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https://issues.apache.org/jira/browse/PDFBOX-2681?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14320735#comment-14320735
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John Hewson commented on PDFBOX-2681:
-------------------------------------
{quote}
I was wondering if I somehow converted all color values to RGB first, whether
I'd come out with a gradient that looked like Adobe's rendering since
gradient-test-photo-rgb-pdfbox.jpg which was rendered from the RGB PDF looks
fine.
{quote}
We know that non-gradient CMYK conversion works fine, so what you've found
could indicate a bug with the gradient (shading) calculation. Shadings are
rather complex and involve a multi-dimensional linear interpolation in the
source color space. The resulting color values are then converted into the
target color space. Note that the interpolation must be done in the source
color space as the results can be different when performed in another color
space and the target color space is unknown to the creator of the PDF.
> 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,
> 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
>
>
> 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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