Empty rearrangements in AAT

Number:rdar://36384131 Date Originated:2018-01-09
Status:Open Resolved:
Product:CoreGraphics Product Version:
Classification: Reproducible:Always
Core Graphics

In CoreText’s implementation of the AAT ‘morx’ table, rearrangement verbs get ignored when the variable-length part of the marked sequence is empty. This behavior does not match the ‘morx’ specification, which says that ‘x’ represents an arbitrary sequence of glyphs. It’s not 100% clear from the spec whether ‘arbitrary sequence’ includes zero-length sequences, but it would certainly make sense to interpret the spec this way — other implementations (eg. fontkit) have taken this path. (It might be worth clarifying the spec).

Steps to Reproduce:
Run the Unicode text rendering test suite [https://github.com/unicode-org/text-rendering-tests] and check test case MORX-4: [https://rawgit.com/unicode-org/text-rendering-tests/master/reports/CoreText.html#MORX-4]. The test fonts are linked from the test descriptions.

Expected Results:
Tests should pass, as they do with fontkit: [https://rawgit.com/unicode-org/text-rendering-tests/master/reports/fontkit.html#MORX-4]

Actual Results:
Tests for CoreText are failing. See [https://rawgit.com/unicode-org/text-rendering-tests/master/reports/CoreText.html#MORX-4] for test report.

macOS 10.12.6 (16G1114)


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