x/image/font/sfnt: GlyphName returns empty string on OpenType font

[andybalholm]

What version of Go are you using (go version)?

$ go version
go version go1.13beta1 darwin/amd64

What did you do?

package main

import (
	"fmt"
	"io/ioutil"
	"log"

	"golang.org/x/image/font/sfnt"
)

func main() {
	b, err := ioutil.ReadFile("texgyrepagella-bold.otf")
	if err != nil {
		log.Fatal(err)
	}

	f, err := sfnt.Parse(b)
	if err != nil {
		log.Fatal(err)
	}

	var buffer sfnt.Buffer
	g, err := f.GlyphIndex(&buffer, '!')
	if err != nil {
		log.Fatal(err)
	}

	name, err := f.GlyphName(&buffer, g)
	if err != nil {
		log.Fatal(err)
	}

	fmt.Printf("The glyph name is %q.\n", name)
}

texgyrepagella-bold.otf.zip

What did you expect to see?

The glyph name is "exclam".

What did you see instead?

The glyph name is "".

[smasher164]

sfnt doesn't support post table version 3.0, which is why GlyphName returns an empty string here. See https://docs.microsoft.com/en-us/typography/opentype/spec/post#version-30
/cc @nigeltao

[andybalholm]

In an OpenType PS font, I think the glyph names are stored in the CFF table along with the outlines instead of in the post table. (But I could be wrong on this; it's hard to find solid information without digging way down into the specifications.)