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https://issues.apache.org/jira/browse/PDFBOX-4184?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16429707#comment-16429707
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Tilman Hausherr commented on PDFBOX-4184:
-----------------------------------------
I found the cause of the bug from the github issue, it is in
{{createAlphaFromARGBImage}}, the line {{bos.write(pixel)}}. For 16 bit images
it should be changed to {{bos.write(pixel / 256)}}. So the existing code should
be changed to
{code}
else
{
bpc = 8;
int dataType = alphaRaster.getDataBuffer().getDataType();
if (dataType == DataBuffer.TYPE_USHORT)
{
for (int pixel : pixels)
{
bos.write(pixel / 256);
}
}
else
{
for (int pixel : pixels)
{
bos.write(pixel);
}
}
}
{code}
Sadly this doesn't explain why I can't produce a test that fails... I did make
tries with alpha values and nothing weird happened.
> [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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