f401f85a5a
Core Text doesn't actually have a concept of DPI internally, as it doesn't rasterize anything by itself, it just generates vector paths that get passed along to Core Graphics. In practice this means Core Text operates in the classical macOS logical DPI of 72, with one typographic point corresponding to one point in the Core Graphics coordinate system, which for a normal bitmap context then corresponds to one pixel -- or two pixels for a "retina" context with a 2x scale transform. Scaling the font point sizes given to HarfBuzz to an assumed DPI of 96 is problematic with this in mind, as fonts with optical features such as 'trak' tables for tracking, or color glyphs, will then base the metrics off of the wrong point size compared to what the client asked for. This in turn causes mismatches between the metrics of the shaped text and the actual rasterization, which doesn't include the 72 to 96 DPI scaling. If a 96 DPI is needed, such as on the Web, the scaling should be done outside of HarfBuzz, allowing the client to keep the DPI of the shaping in sync with the rasterization. The recommended way to do that is by scaling the font point size, not by applying a transform to the target Core Graphics context, to let Core Text choose the right optical features of the target point size, as described in WWDC 2015 session 804: https://developer.apple.com/videos/play/wwdc2015/804/ |
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|---|---|---|
| .. | ||
| data | ||
| texts/in-house | ||
| CMakeLists.txt | ||
| hb_test_tools.py | ||
| hb-diff | ||
| hb-diff-colorize | ||
| hb-diff-filter-failures | ||
| hb-diff-stat | ||
| hb-unicode-decode | ||
| hb-unicode-encode | ||
| hb-unicode-prettyname | ||
| Makefile.am | ||
| README.md | ||
| record-test.sh | ||
| run-tests.py | ||
Adding tests
You can test shaping of a unicode sequence against a font like this:
$ ./hb-unicode-encode 41 42 43 627 | ../../util/hb-shape font.ttf
assuming an in-tree build. The 41 42 43 627 here is a sequence of
Unicode codepoints: U+0041,0042,0043,0627. When you are happy with
the shape results, you can use the record-test.sh script to add
this to the test suite. record-test.sh requires pyftsubset to
be installed. You can get pyftsubset by installing
FontTools from https://github.com/behdad/fonttools.
To use record-test.sh, just put it right before the hb-shape invocation:
$ ./hb-unicode-encode 41 42 43 627 | ./record-test.sh ../../util/hb-shape font.ttf
what this does is:
- Subset the font for the sequence of Unicode characters requested,
- Compare the
hb-shapeoutput of the original font versus the subset font for the input sequence, - If the outputs differ, perhaps it is because the font does not have
glyph names; it then compares the output of
hb-viewfor both fonts. - If the outputs differ, recording fails. Otherwise, it will move the
subset font file into
data/in-house/fontsand name it after its hash, and print out the test case input, which you can then redirect to an existing or new test file indata/in-house/testsusing-o, e.g.:
$ ./hb-unicode-encode 41 42 43 627 | ./record-test.sh -o data/in-house/tests/test-name.test ../../util/hb-shape font.ttf
If you created a new test file, add it to data/in-house/Makefile.sources
so it is run. Check that make check does indeed run it, and that the
test passes. When everything looks good, git add the new font as well
as the new test file if you created any. You can see what new files are
there by running git status data/in-house. And commit!
Note! Please only add tests using Open Source fonts, preferably under OFL or similar license.