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https://issues.apache.org/jira/browse/PDFBOX-4184?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16430535#comment-16430535
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ASF subversion and git services commented on PDFBOX-4184:
---------------------------------------------------------

Commit 1828715 from [~tilman] in branch 'pdfbox/trunk'
[ https://svn.apache.org/r1828715 ]

PDFBOX-4184: divide 16 bit alpha values by 256

> [PATCH]: Support simple lossless compression of 16 bit RGB images
> -----------------------------------------------------------------
>
>                 Key: PDFBOX-4184
>                 URL: https://issues.apache.org/jira/browse/PDFBOX-4184
>             Project: PDFBox
>          Issue Type: Improvement
>          Components: Writing
>    Affects Versions: 2.0.9
>            Reporter: Emmeran Seehuber
>            Priority: Minor
>             Fix For: 2.0.10, 3.0.0 PDFBox
>
>         Attachments: pdfbox_support_16bit_image_write.patch, 
> png16-arrow-bad-no-smask.pdf, png16-arrow-bad.pdf, 
> png16-arrow-good-no-mask.pdf, png16-arrow-good.pdf
>
>
> The attached patch add support to write 16 bit per component images 
> correctly. I've integrated a test for this here: 
> [https://github.com/rototor/pdfbox-graphics2d/commit/8bf089cb74945bd4f0f15054754f51dd5b361fe9]
> It only supports 16-Bit TYPE_CUSTOM with DataType == USHORT images - but this 
> is what you usually get when you read a 16 bit PNG file.
> This would also fix [https://github.com/danfickle/openhtmltopdf/issues/173].
> The patch is against 2.0.9, but should apply to 3.0.0 too.
> There is still some room for improvements when writing lossless images, as 
> the images are currently not efficiently encoded. I.e. you could use PNG 
> encodings to get a better compression. (By adding a COSName.DECODE_PARMS with 
> a COSName.PREDICTOR == 15 and encoding the images as PNG). But this is 
> something for a later patch. It would also need another API, as there is a 
> tradeoff speed vs compression ratio. 



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