PDFsharp & MigraDoc Foundation

I've been playing around with the ExportImages sample project. When I extracted images from a PDF file that used color space /DeviceCMYK for its JPEGs (and yes, I checked, and these images are using /DTCDecode), the colors were all screwed up. What do I need to do to get the colors right? Clearly simply writing the dictionary stream directly to disk wasn't the answer. (I even opened these images in PhotoShop and the colors were still screwy.)

All the information that Adobe Reader gets about the image is in the JPEG file and in the filter parameters of the PDF file.
Does the JPEG file contain a comment that names the creating application?

Does the image look inverted (negated)? This can be done by using filter parameters.
What do you mean with "screwy"?

_________________
Regards
Thomas Hoevel
PDFsharp Team

Here's the entire dictionary describing one of the images:

<</Subtype/Image
/Length 536739
/Filter/DCTDecode
/BitsPerComponent 8
/ColorSpace/DeviceCMYK
/Width 782
/Height 650
/Type/XObject>>

Yes, the colors look inverted, but if I open the image in an editor and invert the colors, the result looks extremely overexposed, like there's not enough black.

We only have few CMYK JPEGs we used to test PDFsharp.
Those files look correct in Photoshop, but appeared inverted in PDF.
Therefore we added

to make them look correct in PDF.

It seems your PDFs were created without this Decode trick - they changed the JPEG files to make them look correct.
I don't know how to help you.

You can try inverting CMY while keeping K.

_________________
Regards
Thomas Hoevel
PDFsharp Team

It appears that this is a very complex PDF. Its purpose is to test color digital presses, so I guess the color encoding is very fine-tuned. As I look at other objects inside the PDF I see some functions which have /Decode entries similar to what you wrote. Of course, it's kind of hard to work backwards from objects to see exactly how they're being used, so I can't say for sure that the images are using these functions.

If it means anything to you, these are the functions:

46 0 obj
<</Length 147/FunctionType 0/Filter/FlateDecode/Encode[0 63]/BitsPerSample 8/Domain[0 1]/Size[64]/Range[0 1 0 1 0 1 0 1]/Decode[0 1 0 1 0 1 0 1]>>

47 0 obj
<</FunctionType 3/Encode[0 1]/Domain[0 1]/Functions[46 0 R]/Bounds[]>>

48 0 obj
<</ColorSpace/DeviceCMYK/Coords[0 0 0 0 0 1]/Function 47 0 R/Extend[true true]/ShadingType 3>>

Here's a hex dump of the stream for object 46 0:


The other two objects don't have streams.

I never studied functions very closely in Abode's PDF reference. Maybe I should....

(EDIT: These aren't the only functions in the PDF; I just posted a sample of three that seem to relate to each other.)