[font] resolve the initial viewport of SVG glyphs

font/renderer.go

@@ -65,6 +65,13 @@ type GlyphSVG struct {
 	// and several glyphs may share the same Source
 	Source []byte
 
+	// ViewBox is the initial viewport of the SVG document:
+	// the rectangle in SVG user space mapped to the em square
+	// when rendering.
+	// It is resolved from the root <svg> element attributes,
+	// defaulting to (0, 0, upem, upem) as required by the specification.
+	ViewBox SVGViewBox
+
 	// According to the specification, a fallback outline
 	// should be specified for each SVG glyphs
 	Outline GlyphOutline
@@ -179,7 +186,7 @@ func (bt bitmap) glyphData(gid gID, xPpem, yPpem uint16) (GlyphBitmap, error) {
 	return out, nil
 }
 
-func (s svg) glyphData(gid gID) (GlyphSVG, bool) {
+func (s svg) glyphData(gid gID, upem uint16) (GlyphSVG, bool) {
 	data, ok := s.rawGlyphData(gid)
 	if !ok {
 		return GlyphSVG{}, false
@@ -193,7 +200,7 @@ func (s svg) glyphData(gid gID) (GlyphSVG, bool) {
 		}
 	}
 
-	return GlyphSVG{Source: data}, true
+	return GlyphSVG{Source: data, ViewBox: svgViewBox(data, upem)}, true
 }
 
 // this file converts from font format for glyph outlines to
@@ -447,7 +454,7 @@ func (f *Face) GlyphDataOutline(gid GID) (GlyphOutline, bool) {
 
 // GlyphDataSVG looks for glyph data in the 'SVG ' table.
 func (f *Face) GlyphDataSVG(gid GID) (GlyphSVG, bool) {
-	outS, ok := f.svg.glyphData(gID(gid))
+	outS, ok := f.svg.glyphData(gID(gid), f.upem)
 	if !ok {
 		return GlyphSVG{}, false
 	}

font/svg.go

@@ -3,7 +3,12 @@
 package font
 
 import (
+	"bytes"
+	"encoding/xml"
 	"fmt"
+	"math"
+	"strconv"
+	"strings"
 
 	"github.com/go-text/typesetting/font/opentype/tables"
 )
@@ -52,3 +57,109 @@ func (s svg) rawGlyphData(gid gID) ([]byte, bool) {
 	}
 	return nil, false
 }
+
+// SVGViewBox is a rectangle in the "user" coordinate space
+// of an SVG document.
+type SVGViewBox struct {
+	MinX, MinY, Width, Height float32
+}
+
+// svgViewBox returns the initial viewport of the SVG document [doc],
+// resolved from the root <svg> element attributes, defaulting to the
+// em square given by [upem], as required by the specification.
+func svgViewBox(doc []byte, upem uint16) SVGViewBox {
+	viewBox, width, height, ok := svgRootAttributes(doc)
+	if ok {
+		if vb, ok := parseSVGViewBox(viewBox); ok {
+			return vb
+		}
+		if w, okW := parseSVGLength(width); okW {
+			if h, okH := parseSVGLength(height); okH {
+				return SVGViewBox{0, 0, w, h}
+			}
+		}
+	}
+	em := float32(upem)
+	return SVGViewBox{0, 0, em, em}
+}
+
+// parseSVGNumber parses a finite number, rejecting the "NaN" and
+// "Inf" forms accepted by [strconv.ParseFloat].
+func parseSVGNumber(s string) (float32, bool) {
+	v, err := strconv.ParseFloat(s, 32)
+	if err != nil || math.IsNaN(v) || math.IsInf(v, 0) {
+		return 0, false
+	}
+	return float32(v), true
+}
+
+// parseSVGViewBox parses the value of a viewBox attribute:
+// four numbers separated by whitespace and/or commas.
+func parseSVGViewBox(s string) (SVGViewBox, bool) {
+	fields := strings.FieldsFunc(s, func(r rune) bool {
+		return r == ',' || r == ' ' || r == '\t' || r == '\r' || r == '\n'
+	})
+	if len(fields) != 4 {
+		return SVGViewBox{}, false
+	}
+	var nums [4]float32
+	for i, field := range fields {
+		v, ok := parseSVGNumber(field)
+		if !ok {
+			return SVGViewBox{}, false
+		}
+		nums[i] = v
+	}
+	if nums[2] <= 0 || nums[3] <= 0 {
+		return SVGViewBox{}, false
+	}
+	return SVGViewBox{nums[0], nums[1], nums[2], nums[3]}, true
+}
+
+// parseSVGLength parses a positive width or height attribute value
+// expressed in user units, such as "12" or "12px".
+func parseSVGLength(s string) (float32, bool) {
+	s = strings.TrimSpace(s)
+	s = strings.TrimSuffix(s, "px")
+	v, ok := parseSVGNumber(s)
+	if !ok || v <= 0 {
+		return 0, false
+	}
+	return v, true
+}
+
+// svgNamespace is the namespace of SVG elements.
+const svgNamespace = "http://www.w3.org/2000/svg"
+
+// svgRootAttributes returns the raw viewBox, width and height attribute
+// values of the root element of the XML document [doc], or false if the
+// document has no root element or if it is not an svg element.
+func svgRootAttributes(doc []byte) (viewBox, width, height string, ok bool) {
+	dec := xml.NewDecoder(bytes.NewReader(doc))
+	var root xml.StartElement
+	for {
+		tok, err := dec.Token()
+		if err != nil { // includes io.EOF, i.e. no root element
+			return "", "", "", false
+		}
+		if start, isStart := tok.(xml.StartElement); isStart {
+			root = start
+			break
+		}
+	}
+	// A missing namespace is tolerated, but a foreign one is rejected.
+	if root.Name.Local != "svg" || (root.Name.Space != "" && root.Name.Space != svgNamespace) {
+		return "", "", "", false
+	}
+	for _, attr := range root.Attr {
+		switch attr.Name.Local {
+		case "viewBox":
+			viewBox = attr.Value
+		case "width":
+			width = attr.Value
+		case "height":
+			height = attr.Value
+		}
+	}
+	return viewBox, width, height, true
+}

font/svg_test.go

@@ -0,0 +1,70 @@
+// SPDX-License-Identifier: Unlicense OR BSD-3-Clause
+
+package font
+
+import (
+	"testing"
+
+	tu "github.com/go-text/typesetting/testutils"
+)
+
+func TestSVGViewBox(t *testing.T) {
+	const upem = 1000
+	for _, test := range []struct {
+		doc      string
+		expected SVGViewBox
+	}{
+		// no viewport information: default to the em square
+		{`<svg xmlns="http://www.w3.org/2000/svg" id="glyph1"></svg>`, SVGViewBox{0, 0, upem, upem}},
+		{``, SVGViewBox{0, 0, upem, upem}},
+		{`not xml`, SVGViewBox{0, 0, upem, upem}},
+		{`<svg`, SVGViewBox{0, 0, upem, upem}}, // malformed
+		// the root element must be an svg element, in the SVG namespace or none
+		{`<html viewBox="0 0 128 128"></html>`, SVGViewBox{0, 0, upem, upem}},
+		{`<svg xmlns="http://example.com/wrong" viewBox="0 0 128 128"></svg>`, SVGViewBox{0, 0, upem, upem}},
+		{`<w:svg xmlns:w="http://example.com/wrong" viewBox="0 0 128 128"></w:svg>`, SVGViewBox{0, 0, upem, upem}},
+		{`<svg:svg xmlns:svg="http://www.w3.org/2000/svg" viewBox="0 0 128 128"></svg:svg>`, SVGViewBox{0, 0, 128, 128}},
+		// viewBox attribute (a missing namespace is tolerated)
+		{`<svg viewBox="0 0 128 128"></svg>`, SVGViewBox{0, 0, 128, 128}},
+		{`<svg viewBox="-10,-20 30,40"></svg>`, SVGViewBox{-10, -20, 30, 40}},
+		{`<svg viewBox=" 0 , 0 , 1e2 , 50.5 "></svg>`, SVGViewBox{0, 0, 100, 50.5}},
+		{`<svg viewBox='0 0 128 64'/>`, SVGViewBox{0, 0, 128, 64}},
+		// invalid viewBox attributes
+		{`<svg viewBox="0 0 128"></svg>`, SVGViewBox{0, 0, upem, upem}},
+		{`<svg viewBox="0 0 0 128"></svg>`, SVGViewBox{0, 0, upem, upem}},
+		{`<svg viewBox="0 0 128 -1"></svg>`, SVGViewBox{0, 0, upem, upem}},
+		{`<svg viewBox="a b c d"></svg>`, SVGViewBox{0, 0, upem, upem}},
+		{`<svg viewBox="NaN 0 128 128"></svg>`, SVGViewBox{0, 0, upem, upem}},
+		{`<svg viewBox="0 0 Inf 128"></svg>`, SVGViewBox{0, 0, upem, upem}},
+		{`<svg viewBox="0 0 128 +infinity"></svg>`, SVGViewBox{0, 0, upem, upem}},
+		// width and height attributes
+		{`<svg width="100" height="200"></svg>`, SVGViewBox{0, 0, 100, 200}},
+		{`<svg width="100px" height="50px"></svg>`, SVGViewBox{0, 0, 100, 50}},
+		{`<svg width="100"></svg>`, SVGViewBox{0, 0, upem, upem}},
+		{`<svg width="100%" height="100%"></svg>`, SVGViewBox{0, 0, upem, upem}},
+		{`<svg width="12pt" height="12pt"></svg>`, SVGViewBox{0, 0, upem, upem}},
+		{`<svg width="NaN" height="NaN"></svg>`, SVGViewBox{0, 0, upem, upem}},
+		{`<svg width="Inf" height="Inf"></svg>`, SVGViewBox{0, 0, upem, upem}},
+		// viewBox has precedence over width and height
+		{`<svg width="100" height="200" viewBox="0 0 128 128"></svg>`, SVGViewBox{0, 0, 128, 128}},
+		{`<svg viewBox="invalid" width="100" height="200"></svg>`, SVGViewBox{0, 0, 100, 200}},
+		// content before the root element is skipped
+		{`<?xml version="1.0"?><!-- comment --><!DOCTYPE svg><svg viewBox="0 0 128 128"></svg>`, SVGViewBox{0, 0, 128, 128}},
+	} {
+		got := svgViewBox([]byte(test.doc), upem)
+		if got != test.expected {
+			t.Fatalf("document %q: expected %v, got %v", test.doc, test.expected, got)
+		}
+	}
+}
+
+func TestGlyphDataSVGViewBox(t *testing.T) {
+	// this font has no viewBox attribute in its SVG document:
+	// the viewport defaults to the em square
+	font := loadFont(t, "toys/chromacheck-svg.ttf")
+	face := Face{Font: font}
+
+	glyph, ok := face.GlyphDataSVG(1)
+	tu.Assert(t, ok)
+	tu.Assert(t, glyph.ViewBox == SVGViewBox{0, 0, 1024, 1024})
+}