How best to convert from LAB to CMYK?

Vysionous

Hi folks. I am facing a problem when converting from Lab back to cmyk colourspace. I do lose the Lab special colours—I do know that many of the Lab colours are out of the cmyk gaumet. But what is the best conversion technique?

I do add some levels after converting and it helps, but I was wondering if there is a better way. Thanks in Advance.

pathfinder

My first question is why are you going from LAB to CMYK?

Are you files going to be printed on a four ink CMYK printer and the printing firm specifies CMYK color profiles.

As you say, LAB encompasses a MUCH larger color space than CMYK - there is no way all of LAB values are going to be expressed in CMYK, they will be out of gamut, and truncated -

When printed how they are handled will be determined by the Rendering Intents - Absolute Colorimetric, Perceptual, Relative Colorimetric, or Saturation, etc. This is also how the colors will be handled in changing color spaces also......

DavidTO

Go through RGB on your way from LAB to CMYK.

BinaryFx

When working in an "intermediate" editing space or mode such as LAB (or any colour mode or space not the same as the final one required) - it is very useful to set-up a custom softproof of the next or final condition. This is stops a lot of surprises (LAB being potentially more extreme than say RGB to CMYK in the gamut differences).

Regards,

Stephen Marsh.
http://members.ozemail.com.au/~binaryfx/

Vysionous

pathfinder wrote:

My first question is why are you going from LAB to CMYK?

Well, yes pathfinder, I am a print designer so I have to output all my work in CMYK. Thing is, though, even if I go back to RGB then to CMYK, I still lose specially the striking blues. Which in my opinion are the best about the Lab colourspace. All apart from the greatness of the L channel itself, of course.

Vysionous

BinaryFx wrote:

When working in an "intermediate" editing space or mode such as LAB (or any colour mode or space not the same as the final one required) - it is very useful to set-up a custom softproof of the next or final condition. This is stops a lot of surprises (LAB being potentially more extreme than say RGB to CMYK in the gamut differences).

by that you mean a feasable colour output CMYK or RGB would handle, no?

DavidTO

Vysionous wrote:

even if I go back to RGB then to CMYK, I still lose specially the striking blues.

Such is life.

BinaryFx

Vysionous wrote:

by that you mean a feasable colour output CMYK or RGB would handle, no?

Yes, by limiting the visual display of the LAB image (which is clipped at your monitor gamut anyway, so thus the softproof to an output space being even more critical) - via a custom softproof to another device (SWOP for example) - one is not surprised when the conversion is later performed.


Stephen Marsh.
http://members.ozemail.com.au/~binaryfx/

Vysionous

BinaryFx wrote:

Yes, by limiting the visual display of the LAB image (which is clipped at your monitor gamut anyway, so thus the softproof to an output space being even more critical) - via a custom softproof to another device (SWOP for example) - one is not surprised when the conversion is later performed.

I am honestly unaware about SWOP but I have to dig that up. Thanks Binary.

BinaryFx

Vysionous wrote:

I am honestly unaware about SWOP but I have to dig that up. Thanks Binary.

SWOP is a web press condition that is common for magazine printing, when most people in the USA or Australia say "just give me CMYK" - they often mean SWOP CMYK as the aimpoint (there being many different CMYK conditions with SWOP just one of them and not just one magical CMYK that suits all, it depends on the press, paper, country etc).

What I meant was that an output device profile for a RGB input based inkjet multi ink printer or a four colour CMYK press like SWOP is a good softproof as it is a final condition, or one close to the final that is similar...thus the softproofing will show things that may not be shown if using an RGB editing space.

More on SWOP here:
http://www.swop.org/
http://www.swop.org/specification/

There are many other 'industry bodies', 'standards' or 'guidelines' for CMYK press printing, such as GRACOL, FOGRA, IFRA etc.

Photoshop ships with a CMYK profile named "U.S. Web Coated (SWOP) v2" which also may be known as "TR001" CMYK.


Stephen Marsh.
http://members.ozemail.com.au/~binaryfx/

Vysionous

Thanks alot again Binary, you've been greatly helpful.