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colour mode conversion

Discussion in 'Printing & Print Design Forum:' started by cutcopypaste, Mar 14, 2013.

  1. cutcopypaste

    cutcopypaste New Member

    Hi guys, just a quick question about colour conversion: Is there a difference in the way different applications convert from cmyk to rgb and the vice versa? For example Photoshop, Gimp, and those online tools such as Online RGB to CMYK conversion ? So that if I use dofferent tools/apps to convert the same image, is the outcome going to be exactly the same?

  2. Theory Unit

    Theory Unit Member

    I've only a little experience of this, but I used a couple of online converters in the days before I had CS and I found the colour outputs would vary quite considerably between them. Not sure if it was the quality of the code or if there are simply different methods/algorithms for converting CMYK to RGB - it's a very tricky business is colour!

    I just put everything in Photoshop and output from there. Not sure this is much help but there you have it... =)
    cutcopypaste likes this.
  3. cutcopypaste

    cutcopypaste New Member

    Thanks mate that was actually helpful. The thing is I dont always have photoshop on the computer Im using and so have to use some other tools. But I guess Ill try try to use the same tool/app for the files in the same project when ever I can. Sometimes Im so busy that I have to send rgb files to my printer (print24) who I think use photoshop to make the conversion, not sure but I think so. Maybe Ill just buys a few old lisences of PS for the other computers too ans save myself some trouble headache. :D

    Colour is really tricky thats true, it can be a nightmare sometimes figuring out if everything is right before sending a big project off to print.


  4. RGB and CMYK are just ways of describing a colour as percentages of colourants. What's the difference between converting RGBtoCMYK and oneRGBspace to anotherRGBspace? Nothing - absolutely nothing at all. When you convert from one colour space to another it doesn't matter if it's RGB to CMYK or not - what you're actually doing is taking all the colours in your starting space, mapping them to the profile connection space (PCS) normally lab, then mapping from the PCS to your output space. If you're using the AdobeCS defaults, your file might start life as sRGB, map to LAB then map out to USWebcoatedSWOPv2. Out of gamut colours are mapped according to the Rendering Intent selected. The rendering intent, when mapping from a larger gamut to a smaller one makes a big difference to your final image.

    So to answer the initial question - different PCS might lead to different conversions; different rendering intents WILL lead to different conversions.

    Of course - converting to CMYK before sending to the printer is largely unnecessary and generally a cop out on the printer's part. It's the accepted norm, but it's doesn't pass for accepted wisdom among those that understand ICC colour management. If anyone is aware of an argument for designers converting to CMYK before submitting to an external printer that does hold up in the face of questioning I'd be interested in hearing it. Only one I've heard so far is "it makes our life easier", but that's from the printer not the customer, which seems a bit backwards to me. Here's my argument against. RGB or CMYK? Colour Spaces - what should you work in? | Hudson
  5. Katedesign

    Katedesign Well-Known Member

    I suppose I'd should put my brain in and read your article Craig, but I have always converted to CMYK and have rarely/never had a job rejected or had to reject a job because of colour... lucky I guess!!
  6. Converting to CMYK isn't "wrong" but it often unnecessarily limits your colour. Because you're not converting to CMYK - you're converting to A device SPECIFIC CMYK. The default Adobe CMYK is USWebCoatedSWOP. That's a CMYK space that represents the output of a litho press set to particular standards on a particular stock viewed in a particular light. It's a very small gamut. Today's UK presses are more likely to be set to a standard represented by FOGRA39 for example. Draw a small circle inside a larger circle. The smaller circle represents SWOP. When you convert into SWOP all of the colours in your artwork are inside that small circle. The press is capable of printing all of the colours in the FOGRA39 larger circle, but you've converted the artwork into the much smaller set of colours. From that point a few different things can happen.

    If you send those SWOP CMYK numbers to a colour managed printer, she'll produce them exactly as you've asked and you'll get the colour you're expecting. You won't be using the full capabilities of the press which has a larger gamut available.

    If you send those SWOP CMYK numbers to a printer whose press has a larger gamut but the printer doesn't understand colour management, then your SWOP CMYK numbers will likely be used directly and your colours will be wrong. Think about the two circles again. Applying the numbers to the wrong gamut is like stretching the inner circle to the outer one - pulling all the colours outwards - making skin tones too intense, grass and skies becoming unnatural. The memory colours are the easiest place to see this in action. (In signage and graphics this is often moronically mistaken and referred to as "making colours pop" when in fact it's inaccurate printing! Specify the bright colours in the first place if you want them!)

    If you want to use the full capabilities of any press whilst maintaining accurate control of all colours, then you need to create files that use the extremes. I'm not just talking about bright colours. I'm talking about not limiting your shadows to the charcoal grey of SWOP, not limiting your blues to the "that can't be hit in process" dull black/blues as you approach reflex.

    When you understand that files are just a list of numbers representing colours - like words on a page - you realise it doesn't matter a damn, CMYK or RGB. Like a page of a book translated into two different languages - you realise that it really makes no odds which language it's in provided the meaning is still accurately understood. EXCEPT if one of the languages has a smaller vocabulary than the other of course. And that's what I'm talking about.
  7. TDesignCo

    TDesignCo Member

    For vocabulary read ICC profile Hudson Display? At a lot of printers I use prefer to accept CMYK and I think a lot of printers choose to do this to minimise colour conversion shifts, but as you are saying reducing the colour gamut (big circle to little circle!). As mentioned this does reduce the range of colours you can hit on press. Much better is using the correct ICC profile for the device/press or RIP you are sending the file too. This guarantees better conversion and output. Most good printers should offer advice on this, and if in doubt get a colour managed proof from them before the press run starts I'd say!
    Last edited: Mar 19, 2013
  8. Well, for vocabulary read "colour gamut" which an ICC Profile describes. As for using the right profile for the output press... Actually that's not it either, although it's much closer.

    A press doesn't have A profile. a rip doesn't have a profile. A profile describes all the colours that can be created on a machine/ink/media/settings combination. I have a dozen profiles for one single product. Although my clients are welcome to those profiles (useful for soft proofing) I don't suggest they work in those colour spaces. The same reason applies as I offered above. Let's say a design is today being printed on a media that takes little ink, has a small gamut. If you work in that space you've limited your artwork to that colour gamut. That only makes sense if you're never going to want that design on anything else, or have it printed anywhere else. Otherwise you're limiting colour for no reason.

    This is where colour standards come in. Using something like FOGRA39 makes sense if you're sending to printers that calibrate their presses to match the output standard that describes.

    But, and here's the interesting thing for us nerds, any press that can be calibrated to that standard is presumably capable of hitting colours that are outside of the gamut of that specification but is being restricted so as to not do. That's giving away some colour capability isn't it? Anyway - I've digressed from the thread.

    I think the conversion responsibility should always lie with the printer and this is why. The printer will be converting to their output gamut anyway! So if the designer does a conversion it's an added, entirely unnecessary stage in the process. A printer who knows what they're about is going to do a conversion on your converted file... I think you're right that good printers should offer advice on colour communication. I don't think asking the designer to do the conversion minimises shifts - I think it minimises surprises and awkward conversations when the blind are leading the blind. Most arguments seem to be about making the printer's life easier, not the designer's. Isn't that just wrong? Who is supposed to be the expert on the printing part?

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