Possibility of implementing YCCK color transform as available in some jpeg files?

[richardpl]

For more info read:
http://halicery.com/Image%20Decoders/JPEG/JPEG%20notes/CMYK.html

[sekrit-twc]

After studying this issue, it appears that CMYK is functionally equivalent to RGBA, as K functions as an inverse alpha mask. The missing capability is CMY<-->RGB conversion, as YCC is merely YUV transform applied to CMY. Support for RGBA has been added since z.lib 3.0, so this feature is tractable.

[sekrit-twc]

Still debating whether to implement this internally. It is possible to transform YCCK/CMYK to YUV/RGB with an I/O callback:

YUV:
Y = 1 - ((1 - K) * Y + K)
U = -((1 - K) * C0)
V = -((1 - K) * C1)

RGB:
R = 1 - ((1 - K) * C + K)
G = 1 - ((1 - K) * M + K)
B = 1 - ((1 - K) * Y + K)

or more abstractly

Negative = (1 - K) * Negative_K + K * Black
Positive = Black - Negative

where Black = [1 0 0] for YCC and [1 1 1] for CMY

[richardpl]

what are C0 and C1 values?

[sekrit-twc]

First and second negative chroma channels. Per "YCC".

[sekrit-twc]

@richardpl Can you provide a raw dump of YCCK or CMYK image data?

[richardpl]

Relevant file can be downloaded from here:

http://trac.ffmpeg.org/ticket/9085

[sekrit-twc]

The FFmpeg tracker only has a JPEG file. If I open it in an image viewer, I only get the decoded RGB. Please upload the raw YCCK pixel data.

[Artoria2e5]

The JPEG is fine; the viewer is the problem. Unfortunately there are very few formats that hold CMYK, so converting is tough.

djpeg produces the attached pnm, but I cannot verify whether it is correct. It is probably CMYK and not YCCK still. I mean, you expect djpeg to give you RGB and not YCC, and it's the same here. If you want the guts, you need to get the library out.
Channel.pnm.zip

The YCCK thing is also just a big hack given the CMYK/RGB transform, but we all know that. The point is that it gives coding gain.