[XeTeX] Kerning flaw with MinionPro & XeTex

[Adam Twardoch (List)]

On 11-02-07 09:53, Jonathan Kew wrote:
> Adam,
>
> Thanks for the detailed analysis and testcase.
>
> It looks like this is an instance of http://bugs.icu-project.org/trac/ticket/7753.

Yes, seems like it. I've posted my testcase files there as well.

Best,
Adam


> JK
>
> On 7 Feb 2011, at 01:31, Adam Twardoch (List) wrote:
>
>> I have identified this issue as a serious bug in XeTeX.
>>
>> In MinionPro-Regular.otf (version 2.068 is the one I have, but it's
>> likely that it applies to all versions), the relevant kerning definition
>> excerpt looks like the following:
>>
>> feature kern {
>>  lookup kern1 {
>>      pos uni0423 uni0431.ital -53;
>>    subtable;
>>      pos [uni040E uni0423 uni0474] [uni0435 uni043E uni0441
>>      uni0444 uni0451 uni0454 uni0473 uni0431.ital] -118;
>>   } kern1;
>> } kern;
>>
>> This means that there is an individual kern value between uni0423
>> uni0431.ital with the value of -53, then a subtable break, then a
>> class-based value for the same pair of -118.
>>
>> Adobe InDesign CS5 correctly identifies the pair in the first subtable
>> (-53) as the one that it should apply, and ignores the second value. So
>> the kern comes out as -53.
>>
>> It very much looks like XeTeX *incorrectly* parses *both* subtables, and
>> applies a sum of both the individual and the class value (-53-118 =
>> -171), which results in a very deep kern, and therefore an overlap.
>>
>> This seems to be a serious, systematic bug in the way XeTeX (or its
>> underlying ICU Layout library) processes the GPOS table.
>>
>> I've made an isolated test case that confirmes it.
>>
>> I've created a test OpenType font. It defines a kerning pair A B -300, a
>> subtable break, and again the same pair. InDesign correctly applies only
>> the first value (-300), which is shown in the
>> SubtableKernTest-InDesign.pdf file. XeTeX applies both values
>> consecutively, which is shown in SubtableKernTest-XeTeX.pdf
>>
>> All the test files (.ttf, .fea feature definitions, .tex and two PDFs)
>> are available at
>> http://www.twardoch.com/tmp/SubtableKernTest.zip
>>
>> I've also sent the test files offline to Jonathan Kew.
>>
>> Many thanks to Alessandro Ceschini for raising the issue.
>>
>> Regards,
>> Adam
>>
>>
>>
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