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https://issues.apache.org/jira/browse/PDFBOX-2681?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14324042#comment-14324042
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Jon Wu commented on PDFBOX-2681:
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[~tilman] good intuition. I think you're right that this is a transparency
problem. I rendered the simple examples you added and found consistent results
between Adobe (Photoshop) and PDFBox. I also did some tests where I added
opacity to the gradient with colors only and that worked fine as well.
It seems as if there's some interaction between a transparent CMYK color and
the RGB photo. In photo-rectangle-overlay-opacity-30.pdf, I took the sample
photo and drew a gray rectangle over it with 30% opacity. In
photo-rectangle-overlay-opacity-30-photoshop-comparison.jpg you can see a
comparison between Photoshop and PDFBox (w/ US Web Coated SWOP v2) for
rendering this image.
> Colors inaccurate rastered CMYK source w/ gradient + photo
> ----------------------------------------------------------
>
> Key: PDFBOX-2681
> URL: https://issues.apache.org/jira/browse/PDFBOX-2681
> Project: PDFBox
> Issue Type: Bug
> Components: Rendering
> Affects Versions: 2.0.0
> Environment: Tested on OS X 10.10, Java 1.6 / 1.8
> Reporter: Jon Wu
> Attachments: Shading2Function2CMYK.pdf, Shading2Function2CMYK.ps,
> acrobat-vs-pdfbox-swop.png, gradient-test-illustrator.jpg,
> gradient-test-pdfbox.jpg, gradient-test-photo-illustrator.jpg,
> gradient-test-photo-pdfbox-us-web-coated-swop-v2.jpg,
> gradient-test-photo-pdfbox.jpg, gradient-test-photo-rgb-illustrator.jpg,
> gradient-test-photo-rgb-pdfbox.jpg, gradient-test-photo-rgb.pdf,
> gradient-test-photo.pdf, gradient-test.pdf,
> photo-rectangle-overlay-opacity-30-photoshop-comparison.jpg,
> photo-rectangle-overlay-opacity-30.pdf, purple-gradient-comparison.jpg
>
>
> In the attached gradient-test-photo.pdf, there's a photo with a gradient on
> top of it. The colors in the gradient are CMYK. When rasterizing this test
> file with PDFBox 2.0.0 SNAPSHOT 20150209, the resulting image's gradient is
> different from the original and the colors of the photo under the gradient
> look significantly different vs the display in Adobe tools like Illustrator
> or other compliant PDF readers.
> Issues:
> - Color hue is wrong. Is this a CMYK to RGB issue?
> - Opacity of gradient is wrong and much different. The jpg from Illustrator
> shows much more of the photo without a gradient over it.
> Without a photo, there's still a difference, but it's much harder to see.
> Example in gradient-test.pdf where the same gradient is drawn over a grey
> rectangle.
> If I convert the PDF to RGB in Illustrator first and rasterize the RGB PDF
> with PDFBox, the result is nearly identical between Illustrator and PDFBox so
> this seems like a CMYK conversion issue in both the general color conversion
> and maybe how the gradient is calculated.
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