chws_tool 1.4.5
Utility for OpenType chws/vchw features
Requires Python: >=3.10

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# chws_tool

This tool adds the OpenType
[`chws`], [`vchw`], [`halt`], and [`vhal`]
features to OpenType/TrueType fonts when any of these features are missing.
Please see [east-asian-spacing] for details of these features.

This tool uses the [east-asian-spacing] package as its core engine,
and has following advantages:
* Simpler API and command line options.
* Supports CJK fonts at [fonts.google.com] in its built-in [config].

To add new fonts to the supported font list,
please see the [Adding Fonts] section below.

[east-asian-spacing]: https://github.com/kojiishi/east_asian_spacing
[`chws`]: https://docs.microsoft.com/en-us/typography/opentype/spec/features_ae#tag-chws
[`halt`]: https://docs.microsoft.com/en-us/typography/opentype/spec/features_fj#tag-halt
[`vchw`]: https://docs.microsoft.com/en-us/typography/opentype/spec/features_uz#tag-vchw
[`vhal`]: https://docs.microsoft.com/en-us/typography/opentype/spec/features_uz#tag-vhal
[fonts.google.com]: https://fonts.google.com/

## Install

You can install this tool by [pipx] or [uv].
```shell-session
pipx install chws-tool
```
```shell-session
uv tool install chws-tool
```
Using [pip] is also supported, but please be aware that,
if you install with [pip] in the global environment,
its dependencies may cause conflicts with other packages.
If all what you need is the command line tool,
[pipx] or [uv] can install it globally
while still isolating it in a virtual environment.
```shell-session
pip install chws-tool
```

[pip]: https://pip.pypa.io/en/latest/
[pipx]: https://pipxproject.github.io/pipx/
[uv]: https://docs.astral.sh/uv/

## Command Line Usage

The following example adds the features to `input.otf`
and saves it to the `build` directory.
If the argument is a directory,
the tool expands it to all fonts in the directory recursively.
```shell-session
add-chws input.otf
```
Use the `-o` option to change the output directory,
or the `--help` option for the full list of options.
```shell-session
add-chws input_dir -o output_dir
```

## API

The following example creates a font with the features
in the "`build`" directory
if the features are applicable:
```python
import chws_tool

def main():
    output_path = chws_tool.add_chws("fonts/input.otf", "build")
    if output_path:
        print(f"Success! saved to {output_path}")
    else:
        print("Skipped")
```
If you prefer to overwrite existing fonts, you can omit the output directory.
```python
import chws_tool

def main():
    chws_tool.add_chws("fonts/input.otf")
```
If your program uses [asyncio]:
```python
import asyncio
import chws_tool

async def main_async():
    output_path = await chws_tool.add_chws_async("fonts/input.otf", "build")
    if output_path:
        print(f"Success! saved to {output_path}")
    else:
        print("Skipped")

asyncio.run(main_async())
```

[asyncio]: https://docs.python.org/3/library/asyncio.html

## Advanced Topics

### Clone and Install

If you want to clone the repository and install
in the [editable mode] with the development packages,
using [uv]:
```shell-session
git clone https://github.com/googlefonts/chws_tool.git
cd chws_tool
uv sync
. .venv/bin/activate
uv tool install -e .
```
If you prefer using [pip]:
```shell-session
git clone https://github.com/googlefonts/chws_tool.git
cd chws_tool
pip install -e '.[dev]'
```

[editable mode]: https://pip.pypa.io/en/stable/cli/pip_install/#install-editable

### Adding Fonts
[adding fonts]: #adding-fonts

This package has a built-in list of supported fonts in its [config].

Fonts not in the known list are still processed
with the default configuration,
but this package shows a warning message.

When adding new fonts to the known font list,
the following process is recommended:

1. Find the font names.
   Running the `add-chws` with `--print-name` option can print them.
2. Add them to the [config].
3. (Optional) Build the font and run the [Visual Test].
   This step is optional
   because this package automatically avoids glyph collisions
   by computing glyph outlines.
4. (Optional) Tweak the [config] if needed.

[config]: src/chws_tool/config.py

### Visual Test
[Visual Test]: #visual-test

The primary purpose of this process is to find
too tight spacings or glyph collisions caused by the kernings.

This tool has heuristic rules to determine
the applicability of the spacings using the glyph metrics,
but assumes that full-width punctuation glyphs have enough internal spacings
according to linguistic conventions
as in [UAX#50](http://unicode.org/reports/tr50/#vertical_alternates)
or in [CLREQ](https://w3c.github.io/clreq/#h-punctuation_adjustment_space).
Unfortunately, not all fonts follow the conventions.

To run the visual test:

1. Add the test font to the font list
   in the top `<script>` block of [`tests/test.html`](tests/test.html).
2. Open it in your browser.
3. Check "Fullwidth", "Upright", and "Hide same size" check boxes.

Here is the check list and tips:

* Find where glyphs are too tight or collide
  when the feature is on but look fine without the feature.
  - Quotation marks can collide most often,
    especially in vertical flow but sometimes in horizontal flow too.
  - "Open+Open" and "Close+Close" are more likely to collide than other pairs.
  - Select glyphs to check how much inks overflow the glyph metrics.
* Test both horizontal and vertical flows
  by flipping the "Vertical" checkbox.

Other controls in the test are not mandatory,
but they may be useful in following cases.

* Uncheck "Hide same size" to see cases where kernings are not applied
  by the built-in rules.
  They are usually fine.
  In most cases, they are because the glyphs are missing,
  they are not full-width,
  or the pair should not apply for the script of the font.
  The test file contaisn code poitns for all East Asian scripts,
  but not all code points should apply spacings for all script.
* The "Language" list does not matter in most cases,
  except when the font supports multiple locales,
  such as Noto CJK.
* The "Fullwidth" feature can change glyphs,
  but in most cases,
  if glyphs look ok with the feature on,
  they should be ok with the feature off.
* The "Upright" feature is only valid in the vertical flow.
  You can just check it on,
  for the same reason as the "Fullwidth" feature.
* The "Characters" input fields can change the test cases.
  You don't have to change them unless you have specific needs.

### Releasing

See https://googlefonts.github.io/python#make-a-release.