Talk:CMYK color model

Old discussion

Adopted orphan redirects for searching: CMYK colour model

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Can you convert the following RGB coordinates to CMYK??

Color    RGB         CMYK
Gray     128 128 128
Black    0   0   0
White    255 255 255
Red      255 0   128
Orange   255 83  0
Yellow   255 255 0
Lime     83  255 0
Green    0   255 128
Sea blue 0   172 255
Blue     0   0   255
Purple   172 0   255

66.32.71.96 02:21, 2 Apr 2004 (UTC)

Note: (or note to self :-P ) we need an RGB -> CMYK conversion formula, like on HSV color space , that'd be handy. Kim Bruning 12:38, 3 Apr 2004 (UTC)

Okay, let's snarf some formulas off of : http://www.easyrgb.com/math.php?MATH=M12#text12

(mathematics isn't copyrightable AFAIK, and I'll be rewriting anyway :-) ) and then rewrite them to 1 form:

// RGB -> CMY
//RGB values = From 0 to 255
// CMY values = from 0 to 1

C = 1 - ( R / 255 )
M = 1 - ( G / 255 )
Y = 1 - ( B / 255 )
//CMY values = From 0 to 1 

// and CMY-> CMYK
var_K = 1 

if ( C < var_K )   var_K = C
if ( M < var_K )   var_K = M
if ( Y < var_K )   var_K = Y

C = ( C - var_K ) / ( 1 - var_K )
M = ( M - var_K ) / ( 1 - var_K )
Y = ( Y - var_K ) / ( 1 - var_K )
K = var_K

Okay, so we'd better write that out tidily :-)

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so let's see

${\begin{matrix}C_{CMY,float}=1-(R/255)\\M_{CMY,float}=1-(G/255)\\Y_{CMY,float}=1-(B/255)\end{matrix}}$

$var_{K}=\left\{{\begin{matrix}C_{CMY,float},&{\mbox{if }}C_{CMY,float}<var_{K}\\M_{CMY,float},&{\mbox{if }}M_{CMY,float}<var_{K}\\Y_{CMY,float},&{\mbox{if }}Y_{CMY,float}<var_{K}\end{matrix}}\right.$

${\begin{matrix}C_{CMYK,float}=(C_{CMY,float}-var_{K})/(1-var_{K})\\M_{CMYK,float}=(M_{CMY,float}-var_{K})/(1-var_{K})\\Y_{CMYK,float}=(Y_{CMY,float}-var_{K})/(1-var_{K})\\K_{CMYK,float}=var_{K}\end{matrix}}$

finally:

for: $C,M,Y,K\in (\mathbb {Z} ,[0,255])$

we can simply do:

${\begin{matrix}C=255\cdot C_{CMYK,float}\\M=255\cdot M_{CMYK,float}\\Y=255\cdot Y_{CMYK,float}\\K=255\cdot K_{CMYK,float}\end{matrix}}$

Okay, as a sucky biologist, my math sucks. Can a real mathematician look this over before adding it to the main article? Else I'll add it in a week or so. Note that I really don't like the float inbetween step, and it's not really nescesary :-)

Kim Bruning 13:21, 3 Apr 2004 (UTC)

Oh heck, let's be bold and add it, some mathematician will just come along and simplify the float step I'm sure :-) Kim Bruning 13:27, 3 Apr 2004 (UTC)

Ugh, I think I made some slight errors converting from algorithm to formula. Um I'll leave as is for now, some mathematician can pick up the pieces :-/ Sorry! Kim Bruning 13:32, 3 Apr 2004 (UTC)

Here's rewriting this so it's a little neater. This is going off that site before:

Converting CMYK -> RGB = CMYK -> CMY -> RGB

t_CMYK ∈ [0,1]⁴ is the CMYK quadruple, t_CMY ∈ [0,1]³ is the CMY triple, and t_RGB ∈ [0, 255]³ is the RGB triple. Then:

${\begin{matrix}t_{CMYK}=(c,m,y,k),\mathrm {then} \\t_{CMY}=(c',m',y')=(c(1-k)+k,m(1-k)+k,y(1-k)+k)\ \mathrm {and} \\t_{RGB}=(r,g,b)=(255(1-c'),255(1-m'),255(1-y'))\\\end{matrix}}$

Converting RGB -> CMY, with the same color vectors as before

${\begin{matrix}t_{RGB}=(r,g,b)\\t_{CMY}=(c',m',y')=(1-r/255,1-g/255,1-b/255)\\t_{CMYK}=(c,m,y,k)=(c-\mathrm {min} (c,m,y),m-\mathrm {min} (c,m,y),y-\mathrm {min} (c,m,y),\mathrm {min} (c,m,y))\\\end{matrix}}$

Dysprosia 14:05, 3 Apr 2004 (UTC)

Thanks, looks tidier at least! On computers, CYMK is usually represented in 32 bits (4 bytes: C, M , Y, K), so each C, M, Y, K value is an integer from 0 to 255, and not a floating point value from 0 to 1 as was stated on the referenced webpage. Cutting out the floating point values would improve the formula further. (something to sort out in my copious free time :-) Kim Bruning 14:26, 3 Apr 2004 (UTC)

• Can you put the coordinates in the above table that already shows RGB?? 66.32.123.29 15:36, 3 Apr 2004 (UTC)
  • Instead of giving you a fish, we gave you a fishing rod. :-) Calculate the cmyk values using the formulas, or the webpage or what have you, and multiply by 255 (and round off) to get the compuer representation. Kim Bruning 16:00, 3 Apr 2004 (UTC)
    • What do fish and fishing rods have to do with RGB and CMYK?? Is this just an expression?? What does it mean?? Does Wikipedia have an article for expressions like this??66.32.113.34 16:55, 3 Apr 2004 (UTC)
      • "give a man to fish, and he will eat for a day, teach a man to fish, and he will eat for the rest of his life". The idea is that we just explained how you can figure out cmyk values for yourself, into perpetuity :-) Unfortunately the formula given by dysprosia gives values from 0 to 1, and we want to scale that to 0-255. (so just multiply by 255, easy enough). Well I'll do some examples to see if the formula is actually correct...

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some tests

${\begin{matrix}(128,128,128)_{RGB}=(0,0,0,0.5)_{CMYK}=(0,0,0,128)_{CMYK,scaledtobytes}\\(255,0,0)_{RGB}=(0,1,1,0)_{CMYK}=(0,255,255,0)_{CMYK,scaledtobytes}\\(255,255,0)_{RGB}=(0,0,1,0)_{CMYK}=(0,0,255,0)_{CMYK,scaledtobytes}\end{matrix}}$

Wow! This formula is really easy to use too! :-) I'm sure there's a way to skip the float phase , but this is nice :-)

• But 255 0 0 was not among the colors in the above table. 66.245.99.11 18:20, 3 Apr 2004 (UTC)
  • But I'm not going to calculate your table for you, I was figuring some edge cases to see if the formula actually works. Kim Bruning 18:33, 3 Apr 2004 (UTC)

Hmm, note I was doing mental arithmetic, so values are rounded off. Kim Bruning 21:48, 3 Apr 2004 (UTC)

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okay, one example for 66.32.123.29, though really wikipedia isn't for this. Note that your value for red is really weird?

We're using "red, green blue" so:

${\begin{matrix}red=(255,0,0)_{RGB}\\green=(0,255,0)_{RGB}\\blue=(0,0,255)_{RGB}\end{matrix}}$

tell you what, let's find blue in CMYK

• disprosia said:

${\begin{matrix}t_{RGB}=(r,g,b)\\t_{CMY}=(c',m',y')=(1-r/255,1-g/255,1-b/255)\\t_{CMYK}=(c,m,y,k)=(c-\mathrm {min} (c,m,y),m-\mathrm {min} (c,m,y),y-\mathrm {min} (c,m,y),\mathrm {min} (c,m,y))\\\end{matrix}}$

• Okay, well, $blue=(0,0,255)_{RGB}$

so we do:

$t_{RGB}=(0,0,255)$
$t_{CMY}=(c',m',y')=(1-0/255,1-0/255,1-255/255)$
$t_{CMY}=(c',m',y')=(1,1,0)$
$t_{CMYK}=(c,m,y,k)=(c-\mathrm {min} (1,1,0),m-\mathrm {min} (1,1,0),y-\mathrm {min} (1,1,0),\mathrm {min} (1,1,0))$

• Well ... $\mathrm {min} (1,1,0)=0$ , so:

$t_{CMYK}=(c,m,y,k)=(1-0,1-0,0-0,0)$
$t_{CMYK}=(c,m,y,k)=(1,1,0,0)$

• scaling:

$t_{CMYK,scaled}=255\cdot t_{CMYK}$

• So in the end we get:

$t_{CMYK,scaled}=(255,255,0,0)$

Which I happen to know is the right value for blue. :-)

Easily understood

Doesn't anyone think this page should be made more simpler so people can understand or add more explanations to the algorithm-ishs given in the end? My thought since I still do not understand how. -- WB 02:30, Mar 18, 2005 (UTC)

I think the algorithms given should be removed, as they are of little practical use. They do not reflect the real-world conversions actually needed, and by putting them into a Wikipedia article, it adds to the confusion and errors which can produce very expensive printing mistakes. Views? Notinasnaid 09:34, 11 Apr 2005 (UTC)

It is less productive to remove inaccurate algorithms altogether as opposed to improving the algorithms in the first place. Dysprosia 12:19, 11 Apr 2005 (UTC)

The crucial point is that there is no algorithm. There is no single conversion from RGB to CMYK, so - much as people yearn for there to be a simple algorithm - it is doing a disservice to put one there. Notinasnaid 16:31, 11 Apr 2005 (UTC)

I have tried to put some background in the new article absolute color space. Neither RGB nor CMYK are, by themselves, absolute, so there is no single conversion between them. Notinasnaid 20:27, 11 Apr 2005 (UTC)

It seems to me that the lack of reversibility in RGB to CMYK to RGB conversions isn't an issue which should prevent presentation of an algorithm. Such an algorithm is a necessity in many applications; for example, rendering an RGB-based image into CMYK inks on paper. CoyneT 00:54, 12 Apr 2005 (UTC)

To my mind, that's exactly the reason to not have it, because, frankly, it's rubbish. People have written software using this algorithm, and the results of the conversion don't really satisfy anyone, because the colors are different. Notinasnaid 07:43, 12 Apr 2005 (UTC)

The discretionary note added is sufficient enough. Those who want to make use of such an algorithm are warned for its use, and those who need something professional will be warned of the consequences for using the algorithm. Dysprosia 08:00, 12 Apr 2005 (UTC)

That makes sense. If I ever manage to find a way to express how it should be done, I'll add that too. (It won't be an algorithm). Notinasnaid 08:28, 12 Apr 2005 (UTC)

Still doesn't make sense

The sentence still doesn't make sense. Suppose that the conversion from RGB to CMYK is reversible. I take this to mean that there exist a conversion from RGB to CMYK, and from CMYK to RGB. But if we suppose now, seperately, that the conversion from CMYK to RGB is not reversible. Then I take this to mean that there is a conversion from CMYK to RGB, but not one from RGB to CMYK. This clearly contradicts the first supposition. One of the suppositions is wrong. Which is it? Dysprosia 10:20, 20 July 2005 (UTC)

Are we talking about colour-for-colour matching? Please clarify the sentence if this is the case. Dysprosia 10:27, 20 July 2005 (UTC)

This is nothing to do with colour matching, just simple mathematics.

Clearly it either (a) isn't expressed clearly or (b) is expressed clearly but is so unexpected it seems to be wrong. Yes, you can start with any RGB value, use the formula to convert to "CMYK" and then back to RGB. You will have the same value, in every case. But if you start with a CMYK value, convert to "RGB" and go back again, you will (except in particular lucky cases) not have the same value. If you want a worked example, start with the two CMYK values 0,0,0,1 and 1,1,1,1 and see what you get when converted to RGB then back to CMYK. Notinasnaid 10:31, 20 July 2005 (UTC)

Let me try and clarify the wording based on this, so it doesn't sound so paradoxical. Dysprosia 10:36, 20 July 2005 (UTC)

That's pretty good, thanks! We might also want to speak of gamut here, as one of the reasons that RGB and CMYK aren't completely inteconvertable is that certain colors are "out of gamut" for one color space or the other and simply can't be represented (accurately) in the other color space.

Atlant 11:33, 20 July 2005 (UTC)

No, gamut doesn't apply here. These are artificial conversions (which I personally would prefer to see removed) rather than anything that does accurate colour. But people do seem to want to see formulae, even if they are rubbish. There is no gamut mapping implied. I'd like to see a separate subsection about the principles of an accurate conversion. Notinasnaid 11:48, 20 July 2005 (UTC)

I'm sorry, but you're wrong. "Out of gamut" colors are one of the principal reasons the conversions aren't reversible. While a theoretical CMY/RGB system might use identical primaries (if you think of C as -R, M as -G, and Y as -B), but we live in the real world and real-world CMY(K) and RGB use different primaries that enclose different gamuts.

Atlant 12:31, 20 July 2005 (UTC)

One more point: there cannot exist any RGB <-> CMYK mapping that is invertible in the CMYK -> RGB -> CMYK case. I don't know how to express it mathematically, but it's something to do with mapping four unknowns into three, and back to four; it's impossible to preserve all information. Or is it? Now, it may be invertible in terms of colour display, but that's a different story; the actual CMYK numbers are the point of the discussion. Notinasnaid 11:54, 20 July 2005 (UTC)

What counts is the size of the sets you're mapping between and/or the domain, range, and codomains of the conversions, depending on what you're talking about, but that is another story. What is being said currently in the article includes the case you just mentioned, I believe, though somewhat implicitly. I could give a mathematical reasoning here to support that or to further clarify that but it may be a little overcomplicated and I would be liable to make mistakes anyway ;) Dysprosia 12:27, 20 July 2005 (UTC)

(resonding to Notinasnaid) This would be true if "K" were really an independent variable. But it's not; it's simply a way of saving ink and accounting for the fack that in the real world, C+M+Y doesn't quite make K. See all the mentions of Grey Color Removal, Under-Color Removal, and the like.

Atlant 12:31, 20 July 2005 (UTC)

Let's try and make it clear what I'm talking about.

1. I am specifically talking about the formulae on the article page, not about the general case of CMYK <-> RGB conversion. These formulae make no allowance for gamut (work through them if you think they do), so gamut is not relevant to that discussion.

Okay. But let's make clear what's real-world and what's theoretical. Atlant 14:56, 20 July 2005 (UTC)

2. When I talk about reversibility I mean, specifically, a return to the same four variable values. I am not talking about whether the colour they represent happen to be the same. While it might in some situations be interchangeable, there are other situations where the conversion would produce bad results. The conversion – this conversion – applies only one rule for GCR/UCR etc.

I understood that. Atlant 14:56, 20 July 2005 (UTC)

3. These are bad formulae but if the article doesn't make it clear the preceding paragraph is only about them, then it should.

Maybe it would clarify the article to give specific examples of inputs and outputs for the formula, showing the non-reversibility. Notinasnaid 14:43, 20 July 2005 (UTC)

Please try and limit your use of the word "reversibility", as it can imply "invertibility" and thus needs to be properly qualified otherwise, and it caught me once already. Thanks Dysprosia 14:52, 20 July 2005 (UTC)

An example or three would be very helpful. Atlant 14:56, 20 July 2005 (UTC)

I am surprised by this whole bad formula discussion. When you convert CMYK to RGB, you are mapping a 4-dimension space onto a 3-dimension space, i.e. you lose 1 dimension (you can compare it to converting a 3D-design of a house to a floorplan -you cannot convert that floorplan to a 3D again). So technically it is not a conversion, but a 'projection' that is not reversible/invertable.

Evenso, one of the questions people have most often concerning CMYK is "but how can I show it on my screen?" so they need the RGB equivalent. So why was the CMYK-2-RGB converter (external link) deleted? Anyway, what comes out of your printer in most cases (most document/image software uses RGB) is an CMYK approximation of an RGB colour, done either by your printer or printer driver. So RGB-2-CMYK inverse projection is far from useless. The extra parameter you use is: do you want to use more black ink (K) or more of the three colour inks (CMY)? So the issue is more economical. Pforret

My argument against this formula is that it is not a good formula. It suggests that RGB cyan (100% blue+100% green) is best represented by 100% CMYK cyan. This is not the case: it is a fallacy. I think it is very important that this article educate people that there is no simple conversion that will produce a satisfactory matching from RGB to CMYK and vice versa. That what they seek does not exist. Otherwise, they will go away from this article and implement things on that basis, to nobody's good. Converting between RGB and CMYK requires knowing what color space is in use for RGB and CMYK, and doing a color mapping based on those color spaces (e.g. using ICC profiles). I have to ask this, I hope it does not come over too negative: is anyone who promotes this formula actually responsible for high quality color matching? Notinasnaid 15:45, 29 September 2005 (UTC)

I would expect that if someone were to develop a high-quality color matching system, he/she would dive into the whole Pantone/RGB/CMYK matter more deeply. For the Average Joe (i.e. 95% of your Wikipedia public) it is enough to know that the best approximation of a certain CMYK colour is RGB #XXXXXX. They don't care about color temperature, gamut and ICC profiles. If they get a color specified in CMYK, they want to know: is it greenish, redish or brownish? Oh, it's pink!

Again: the main purpose of an online converter is conversion from CMYK to RGB, not the inverse. Most of us live in a RGB world, we need our stuff in RGB and all conversion back to CMYK is done for us (by a printer driver or some piece of software). So if "Cyan" is (1,0,0,0) in CMYK, what is the best approximation of that color in RGB? Probably #00FFFF is a pretty good option. If you have a better formula, be my guest and share it. If not, please add the "not for professionals" warning and let Average Joe convert his CMYK to RGB.

--Pforret 08:13, 3 October 2005 (UTC)

No, sorry, I cannot agree. Leaving aside that Pantone has nothing to do with it... you say "it is enough to know that the best approximation" – no. It is not the best approximation. It is not even a good approximation. So the average Joe will go away with a bad approximation, which he does not understand enough to know is bad. So I argue that we should certainly not link to a calculator which people will certainly use in preference to actually reading the difficult and unsavoury facts in the article. "If you have a better formula" – - there is no formula. That's my point.

If this were an introductory piece I'd have no problem with leaving the link in. But this is supposed to be an encyclopedia; that means that I don't think that oversimplifications have any place; if the facts are complicated, it's the job of an encyclopedia to say so, hopefully in a way that can be understood, not to gloss over them. Notinasnaid 08:33, 3 October 2005 (UTC)

By the way, there is nothing impossible about a web site which does this properly while starting the education of people in the issues. For example, a site which offered a choice of RGB and CMYK profiles (ideally with the opportunity to upload a custom one), and then did the conversion on this basis. On the other hand the site to which you posted a link not only does not do this, it does not even carry warnings about how inaccurate the result is. I do not think that simplicity is better than accuracy. Also, please do not change the page while this is still under discussion. I observe that this is your own site (or there is a remarkable coincidence of names): I can understand the emotional attachment you have to it, but I do not agree it has any place on Wikipedia. It may be that this page needs to go to dispute resolution. Notinasnaid 08:48, 3 October 2005 (UTC)

I will try to construct a better formula using CIE-XYZ

--Pforret 10:42, 3 October 2005 (UTC)

Removing the picture

I propose removing the picture . It contains pure RGB cyan, magenta and yellow, which look very little like printing inks. I propose not replacing it because there is a wide variation in printing inks, and even if there wasn't, there is a wide variety in monitors. In general, I don't think any reference colors belong in wikipedia. Notinasnaid 11:21, 5 September 2005 (UTC)

Don't do that. While everyone knows what "Yellow" is, many people don't know what "Magenta" is and nearly everyone has no idea what "Cyan" is. The picture is a good start.

Of course, you should feel free to add another picture (of 55-gallon drums of ink or what-have-you).

Atlant 12:34, 7 September 2005 (UTC)

Well, that isn't what cyan and magenta look like. That's the problem... Notinasnaid 17:23, 7 September 2005 (UTC)

Funny, when displayed on an additive-color device, I was pretty sure that cyan was 100% green + 100% blue and magenta was 100% red and 100% blue. What do you think they look like (when displayed on an additive color device)? Or maybe you should print the Wiki page and see if your color printer shoots anything other than cyan ink for the cyan swatch and magenta ink for the magenta swatch; it ought to be pretty easy to tell, even without a magnifying lens.

Atlant 14:06, 8 September 2005 (UTC)

The article isn't about additive color. Certainly, additive cyan/magenta look like the picture. But I have never seen a cyan/magenta ink for use in CMYK that was anything like that. Both the cyan and magenta in CMYK are significantly darker, but vary depending on the ink mix. That's why CMYK has a significantly different gamut to RGB. This picture can only mislead and confuse people in an area where there is already substantial confusion about what these colors really look like. Should this article be about real CMYK or an idealised and nonexistent version of it? Notinasnaid 21:59, 8 September 2005 (UTC)

No, you're correct: this article isn't about additive color. Unfortunately, 99.999% of the time, this article will be viewed on a device that uses additive color, and you're just going to have to live with that. As I said, if you don't like those color swatches, find a photo of gobs of ink that pleases you, and add that to the article. But don't remove something that is technically accurate just because you don't like it.

(By the way, you're dodging the suggestion I made about printing the page using an inkjet printer. I assure you that the C, M, Y, and K inks used by those printers pretty-accurately match the color swatches inthis article as displayed on my various CRT and LCD displays.)

Atlant 12:00, 9 September 2005 (UTC)

I did your test, printing on an Epson 880 with Epson inks and default Windows 2000 driver settings. The result was nothing like the strong primaries in the original graphic. They were rather more like but of course that was created with the wrong CMYK profile: SWOP Coated rather than a profile for the Epson inks. The colors were certainly duller than that. And when I printed this new graphic they were duller still, a result of continuing gamut compression which means that they never can match with these printer settings.

Examining with a loupe I can see that in the first printout, the cyan contains white and magenta dots, the magenta has white and cyan dots and the yellow has white and cyan dots. In the second picture, the cyan seems to have only a little white, the magenta only a little cyan, and the yellow is very pure.

I feel that would be a better graphic - it at least looks something like some real world colors. But it is misleading, just like the formula for conversion which I would also like to see removed, because it reinforces wrong ideas. Would the entry for sparrow allow a picture that was sometimes a cormorant? I don't think so. This is why I think color swatches (except RGB swatches designed to show your own monitor's behaviour) have any places in an online resource that has pretensions to accuracy. Notinasnaid 13:07, 9 September 2005 (UTC)

Do I understand this correctly?

I have been reading about the RYB model and the CMYK model. First I'll try to explain my train-of-thought: in my mind, magenta is created by adding red and blue, this works for both paint and RGB values. So I was wondering how CMY is concidered a good base colour triad. So I started reading and found out about the principle of additive and substractive models. I understand these. I can see why CMY is a good model. Then I read this page and my initial thought were confirmed when I read: "dark blue reproduces poorly in CMYK, coming out rather murky (as it has to be approximated by darkening cyan with black and a little magenta)". So here is my theory: RGB works on a monitor because the monitors physically do like the theory: paint with light. The CMY model is a theory that translates poorly to printing practice, because we try to print using physical materials (ink/paint) to mix up a new material that absorbs light like the CMY model would. It is this that results that shades of blue and purple are poorly reproduced? Or do I miss something here?

You're not missing anything. The fact is that it is impossible to get close to a typical monitor gamut on a printer, especially using only four inks. But when four inks are used, although they are *called* cyan, magenta, yellow, and black, the cyan and magenta in particular are very different from the colors that are called by those names in an RGB model. By convention, any roughly blue-greenish ink is called "cyan", and any roughly pinkish ink is called "magenta". For example, here is a rough approximation of the ink colors used in a printer I'm currently working on (with the usual caveats about color reproduction on the web):

As you can see, this is something of a compromise between CMY and the RYB traditionally used with paints. This printer has an excellent gamut of blues, because the "cyan" ink is very blueish. Also, its C+M+Y is not "murky" at all; you can hardly distinguish it from black. It still can't produce anything like the intense blues you can get on a monitor, and it does less well with reds (despite the magenta being more reddish than most).

Using five or six inks can produce a significantly better gamut -- of course with higher costs for heads and ink. Some high-end printers use CMYK+Red+Blue; others use CMYK+Orange+Green. There are also a handful of 8-color printers, generally allowing some of the colors to be replaced by specialist inks for particular jobs. DavidHopwood 04:43, 19 January 2006 (UTC)

When I went shopping last summer, I saw some that were CYMKcm. (lowercase meaning pale colors) The light colors helped to avoid speckles when printing pale things. AlbertCahalan 04:42, 26 April 2006 (UTC)

Just for fun, I converted the L*a*b* values provided by the ink manufacturer to sRGB, using the icctrans utility from http://www.littlecms.com/newutils.htm (with colorimetric rendering intent):

Hmm, the yellow and magenta don't look right at all, and I trust my eyes more than icctrans (although maybe I was overestimating the brightness in the first image). Oh well. DavidHopwood 05:34, 19 January 2006 (UTC)

Why no formula

I have been asked to explain why I say "there is no formula" for converting between CMYK and RGB. It does on the face of it sound like nonsense. Here's why:

I'll try to explain where this "there is no formula" comes from. There are two distinct phases (a) why there isn't one in theory and (b) why there isn't one in practice if the problem is reduced to one where calculation is possible.

1. There is no formula because the problem is not well defined. Neither RGB nor CMYK are absolute color spaces, so to talk of conversion is not really meaningful until both spaces are actually defined. For instance, if you were talking about sRGB and a particular CMYK profile (derived for ink and paper conditions), then the conversion becomes meaningful. Since each combination of RGB and CMYK profile has a different answer, you can't say anything about the conversion until those are decided. Rendering intent is relevant too.

PostScript version 1 used only CYMK. Was it well-defined? If yes, then I suggest using that with sRGB. AlbertCahalan 04:45, 26 April 2006 (UTC)

CMYK appeared in a supplement to level 1 PostScript, yes. The actual conversion was well documented (I think this is one reason it has been popular) but did depend on what values were passed to setundercolorremoval and setblackgeneration. But whatever values were passed it was a very poor match colorimetrically (this color becomes something like this color). So why use this, particularly poor, conversion? Notinasnaid 21:33, 28 April 2006 (UTC)

2. Once the color spaces and rendering intent are defined, clearly a calculation can and does take place (e.g. every time the screen is redrawn in Photoshop). But in this case I say there is no formula because nobody has expressed this as a mathematical formula, and it isn't really practical to do so. Let's look at what really happens. To oversimplify, the RGB profile contains information on the white point, gamma, and a matrix. This is used to convert the RGB value to Lab. So far, this would be practical to express as a formula.

The CMYK profile contains 3-to-4 dimensional lookup tables, up to three of them for different rendering intents. Each table is typically pretty large, though not as large as the 4-to-3 table used for going the other way. A color manager interpolates values in this table to convert the particular color. The table is typically generated by sampling colors with a measuring device. I don't see it practical to express this as a formula, nor every useful as each profile would be different.

The formula in the article is an attractive one, but it is based on a fiction. The fiction is that CMYK cyan and RGB cyan (similarly magenta) are equivalent colors. Notinasnaid 13:21, 5 October 2005 (UTC)

Personal Opinion Used in Technical Description

...The nearest equivalent in CMYK is an ugly purple shade....

The word "ugly" is not strictly appropriate for the context. If there are no objections I will replace this with a more appropriate description, and perhaps add an example of what it looks like, so as readers can draw their own conclusions about whether it is "ugly" or not.

Joseph Crowe 16:50, 6 January 2006 (UTC)

I agree the language is bad. Please don't add a visual example. You CANNOT show CMYK on screen, so you produce an entirely misleading idea that you can, by producing one particular conversion to RGB, which is itself monitor dependent. Notinasnaid 18:10, 6 January 2006 (UTC)

Right. I'll go ahead with the edit then. Joseph Crowe 15:30, 8 January 2006 (UTC)

Typo in history

I changed the text "a RGB color" to "an RGB color," but when I described what I did in the page history, I fat-fingered and put "'changed "a RGB color" to "an RGM color.'" I know this sounds foolish, but I just wanted everyone to know I hadn't actually changed the B to an M. Sorry about the flub in the page history. —CKA3KA (Skazka) 18:40, 13 January 2006 (UTC)

You don't have to apologize so profusely for this. Dysprosia 04:36, 19 January 2006 (UTC)

The K-channel image should be...?

According to the convertion,
RGB=(255,255,255) -> CMYK=(0.0, 0.0, 0.0, 0.0)
That is reasonable that there is no quantity of black color in the white color.
Relatively, the color Black (RGB=(0,0,0)) convert to CMYK is (0.0, 0.0, 0.0, 1.0)
So, if a image in RGB is converted to CMYK domian.
The K-channel image should be like the invert of image of luminosity.
But the K-channel image of the picture on the right side of this page, the CMYK-domain image, seems to be like the image of luminosity, but the invert of image of luminosity.
Is something wrong?

Ignore the formula. It's nonsense. There is no simple formula, and the idea doesn't even make sense unless RGB and CMYK are defines as specific color spaces. However, it is not the case that the K channel should be a mirror of the luminosity. K is there for mixing with the other inks for the desired affect. Consider an image that is entirely magenta. It has luminosity, but there must be nothing in the K channel. Notinasnaid 09:20, 31 August 2006 (UTC)

Drying

Does the pigment dry before the next color is applied or are they all applied wet and dried together?--Energman 22:34, 17 December 2006 (UTC)