Tilman Hausherr commented on PDFBOX-4184:

I committed it anyway, still without the equality test which we didn't have 
before either.

A possible explanation is that converting from RGB to CMYK and back is not 
always perfect. Maybe use the code that you had commented?

> [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, fix_profile_use.patch, 
> fix_profile_use3.patch, fix_profile_use4.patch, 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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