Is there such a thing as a Lab profile that just passes through the colours without changing them. If so where can I get hold of one please? Thanks!
You would need an output device that has the same gamut as the input device. Pretty unlikely that you will find that. Or just don’t use ICC. The whole point of the ICC system is it’s ability to compensate and adjust for the differences in color range between the input and the output. Till we have perfect inks and perfect printers, which will happen when we get some flying pigs… And that may not happen for a while cause they are down with the flu…
On Mon, Dec 14, 2009 at 7:33 AM, mitramin <email@example.com (firstname.lastname@example.org)> wrote:
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What I actually need is a Lab based ICC profile that I can use to convert from lets say RGB to Lab and then from Lab to CMYK that would give me the exact same results as if I used a direct RGB to CMYK conversion. The Lab profile will be used both as an input and an output profile.
What I actually need is a Lab based ICC profile that I can use to convert from lets say RGB to Lab and then from Lab to CMYK that would give me the exact same results as if I used a direct RGB to CMYK conversion.quote]
If you’re using Photoshop, you could just convert to to LAB, is this what you mean? This is how ICC profile’s generally work (RGB->Lab->CMYK with Lab or XYZ as the profile connection space). A direct RGB to CMYK conversion could be done through a device-link profile, omitting the Lab connection space, but this is not the norm. So for example, in Photoshop, if you’re converting from sourceRGB to Lab, then separately from Lab to desitination CMYK, you should get identical results as just converting from source RGB to Destination CMYK, provided the same profiles and rendering intents are used.
I understand all that but what I want to do is outside the Photoshop etc. I would like to go from lets say CMYK to n channels (Ncls) but not directly, instead through an intermediate Lab to Lab colour space.
CMYK->Lab then Lab->Ncls, what I need is the middle bit, an ICC profile that just passes through the colours without altering them.
The reason I need this is that my application does not output to Ncls directly but can to a Lab colour space. I would then want to take this Lab and convert it to other multi-channel spaces. So if I use the same Lab profile for both output and input profile then I presume the conversion will be the same as direct CMYK to Ncls.
Lab is device independent and as such, doesn’t require “characterization by a profile”, so I’m not sure I see where a problem would arise. A image in CIELab doesn’t really need to be tagged with an Lab profile. A Lab to Lab colorspace would be redundant…like taking a flight from Detriot to Detroit.
That would be terrible… I can imagine all the pent up anticipation sitting there in the seat… Then takeoff, landing, only to discover your’re still in Detroit… Then again, the sheer sentiment of being in Detroit might end up making you realise that it is all pointless, and resistance is futile…
If I were to convert RGB into Lab in Photoshop I’d get a cmyk value where k is non zero, and the values of c,m and y similarly so but dependant upon the maximum inl levels. So I assume that there is not one fized and definitive transformation, it depends on the profile used to define the cmyk working space.
To underscore the point, I could simplistically add the k values back into the c, m, and y and use a k value of Zero … unfortunately I’d likely end up with values in cmy of greater than 100 …which I’d not be able to use.
Further, if I make a new custom cmyk profile with photoshop where I select no K and a maximum of 300 ink, I obtain another set of coordinates again … but the sweet thing is that the image looks just the same.
What this says to me is that, if I can get 3 different sets of cmyk values for a single set of RGB, then there is no unique and universal Lab profile … all would be vendor specific.
Now photoshop have one called Lab Color that you could use in converting to cmyk but would that be the same profile that you are using in your conversion from RGB?
That is the rub.