ttf2otf snippet

[ftCLI]

Thanks to @justvanrossum and @anthrotype help in #2836 (and thanks to @behdad for implementing qu2cu!), I made a ttf2otf snippet.

In addition to fontTools, requires:

• click to create the command line interface
• afdko to perform optional otulines quality check with checkoutlinesufo
• cffsubr to subroutinize converted fonts

Usage

Usage: ttf2otf.py [OPTIONS] INPUT_PATH

  Converts TTF fonts (or TrueType flavored woff/woff2 web fonts) to OTF fonts
  (or CFF flavored woff/woff2 web fonts).

Options:
  -t, --tolerance FLOAT RANGE   Conversion tolerance (0-3, default 1). Low
                                tolerance adds more points but keeps shapes.
                                High tolerance adds few points but may change
                                shape.  [0<=x<=3]
  --scale-upm                   Scale units-per-em of source font to 1000
                                before converting to OTF
  --keep-glyphs                 Doesn't remove NULL and CR glyphs from output
                                fonts
  --no-subr                     Do not subroutinize converted fonts with
                                cffsubr
  --check-outlines              Performs optional outline quality checks and
                                removes overlaps with checkoutlinesufo
  -out, --output-dir DIRECTORY  Specify the directory where output files are
                                to be saved. If output_dir doesn't exist, will
                                be created. If not specified, files are saved
                                to the same folder.
  --recalc-timestamp            Keep the original font 'modified' timestamp
                                (head.modified) or set it to current time. By
                                default,  original timestamp is kept.
  --no-overwrite                Overwrite existing output files or save them
                                to a new file (numbers are appended at the end
                                of file name). By default, files are
                                overwritten.
  --help                        Show this message and exit.

Screenshot

Link
https://github.com/ftCLI/TTF2OTF

[behdad]

THanks! We should probably incorporate this in fonttools.

[ftCLI]

There's still a TODO: rounding fractional coordinates (I suppose it's a pathops issue, but if a source TTF has fractional coordinates the outlines of the output font are broken).

As a temporary solution, I convert the font to a temporary OTF using T2CharstringPen, convert if back to TTF and use this last TTF as input font. But looks inelegant: https://github.com/ftCLI/ftCLI/blob/eae80490864a2e0af8f50866f4e5e94951873e9b/ftCLI/Lib/converters/ttf_to_otf.py#L65

(I removed this part of code from the snippet)

[Lorp]

What does "if a source TTF has fractional coordinates" mean?

TTF by definition cannot contain explicit fractional coordinates. However, most TTFs contain implicit fractional coordinates because of the implied on-curve point exactly half way between two consecutive off-curve points. CFF and CFF2 allow non-integer coordinates, but their representation is significantly less efficient than integers.

A 100% accurate integer qu2cu operation is possible by converting each quadratic curve into a cubic curve, but only if the upem is doubled. So all of the n + 0.5 fractional coordinates become 2 n + 1 integer coordinates. It could therefore be a useful option to double the upem in this tool.

[ftCLI]

This is what happens with a TTF of Avenir Std Book:

If the TTF is opened font in FontLab:

[ftCLI]

What does "if a source TTF has fractional coordinates" mean?

TTF by definition cannot contain explicit fractional coordinates. However, most TTFs contain implicit fractional coordinates because of the implied on-curve point exactly half way between two consecutive off-curve points. CFF and CFF2 allow non-integer coordinates, but their representation is significantly less efficient than integers.

A 100% accurate integer qu2cu operation is possible by converting each quadratic curve into a cubic curve, but only if the upem is doubled. So all of the n + 0.5 fractional coordinates become 2 n + 1 integer coordinates. It could therefore be a useful option to double the upem in this tool.

Done. I added the --scale-upm INT.

I moved the script here: https://github.com/ftCLI/TTF2OTF

[behdad]

qu2cu introduces its own fractional values...

[Lorp]

I wrote:

A 100% accurate integer qu2cu operation is possible by converting each quadratic curve into a cubic curve, but only if the upem is doubled.

Scratch that. Coordinates of cubic control points converted from integer quadratics are fractional in general.

[ftCLI]

Workaround:

if self.options.safe_mode:
    # Create a temporary OTF file with T2CharStringPen without using pathops...
    ttf2otf_converter_temp = TrueTypeToCFF(font)
    ttf2otf_converter_temp.options.charstring_source = "t2"
    ttf2otf_converter_temp.options.subroutinize = False
    ttf2otf_converter_temp.options.remove_glyphs = self.options.remove_glyphs
    temp_cff_font = ttf2otf_converter_temp.run()
    temp_cff_font.flavor = None
    temp_cff_file = makeOutputFileName(font.reader.file.name, extension=".otf", suffix="_tmp")
    temp_cff_font.save(temp_cff_file)

    # ... and convert it back to a temporary TTF file that will be used for conversion
    temp_ttf_file = makeOutputFileName(temp_cff_file, extension=".ttf")
    run_shell_command(["otf2ttf", temp_cff_file, "-e", "2.0", "-o", temp_ttf_file])
    font = TTFont(temp_ttf_file, recalcTimestamp=self.options.recalc_timestamp)
    os.remove(temp_cff_file)
    os.remove(temp_ttf_file)

def get_t2_charstrings(self) -> dict:
    """
    Get CFF charstrings using T2CharStringPen without using pathops

    :return: CFF charstrings.
    """
    charstrings = {}
    glyph_set = self.font.getGlyphSet()

    for k, v in glyph_set.items():
        # Draw the glyph with T2CharStringPen and get the charstring
        t2_pen = T2CharStringPen(v.width, glyphSet=glyph_set)
        glyph_set[k].draw(t2_pen)
        charstring = t2_pen.getCharString()
        charstrings[k] = charstring

    return charstrings

ttf2otf.zip

[anthrotype]

maybe disable subroutinization so it's easier to compare the two sets of fonts, subroutinization adds noise to the diff

[ftCLI]

Same. Italics have identical CRC also with --no-subr options, uprights are bigger.

skia_0_7_4.zip
skia_0_8_0.zip

[anthrotype]

strange, it appears as if Qu2Cu isn't actually being run on the outlines inside skia_0_8_0.zip.. it's as if you get one cubic per individual atomic quadratic bezier curve in the input, which is usally too many (ideally you get 1 cubic for each quadratic spline), try to tweak the qu2cu tolerance maybe.. I don't see how it might be connected to the changes I made in skia-pathops. Try to not silently pass over errors which may hide bugs

[ftCLI]

Found. If I don't catch the exception here...

try:
    charstrings = self.get_qu2cu_charstrings(tolerance=self.options.tolerance, all_cubic=True)
except NotImplementedError:
    try:
        generic_info_message("All cubic set to False")
        charstrings = self.get_qu2cu_charstrings(tolerance=self.options.tolerance, all_cubic=False)
    except Exception as e:
        generic_error_message(f"An error occurred while getting qu2cu charstrings {e}")
        return

... and set all_cubic to True

charstrings = self.get_qu2cu_charstrings(tolerance=self.options.tolerance, all_cubic=True)

Raises NotImplementedError from Qu2CuPen.filterContour:

fonttools/Lib/fontTools/pens/qu2cuPen.py

Line 72 in 02a0636

raise NotImplementedError(

[anthrotype]

Ok, thanks. It should not be too difficult to add support for oncurve-less contours in Qu2CuPen, perhaps file an issue so it doest get lost in this long thread

[anthrotype]

I wonder, how hard would it be to make @ftCLI's ttf2otf script also support converting TrueType-flavored VF to CFF2? cc @behdad

[behdad]

Good challenge!

I think there's a CFF2 VF merger pen in cffLib somewhere we can use. I'll take a look.

[ftCLI]

I resume this topic because I think the script is ready to be added as a snippet, but I'm not opening a PR because I don't know if it's ok to use click to build the command line and cffsubr.

The workaround I found to convert oncurve-less contours to cubic is the following:

• Convert the contour to cubic with the T2CharStringPen
• Convert back the charstring back to quadratic with the Cu2QuPen
• Finally, convert the Glyph object to cubic with the Qu2CuPen

    for k, v in glyph_set.items():
        width = v.width

        try:
            t2_pen = T2CharStringPen(width=width, glyphSet={k: v})
            qu2cu_pen = Qu2CuPen(t2_pen, max_err=tolerance, all_cubic=True, reverse_direction=True)
            glyph_set[k].draw(qu2cu_pen)
            qu2cu_charstrings[k] = t2_pen.getCharString()

        except NotImplementedError:
            temp_t2_pen = T2CharStringPen(width=width, glyphSet=None)
            glyph_set[k].draw(temp_t2_pen)
            t2_charstring = temp_t2_pen.getCharString()

            tt_pen = TTGlyphPen(glyphSet=None)
            cu2qu_pen = Cu2QuPen(other_pen=tt_pen, max_err=tolerance, reverse_direction=False)
            t2_charstring.draw(cu2qu_pen)
            tt_glyph = tt_pen.glyph()

            t2_pen = T2CharStringPen(width=width, glyphSet=None)
            qu2cu_pen = Qu2CuPen(t2_pen, max_err=tolerance, all_cubic=True, reverse_direction=True)
            tt_glyph.draw(pen=qu2cu_pen, glyfTable=None)
            log.info(
                f"Failed to convert glyph {k} to cubic at first attempt, but succeeded at second "
                f"attempt."
            )

        charstring = t2_pen.getCharString()

Here's the script: ttf2otf.zip

Usage: ttf2otf.py [OPTIONS] INPUT_PATH

  Convert TrueType flavored fonts to CFF flavored fonts.

  INPUT_PATH argument can be a file or a directory. If it is a directory, all
  the TrueType flavored fonts found in the directory will be converted.

Options:
  -r, --recursive             If INPUT_PATH is a directory, search for fonts
                              recursively in subdirectories.
  -o, --output-dir DIRECTORY  Specify a directory where the output files are
                              to be saved. If the output directory doesn't
                              exist, it will be automatically created. If not
                              specified, files will be saved to the source
                              directory.
  -w, --overwrite             Overwrite the output file if it already exists.
  --recalc-timestamp          Recalculate the font's timestamp.
  --max-err FLOAT             The maximum error allowed when converting the
                              font to TrueType.
  --new-upem INTEGER          The target UPM to scale the font to.
  --correct-contours          If the source TrueType fonts have overlapping
                              contours or incorrect directions, set this flag
                              to correct them.
  --subroutinize              Whether to subroutinize the font.
  --help                      Show this message and exit.