 		draw.Draw(textImg, rects[i], image.White, image.ZP, draw.Src)
-		font.DrawString(textImg, image.ZP, lines[i])
+		font.DrawString(textImg, image.ZP, line)
 
 		scaledImg := imgutil.Scale{Image: textImg, Scale: scale}
 		scaledRect := scaledImg.Bounds()
 
-		target := image.Rect(0, y, imgRect.Max.X, imgRect.Max.Y)
+		y += jumps[i]
+		target := image.Rect(0, y*scale, imgRect.Max.X, imgRect.Max.Y)
 		draw.Draw(img, target, &scaledImg, scaledRect.Min, draw.Src)
-		y += scaledRect.Dy()
+		y += rects[i].Dy()
 	}
 	return img
 }