Following is a small uhh..., tutorial if you will, which, just for fun, shows a conventional RGB to CMYK conversion in the digital realm. It is based on the idea that inverting RGB produces CMY.
1. Convert To Profile: ColorMatch RGB
First we have to bring the image into a more reasonable colorspace. In the early days, the RGB to CMY relation was fixed in the machines available. Later display RGB was assumed. It was certainly not something as big as AdobeRGB or ProphotoRGB.
So there must be some sort of relation between the RGB primaries and the CMYK colorspace. ColorMatch fits the bill, which is a slightly adjusted sRGB to better match the SWOP space.
2. Add a graywedge at the bottom of your image
Next add a graywedge to the image. This will enable us to rebalance the graybalance after conversion, because whereas equal amounts of RGB produce pure gray, equal amounts of CMY does not.
Now Using the Channels Palette:
3. Add an empty channel
4. Select Split Channels
5. Select Merge Channels
Because of the added channel we can now select mode CMYK and select OK.
And because of Photoshop's internal represention of channel values in CMYK, which are the inverse of channel values in RGB, we automatically end up with an inverted RGB image (and an additional empty black channel.)
The image will likely be designated as Untagged CMYK, which means Photoshop uses the CMYK profile from the colorpreferences. If this isn't set to SWOP then make sure to select Assign Profile and assign the SWOP v2 profile.
At this point you should see a very pale and muddy version of your original image.
The image looks muddy because equal amounts of CMY does not produce pure gray. The image is not colorbalanced, more or less equivalent to having an improper whitebalance set in RAW conversion. The image looks pale because C=M=Y=100% is neither gray, nor dark.
To colorbalance the image we can use the graywedge from step 1 and the curves dialog. Open the curves dialog (as opposed to the levels dialog), and select the middle gray pipet. Next select the middle of the graywedge, and sample to let photoshop automatically correct middle gray to pure gray.
7. Increase Color
Next we have to increase the saturation slightly. CMYK does not act linearly as RGB does which is why you don't end up with pure gray above, nor pure black for maximum CMY, and also results in saturation differences. This would normally be part of the internals of the conversion engine. Select Hue/Saturation and increase saturation to taste, in this case +40 is used.
8. Black Generation
The image will still look pale because the darkest color C=M=Y=100% simply doesn't come close to reasonable dark. So now we will introduce a quick & dirty black plate:
9. Select Image->Adjust->Selective Color
10. Set the following values:
C = -35%
M = -50%
Y = -50%
K = +100%
Method = Absolute
This tells the Selective Color command to adjust C=M=Y=100% to a more reasonable under color gray of:
C = 65%,
M = 50%,
Y = 50%
And add 100% black as a substitute.
Selective Color automatically decreases its effect the further away from pure black we get. So 50% gray is hardly affected, as are mildly dark colors. What Photoshop CMYK separation settings are equivalent to this black generation is left as an exercise to the reader.
So, there you have it, a mighty fine CMYK separation. Does it bare any relation to the original? Well, if it does, fine, if it doesn't, tough luck. Make 10 proofs, adjust the freaking heck out of it, and you will have an idea what conventional prepress was all about... (read Andrew's article
for a more rational explanation).