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https://issues.apache.org/jira/browse/PDFBOX-4184?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16551951#comment-16551951
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Emmeran Seehuber commented on PDFBOX-4184:
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I would suggest changing the condition from
{code:java}
if (pdImageXObject.getBitsPerComponent() < 16 &&
image.getWidth() * image.getHeight() <= 50 *
50)
{code}
to
{code:java}
if (pdImageXObject.getColorSpace == PDDeviceRGB.INSTANCE &&
image.getWidth() * image.getHeight() <= 50 *
50)
{code}
as otherwise the LosslessFactory may "random" destroy/reduce the color
information of small images. If e.g. the user has a requirement to always
encode images as CMYK, this would break it. On the other side reducing a 16 Bit
sRGB image to 8 bit is not really losing color information, as sRGB is a rather
small color space. As most image decoders (e.g. TwelveMonkeys) by default
convert every image they decode to sRGB, you can be sure that the user really
wants the non default color space used when he gives a non sRGB image to the
LosslessFactory.
> [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: 2.0.12, 3.0.0 PDFBox
>
> Attachments: 16bit.png, LoadGovdocs.java, images.zip,
> lossless_predictor_based_imageencoding.patch,
> lossless_predictor_based_imageencoding_v2.patch,
> lossless_predictor_based_imageencoding_v3.patch,
> lossless_predictor_based_imageencoding_v4.patch,
> lossless_predictor_based_imageencoding_v5.patch,
> lossless_predictor_based_imageencoding_v6.patch,
> 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,
> size_compare.txt
>
>
> 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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