Add parsing code for the Font Features, as well as new command by bramp · Pull Request #3 · ConradIrwin/font

bramp · 2018-01-21T17:01:27Z

No description provided.

bramp · 2018-01-21T17:03:34Z

For example:

$ go run main.go features ~/path/to/font.woff 

Glyph Substitution Table (GSUB):
	Script "DFLT" (Default):
		Default Language:
			Feature "aalt" (Access All Alternates)
			Feature "c2sc" (Small Capitals From Capitals)
			Feature "case" (Case-Sensitive Forms)
			Feature "crcy" (Currency (Deprecated))
		Language "AZE " (Azerbaijani):
			Feature "smcp" (Small Capitals)
	Script "cyrl" (Cyrillic):
		Default Language:
			Feature "aalt" (Access All Alternates)
			Feature "c2sc" (Small Capitals From Capitals)
			Feature "case" (Case-Sensitive Forms)
			Feature "crcy" (Currency (Deprecated))

Glyph Positioning Table (GPOS):
	Script "DFLT" (Default):
		Default Language:
			Feature "kern" (Kerning)
	Script "cyrl" (Cyrillic):
		Default Language:
			Feature "kern" (Kerning)
	Script "latn" (Latin):
		Default Language:
			Feature "kern" (Kerning)
		Language "AZE " (Azerbaijani):
			Feature "kern" (Kerning)

dmitshur · 2018-01-21T17:43:41Z

+	"github.com/ConradIrwin/font/sfnt"
+)
+
+func layoutTable(font *sfnt.Font, tag sfnt.Tag, name string) {


I’d suggest moving layoutTable to the bottom, under Features, rather than on top.

In Go, it’s common to order code from higher level functions first to lower level helpers second. That way, when reading code, you first encounter higher level, easier to understand code that helps provide context for the helpers. Then, once you're familiar with how a helper is being used and why it’s needed, reading its code is easier.

Thanks for the feedback. Good point. Fixed.

dmitshur · 2018-01-21T19:24:30Z

(The rest of this PR is awesome, nice work!)

dmitshur

Spotted a couple typos.

dmitshur · 2018-01-21T21:20:16Z

+	"github.com/ConradIrwin/font/sfnt"
+)
+
+// Features prints the gpos/gsub tables (contins font features).


s/contins/contains/

dmitshur · 2018-01-21T21:20:20Z

-Usage: font [scrub|info|stats] font.[otf,ttf,woff]
+Usage: font [features|info|metrics|scrub|stats] font.[otf,ttf,woff]

+features: prints the gpos/gsub tables (contins font features)


s/contins/contains/

ConradIrwin · 2018-01-22T17:21:41Z

+	}
+
+	if len(tag) == 4 && tag[0] == 'c' && tag[1] == 'v' && tag[2] >= '0' && tag[2] <= '9' && tag[3] >= '0' && tag[3] <= '9' {
+		return "Character Variants"


@bramp might be worth preserving the number here (e.g. Character Variables 01)

(Also, should we do the same parsing for ss01...? )

Sure, will fix.

Added numeric suffix, plus added support for ss01... and added a test.

ConradIrwin · 2018-01-22T21:46:34Z

+// Features prints the gpos/gsub tables (contains font features).
+func Features() {
+	if len(os.Args) < 2 {
+		panic(fmt.Errorf("Specify a font file"))


Can we start to improve the code for handling these errors, we should make it clear how to fix the problem, and make the error message more legible. (I appreciate the other commands aren't yet very good).

fmt.Printf("Usage: font features <font-file>") os.Exit(1)

I'll do that in a later PR.

ConradIrwin · 2018-01-22T21:48:33Z

+	if font.HasTable(tag) {
+		fmt.Printf("%s:\n", name)
+
+		t := font.Table(tag).(*sfnt.TableLayout)


@bramp This will panic if parsing fails. I'm not sure what to do about that except maybe change the library so that it parses tables on-demand instead of up-front so we can return a sensible error. Any better ideas?

Yes, panic'ng sucks. After this I'd be happy to change the whole library to pass back the errors.

That'd be incredible 😻

ConradIrwin · 2018-01-22T21:49:50Z

+
+		t := font.Table(tag).(*sfnt.TableLayout)
+		for _, script := range t.Scripts {
+			fmt.Printf("\tScript %q (%s):\n", script.Tag, script.String())


When I run this on a font I see a weird:

Script " " ():

Do you know why? Can we make it clearer what it means?

Would you email me that font?

it's here: https://gist.github.com/ConradIrwin/a85f8e432fedf2cd4ea400286a796b5e

If you have the gist tool installed (brew install gist) you can get the file with: gist -r a85f8e432fedf2cd4ea400286a796b5e | base64 --decode > adelle.ttf

So that font has a script who's tag is 0x20202020 (four space characters). The spec says nothing about what this means, so I suspect it is just some weird artefact.

I've changed the tool to not display the () if the name is unknown.

ConradIrwin · 2018-01-22T21:51:02Z

+type scriptTable struct {
+	DefaultLangSys uint16 // Offset to default LangSys table, from beginning of Script table — may be NULL
+	LangSysCount   uint16 // Number of LangSysRecords for this script — excluding the default LangSys
+	// langSysRecords[langSysCount] langSysRecord // Array of LangSysRecords, listed alphabetically by LangSys tag


@bramp Why is this here but commented out? Should we uncomment it or delete it?

Ah yes, perhaps unclear. The ondisk format has a variable length array as the last element. I explicitly deal with that in the code, but left the field commented to indicate it was there.

I can drop the comment, or make it clear why it's there.

ConradIrwin · 2018-01-22T21:54:06Z

+		return nil, io.ErrUnexpectedEOF
+	}
+
+	b = b[record.Offset:]


@bramp Is there a byte length on the record? I'm worried about the parsing code reading beyond the length of the record.

nope, there is sadly no length for any of these fields. I suspect they are all contained within the single table.

ah yes, all the records are within the same table, and that b array only contains the current table:

buffer := io.NewSectionReader(file, int64(entry.Offset), int64(entry.Length)) table, err := parseTable(entry.Tag, buffer) ... b, err := ioutil.ReadAll(buffer) ... b = b[record.Offset:]

ConradIrwin · 2018-01-22T21:55:35Z

Otherwise this looks great.

If you have the time, a test would be great, but happy to merge without.

ConradIrwin · 2018-01-22T21:55:52Z

(Thanks for the other view @shurcooL )

ConradIrwin · 2018-01-22T21:57:54Z

P.S I added you both as collaborators to this repository.

Please notify me for PRs with large changes or API changes — but happy if you want to merge your own PRs for tweaks to code-style / error messages.

…own.

… N" feature names.

bramp · 2018-01-23T01:05:57Z

I've pushed my changes, if you want to make one more pass before merging?

ConradIrwin · 2018-01-23T22:08:59Z

Thanks!

bramp added 3 commits January 21, 2018 08:34

Added mapping of scripts, languages, and features tags to names.

76df659

Add parsing of most of the GPOS/GSUB layout tables.

c7a3e4c

Added a new "features" command for displaying the font features.

b6436c3

dmitshur reviewed Jan 21, 2018

View reviewed changes

Minor tweak due to code review feedback.

980a891

dmitshur reviewed Jan 21, 2018

View reviewed changes

Fixed a couple of typos.

681d414

ConradIrwin reviewed Jan 22, 2018

View reviewed changes

bramp added 2 commits January 22, 2018 17:02

Change feature to only print the description of the field if it is kn…

368e469

…own.

Added support to print out "Character Variants N", and "Stylistic Set…

f0147c0

… N" feature names.

ConradIrwin merged commit 03e4be4 into ConradIrwin:master Jan 23, 2018

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