split to CMYK / combine to CMYK

[kronhead]

I admit I don't really understand color management - but I thought this would work:

1) Take an image - for testing, I used one with just some pure yellow on white
2) Use Image -> Split Channel -> Split to CMYK
3) You get 4 new images masked for the 4 CMYK colors
4) Use Image -> Combine Channel -> Combine from CMYK. make sure the 4 new image names are in the 4 boxes
5) You get a new image - but it is not even CLOSE to being yellow.

I tried the same with cyan - same basic result.

What am I not understanding?

Thanks
Dan

 

[jconingham]

I'm not sure I follow... Are you trying to save an image in CMYK mode?

If that's it, save the image as .tif.

http://teom.coningham.net/psp-x/saving-in-cmyk-mode

 

[kronhead]

Actually, I get the same result when I do that - save it as tif, then open it in PSPX - the yellow has become a dull brown. Could it be something about my system, or some setting I have wrong somewhere in PSPX?

Partly, I am just trying to understand RGB vs CMYK. More specifically, I am designing images for a web service called cafepress.com, which prints your images on t-shirts, etc, when someone orders them. And in reading one of their forums recently about designing for their new black T-shirts (which have some color printing problems), some more expereienced people explained that the printing is done in CMYK, and they suggested converting the image to CMYK to make sure it is correct, then converting it back to RGB before creating the PNG or JPG file to upload.

Actually, I have never had a problem with colors on their white t-shirts - but I thoght I would just try this RGB-CMYK-RGB conversion (the first step, the one I am trying, is needed since PSPX cannot use CMYK directly) to make sure I understood it - and I obviously don't.

Does that make sense? Do you get the same result saving a yellow color as TIF, and then opening it in PSPX (or anything else)?

Thanks.
Dan

PS - my system was set to use a different CMYK profile than yours, but I changed it and got the same result. I also was using the airbrush to create my yellow color - which forced my image to 8-bit RGB for some reason - when I created a yellow rectangle at 16-bit RGB, and saved it as TIF, I get a cyan rectangle instead of a brown one.

 

[jconingham]

Depending on how bright the color is you'll get a much darker and/or washed out version when you convert it to CMYK.

For example: the color 00FF00 turns into 69BD45 when converted.

This happens because printing in CMYK involves adding color (in RGB I believe it is color subtraction), and no color combination results in such a vibrant green.

The trick is to work in CMYK mode from the beginning, so that you can set the amount of yellow to get a green that looks bright enough. This, however, can't be done in PSP.

However, if cafepress.com allows images in PNG and JPG formats then it would be safe to assume that you shouldn't worry too much about this, specially if you are saying that you've never had any problems.
Was it someone from cafepress that said the printing in black t-shirts was done through CMYK? Because black + color = black, so I assume that the printing method is different in this case.

 

[kronhead]

Thanks. I don't remember if it was a Cafepress employee, or a Cafepress user, who said this. My understanding is that they do print a white backing on the black cloth for at least some of the colors - presumaby the brighter ones - but many of the users are dissatisfied with the colors overall - particularly because it is definitely NOT WYSIWYG. They do say this printing-on-black service is still in beta.

But, as you said, I won't worry about it for now - and will just use the results of tests that show empirically what colors do and don't work reasonably well, when designing for black.

Thanks again.
Dan