Sparked by a conversation on channels, I put together an action for creating an RGB file composed of CMYK components (that is, CMYK within RGB, limited only by your RGB color space).  The mechanism is fairly straightforward when you look at the action & result.  The key pieces to understand are that as implemented the action expects that you have not selected a nested layer (picking the top layer would be best), and that you will not darken the default black layer.  You can increase the contrast within the color (CMY) channels by lightening the default ('maximum') K channel as provided for in the action itself, or you can adapt it to use any synthetic K channel of your choosing (L* makes a nice starting point, fwiw), the only requirement being that it never be darker than the source image (makes sense, right?).

More maybe tomorrow, but for now I'm off to bed.

Action.

And for now, please don't redistribute - let this be an advantage to visiting our forums.  But send this thread's link around .


Note: This is not a prepress tool.  It is only a method of getting access to other color channels, losslessly, within a working RGB file & colorspace.  Please don't misunderstand and think that running this action on a ProPhotoRGB file is going to make all your colors printable on newsprint .

I'll check it out

I've updated the action.  As it stands, there are now two versions included with the download.  The first (original) is termed "RGB to CMYK Layers" and generates 4 Solid Color layers of CMYK data (along with a 5th 'paper' layer).  The second is termed "RGB to CMYK Channels" and simply creates CMY & K channels in the channels palette, allowing their use for color correction.  As it stands, please realize that the second action is blind and will perform in an improper separation if you already have channels named 'C', 'M', 'Y', and 'K' - 'K' is the real culprit, but all can conceivably get jumbled.

Hope this is of use to y'all.

Edit: Please note that in both versions, stopping the action to work on the K channel (again, so long as you only lighten it) can be done by any method - Curves, Levels, Shadow / Highlight, Painting, anything.  I simply put a Curve in there because it's familiar and will meet most quick needs.

Edit 2: Further testing shows that there is the potential for a very small amount of data loss.  That is, in a maximally-stressed 16bit file, there can be as much as 64/32k per-channel variation between the original and the result.  For those who don't enjoy math involving powers of 2, that's < 1/256 - but for the sake of completeness needs to be acknowledged.

This is pretty interesting.  And although I have studied a bit of color theory, this is outside my areas of understanding.  Can you explain how this works and when it can be used a little more? 

So, if i run this action on a ProPhoto file, it will immediately make all colors printable even on toilet paper?

Kidding. Thanks a lot for sharing knowledge. I will check it out tomorrow.

Since your excited about channels write us an action that takes RGB and converts it to CYMK, LAB, and greyscale and keeps all the channels from the other modes, including the original RGB channels. 

This is the easy way to find the best channel to add/burn with caculations for makeing masks

I'll go ahead and do a full writeup on how / why this works in a week or two when I've (finally) finished my frequency series for the blog.  It's an interesting discussion for a lot of reasons IMO, and if there's interest, well, why not?

In the mean time, to address a few things:
  -- CMYK is a color mode, not a color space.  So it's entirely possible to have a CMYK file which does encompass the entire ProPhotoRGB gamut if you wanted to - it just wouldn't be very practical, as there are no CMYK output devices capable of reproducing what you can create.  At least not yet.  And probably never on modern toilet paper.  Sorry FLEX .
  -- I discovered early this morning that, as implemented, it doesn't work if you have the "Blend Colors Using Gamma: x.xx" option turned on.  It will only work if the color space's native gamma is in place.  [I'm going to try to work around this eventually]
  -- There are lots of actions out there already for creating 10, 13, 16, and 20+ channels for use in an open file - I see no reason to duplicate that effort.

A bit late, but better than never, I finally got around to writing this up.  Link here.

A good read as usual. For me, the only time I need a cmyk channel in rgb is to poison color in the shadows or as a quick burn for the shadows. Both of which use a custom black channel. I'm a little curious what uses you have.

Added an action set to generate these + all the other favorite channels here.

Nice work