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https://issues.apache.org/jira/browse/FOP-1969?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15398100#comment-15398100
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Glenn Adams commented on FOP-1969:
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I'm not sure why you mean that "this prevents from correctly show some emoji".
The semantics of the elided controls, e.g., ZWJ, ZWNJ, need to be processed by
GSUB/GPOS during the process of creating the output glyph sequence. No further
processing based on ZWJ/ZWNJ should occur after that process. The elision of
controls operates on the output glyph sequence.
The value of retainControls is presently constant, but it is my plan to
introduce a new fox:... property that allows an author to determine whether
control characters are themselves displayed (as control characters). So it's a
placeholder which value is to be determined by the to be implemented fox:...
property. Note that whether controls can be displayed by a font is a per-font
dependency.
> Surrogate pairs not treated as single unicode codepoint for display purposes
> ----------------------------------------------------------------------------
>
> Key: FOP-1969
> URL: https://issues.apache.org/jira/browse/FOP-1969
> Project: FOP
> Issue Type: Improvement
> Components: unqualified
> Affects Versions: trunk
> Environment: Operating System: All
> Platform: All
> Reporter: Glenn Adams
> Attachments: testing.fo, testing.fo, testing.pdf, testing.pdf,
> testing.xml, testing.xsl
>
>
> unicode codepoints outside of the BMP (base multilingual plane), i.e., whose
> scalar value is greater than 0xFFFF (65535), are coded as UTF-16 surrogate
> pairs in Java strings, which pair should be treated as a single codepoint for
> the purpose of mapping to a glyph in a font (that supports extra-BMP
> mappings);
> at present, FOP does not correctly handle this case in simple (non complex
> script) rendering paths;
> furthermore, though some support has been added to handle this in the complex
> script rendering path, it has not yet been tested, so is not necessarily
> working there either;
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