I am having to deal with CMYK JPEGs extracted from a PDF source. The PDFs were created with Photoshop.
The problem is that Photoshop stores JPEG CMYK data in PDF/EPS using "normal" values, whereas in standalone JPEGs it stores inverted values. So, when the DCTDecode streams are extracted bytewise and written to disk, the resulting JPEG files appear inverted.
(The actual extraction is done by an in-house utility, which simply extracts the bytes from the DCTDecode stream and writes them, unmodified, to a file ending in
.jpg It's basically a binary copy-and-paste. The PDFs are available to re-process, should that be required.)
As the images must remain in their JFIF format, is there any way to place a marker into the extracted
.jpg file to make Photoshop open it with the proper encoding? The process must be lossless (not involve further entropy encoding).
The JPEGs already contain the
APP14 marker, and removing it has no effect.
Below is a quote from the
"... it appears that Adobe Photoshop writes inverted data in CMYK JPEG files: 0 represents 100% ink coverage, rather than 0% ink as you'd expect. ... Photoshop 3.0 [and newer]... write uninverted YCCK in EPS/JPEG files... (But the data polarity used in bare JPEG files will not change...)"