Converting RGB to CMYK

[Cion]

Hello Everyone,
This is my first post and I have to say this is the best place to come for advice or help on anyhting PS. Truly a perfect site.
Well my problem is I have little or no printing experience and have looked around the web for a tut or explanation on how to get near the same results in CMYK that I would in RGB. The image I'm trying to convert is being used for a card,you'll find it below (a taste of my handy work). The red keeps turning orange-ish and the ornament completely loses it's saturation and depth. Please help %(.
Thanks %}

[jonbalza]

Just a quick question. Are you printing this on an inkjet printer? If you are, then you don't even need to worry about converting it to CMYK. The printer will (almost all the time) do it for you. As a matter of fact, it may make it look worse converting it to CMYK before you print.

Now... As to the desaturation, - keep in mind that color isn't my strong suit - Mosk CMYK profiles are not as saturated as RGB ones, so that's why your colors aren't as bold.

To test out if your RGB colors aren't going to be reproduced accurately when moved to CMYK, turn on Gamut Preview by going to "View->Gamut Warning" (Shift-Ctrl-Y, if you are interested.)

Hope that helps, and welcome to the forums!

[Scott W.]

Hi Cion and welcome.

Jon's pretty accurate in his post.

if it's an inkjet, dont' bother. Inkjet printers expect to see rgb, if they don't they convert to RGB then to CMYK again. so you will get worse results if you send CMYK to an inkjet printer.


As for the desaturation....

The RGB spectrum is much broader than the CMYK spectrum. Some RGB colors simply are not possible in CKYM. Jon's suggestion of viewing the out of gamut colors is good. Another thing you can do is View > Proof Setup > pick the profile best suited to where you are printing. (you'll need to set up your color preferences first though (control-shift-k/PC or command-shift-K/Mac) This will [b]display[/b[ your image withthat profile. It won't alter your image in any way. It mearly shows you how it will appear. From there you can make adjustments while viewing the proof colors in order to get closer to what you want. Be aware that you simply won't be able to achieve some colors that may be in your RGB document.

[Cion]

That seemed to do the trick, now I can actually tell what might come out of the printer. By the way would it make a difference if it was a laser printer? an HP LaserJet 5500DN.
What did you guys think of the image?

[bopuc]

Think this is my fist post so I'll try to make is useful :)


1. turn on View>Gamut Warning
2. make new layer (on top), paint it black and set opacity to 0%
3. set blend mode to saturate
4. increase opacity of new layer (black one) until you loose all grey areas from gamut warning.

Note that saturation of original image will decrease proportionaly comparing to opacity of 'black' leyer.

[jonbalza]

Excellent Idea, Bopuc. It has lots of potential. That is a great way of getting rid of all non-printable colors... The problem is, as you said, that the entire saturation of the image will be decreased. To fix that, you could always use a layer mask on the saturation layer, which will be able to paint back in the saturation of the areas that aren't causing problems.

BUT, why would you even need to do this, since printing just gets rid of the problem colors, not the entire thing... I wonder if there is a way to just select colors that are out of gamut. If there was a way to do that, you could create a mask that would only affect those colors, and your method would be way more effective.

[Scott W.]

Actually there is Jon.

Select > Color Range > then choose "out of Gamut" from the drop down menu. You could then create a Hue/Saturation adjustment layer ans simply lower the saturation of the out of gamut colors.


Welcome bopuc. Good idea you've got there.

[thereal7]

Very cool approach bo..... I am gonna experiment with this at work.

[venivedi]

Thanks bopuc. That's an great idea with rarely used mode(saturation).

[Phil_The_Rodent]

Yeah for print from RGB, I'll select colour range and shift the result or desaturate. It's not so bad to be *just* over the gamut edge, but you don't want to plug up.

Just a quick lesson in colour theory, RGB and CMY are polar opposites. They actually map to each other. R is the opposite if C, G is the opposite of M, etc.

Now think about these all placed around a colour wheel.

Clockwise.

RYGCBM

The most saturated colours are on the very rim. Now to build Red in CMY, you need to combine Magenta and Yellow (the colours which surround it, as Red isn't a process colour). The unfortunate fact, is that pure margenta absorbs all magenta, but as soon as you add an equal amount of yellow, the ink absorbs all light between Magenta and Yellow. It appears red, but it's significantly dropped in saturation. In fact, it drops to about 70% saturation or so.

If you picture an intersection on your imaginary colour wheel from the pure magenta to the pure yellow straight across the surface, and draw a line out from the center axis towards red, you find that the line only gets to about 70% of the way to the circle edge before you hit the intersection. That's all you can expect from process printing without getting into Pantones.

[Uni]

The tip you gave below was 'spot' on. You just helped my brain wrap around the two modes in a clear concise way. I am grateful. Keep up the good stuff.

You wrote...

Just a quick lesson in colour theory, RGB and CMY are polar opposites. They actually map to each other. R is the opposite if C, G is the opposite of M, etc.

Now think about these all placed around a colour wheel.

Clockwise.

RYGCBM

The most saturated colours are on the very rim. Now to build Red in CMY, you need to combine Magenta and Yellow (the colours which surround it, as Red isn't a process colour). The unfortunate fact, is that pure margenta absorbs all magenta, but as soon as you add an equal amount of yellow, the ink absorbs all light between Magenta and Yellow. It appears red, but it's significantly dropped in saturation. In fact, it drops to about 70% saturation or so.

If you picture an intersection on your imaginary colour wheel from the pure magenta to the pure yellow straight across the surface, and draw a line out from the center axis towards red, you find that the line only gets to about 70% of the way to the circle edge before you hit the intersection. That's all you can expect from process printing without getting into Pantones.[/QUOTE]

[Shaggy]

since printing just gets rid of the problem colors, not the entire thing... I wonder if there is a way to just select colors that are out of gamut. If there was a way to do that, you could create a mask that would only affect those colors, and your method would be way more effective.

Enter - Rendering intents.

Rendering intents do exactly what you guys are talking about, except without any of this masking talk :D Check your PS color settings (make sure the Advanced box is ticked). 3/4 of the way down you'll see the conversion options. There are 4 rendering intents which tell the CMS, in this case Photoshop, how to handle out of gamut colors when converting to CMYK.

The four intents are:
• relative colorimetric
• absolute colorimetric
• perceptual
• saturation

So what is the difference? Remember that the rendering intent's main job is to handle situations where colors are out of gamut. Imagine a circle. Inside the circle are all the colors in your image that are printable. Outside the circle are your 'out of gamut' colors.

Relative colorimetric - takes your out of gamut colors and moves them the the edge of the circle. The colors that were already in gamut will stay where they are (technically they may move a little, but the shift wouldn't be detectable to your eye).

Absolute colorimetric - related to relative except it moves the white point to the white point of the cmyk space, which is the whiteness of the paper. Because of this paper simulation, absolute colorimetric is used for proofing images (simulating how an image will look on a different printer and paper).

Perceptual - Looks at the out of gamut colors and moves ALL of your colors (both in gamut and out) so that the relationships between the colors stay the same. This is a common rendering intent for photographs.

Saturation - takes all of those out of gamut colors and pushes them the the portion of the gamut that will give the brightest, most saturated colors. They may not be accurate, but they will be the most saturated that the destination printer can print.

whew...

So although a mask of the out of gamut colors would get a printable result, rendering intents exist to do this work for you with mathematical precision.

So how do you use them? By default, Photoshop uses the rendering intent selected in your color settings when you use Image > Mode > CMYK. A more flexible solution is to select Image > Mode > Convert to Profile. Select the 'flavor' of CMYK that you'd like to convert to. Tick the preview checkbox and then rotate through the rendering intents to see what gives you the most pleasing result. Most likely, relative colorimetric or perceptual will give you the best looking image.

Hope that clears some things up!