3 Replies Latest reply on Oct 7, 2009 3:28 PM by Printer_Rick

    Basic color management questions

    SarahOwens25

      Having difficult understanding some concepts in color management - would appreciate any guidance to further understand it.

       

      What is the difference in backgrounds between additive and subtractive process color systems?

       

      How is white formed differently in additive and subtractive process systems?

       

      How are the non-process colors like orange formed differently in the additive and subtractive process systems? What colors from each system forms an orange?

       

      Why is the difference in the process of color formation of major concern with the use of computers in the preparation of materials for 4-color process color printing?

       

      Why is the difference in the process of color formation of major concern with the use of computers in the preparation of materials for 4-color process color printing?

       

       

        • 1. Re: Basic color management questions
          Lou Dina Level 3

          Sarah,

           

          Deep subject, so this will only touch the surface.

           

          What is the difference in backgrounds between additive and subtractive process color systems?

           

          How is white formed differently in additive and subtractive process systems?

           

          RGB blends "light" to generate colors.  CMYK blends "inks".  Starting with RGB and light.....pure white blends high levels of red, green and blue wavelengths in approximate equal amounts, which we perceive as white.  In Photoshop parlance, using 8-bit, this translates to 255R, 255G, 255B.  When we see an object, we are actually seeing the light that reflects from that object, not the object itself.  So, a bright white object reflects most of the visible light spectrum and reaches our eye, which the brain interprets to be white.  A black object "absorbs" light so nothing is reflected to the eye.

           

          To print, you need inks on paper, not light.  If you were to use red, green and blue inks, you'd have a big problem printing colors like yellow.  So, they devised the subtractive color process (RGB is additive, ie, adding all three lights together gives you white).  They took the opposite colors of Red, green and blue, which are Cyan, Magenta and Yellow.  In theory, you should be able to create the same spectrum of colors with the CMY subtractive colors that you can with the RGB additive colors, but in practice, that is not the case, for at least two reasons.  The CMY inks are not pure and as intense as the RGB primaries, and second, you have to apply these inks to paper, which drastically limits dynamic range.  To deal with the ink purity problem, (and to give better type), they add Black to the CMY mix, for four inks....Cyan, Magenta, Yellow, and Black.  To get white, you rely on the paper alone, without any ink.  In theory, pure black would be either 100C, 100M, 100Y, or it could be 100K (black), or it could be 100% each of CMYK.  But, these three do not give the same results, due to ink purity, ink limits,  paper, process, etc.

           

          How are the non-process colors like orange formed differently in the additive and subtractive process systems? What colors from each system forms an orange?

           

          Why is the difference in the process of color formation of major concern with the use of computers in the preparation of materials for 4-color process color printing?

           

          In the CMYK world, orange is a mixture of Yellow and Magenta inks, with more yellow than magenta.  For example, 52M, 94Y gives a fairly vibrant orange.  Again, hue, saturation and brightness are dictated by the mix percentages, paper, ink limits, ink purity, etc.  In the RGB world, this same color is defined as 255R, 143G, 33B (depending on the "flavor" (color spaces) of RGB and CMYK you are using.  There is a relationship between RGB and CMYK, and this example is no exception.  When red is maxed out at 255, cyan is the opposite, in this case zero.  In this orange color, Green is 143, near the middle of the range, and so is its opposite, Magenta, which is near the middle of its range, at 52.  Blue is 33, at the very low end of its range, and its opposite, Yellow, is near the top of its range at 94.  Since this is a bright color, there is no black ink used at all.  If it were a darker orange, there would probably be some black in as well.

           

          Some colors can be created in RGB that cannot be duplicated in CMYK.  The opposite is also usually true, that there are colors in CMYK that cannot be duplicated in RGB (depending on the color space you use).  If your intended output is a monitor, the internet, email, or a printer that needs "light" you would generally use RGB.  If the intent is to "print" the job, on a press, inkjet, laser, etc, then CMYK is generally used.  Even if you send an RGB file to your inkjet, the printer driver translates the RGB colors to CMYK in the background before output.  Most, if not all, printers use CMYK subtractive inks for printing,  These can generate a large portion of the printable spectrum.  Some printers add light cyan and light magenta inks, or even red, green, blue, orange, and other colors to help extend the color gamut of the printer so it can achieve colors that would be "out of gamut" using CMYK alone.

           

          Another difference is that CMYK is a four color process, unlike RGB, which is three color.  A given color has only ONE definition when defined in RGB, HSB, L*a*b*, or other 3 component color schemes.  With CMYK, many colors can be created using different mixes of CMYK, which adds complexity, but also offers opportunities and flexibility, especially on press.

           

          Like I said at the beginning, this is a very deep and complex subject, and this only touches on the basics.

           

          Lou

          • 2. Re: Basic color management questions
            Rick McCleary Level 3

            Hi Sarah -

             

            Your opening statement is about color management:

            Having difficult understanding some concepts in color management - would appreciate any guidance to further understand it.

             

            But your questions are more about process:

            What is the difference in backgrounds between additive and subtractive process color systems? etc...

             

             

            I'm wondering if we know what you're really asking. Perhaps you could give a brief description of the problem you're trying to solve. Getting the most useful answer often starts with asking the right question.

             

            Rick

            • 3. Re: Basic color management questions
              Printer_Rick Level 4

              Some of these sound like color theory questions.

               

              In the commercial print world I often hear "why can't we print with RGB?" The reason is pretty simple. R + G + B = White. If you shine white light through a red filter, then a green filter, then a blue filter, you get white light.

               

              If you printed with red, green and blue ink, you would get mud, more or less. So the workaround is the subtractive color model CMY. Shine white light through a Green and Red filter, you get yellow. The other RGB component - Blue - is what is missing. So this is subtracted from RGB white to get yellow.

               

              Logic is the same for Magenta and Cyan. Blue and Red light = Magenta, so Green is subtracted. Green and Blue light = Cyan, so red is subtracted.

               

              In CMY the white component is the base (paper). In RGB the black component is the base (monitor). So if the question "why can't we print with RGB" is one side of the coin, the other is "why can't we have a white monitor?" (maybe one day)

               

              Anyway, the subtractive colors all have empirical opposite colors as I stated - Cyan has Red, Yellow has Blue, and Magenta has Green. If you have RGB filters you can arrive at very crude CMY print components for an image (this is how they did it in the old days, with film and big cameras). Today this can be replicated in Photoshop. Take an RGB image, Mode: Multichannel. The channel data does not change at all, just the channel names. The onscreen color appearance drastically changes, much for the worse. Don't save your image if you do this.

               

              CMY is often confused with the primary colors everyone remembers from grade school. There is common ground, but CMY is not like red, green and yellow paint. One difference is the CMY components are all very light colors. CMY inks are also transparent, and allow other inks to show through them on the same sheet of paper. The color mixing occurs as the inks are printed in sequence (unlike paints, which can be mixed together before being applied to the canvas). You can mix red, green, and blue paints to make all sorts of colors, but if you need light colors, you have to add white. In commercial offset printing white ink is hardly ever used, and it's not designed to mix with other inks in a print sequence (it's opaque).

               

              CMY is very limiting. If it was a perfect world, all we would need is CMY and we could print anything. The problem is the paper is never white enough, and the inks are never vibrant enough, or even dark enough. Well, they addressed the dark problem with black. CMY together do make black, but it's a lot of ink, and you still end up with an impure black. So the black ink (in a traditional color separation) is added to accentuate the darker areas of images, where CMY falls short.

               

              Why not CMYB? Long ago black was referred to as the key color. A lot of history there, but it is helpful to think of black as the "key" in a CMYK image. I mentioned that you can build an image with CMY, and add black to accentuate shadows. But what if I tried a different approach? I could make a black and white image using black ink, and use CMY to add color to that. Today this is done quite a bit - the advantage is a LOT less ink is used. The disadvantage with making black the foundation of a color image - the quality usually isn't as good. But if it's a cheap newsprint job the ink savings can be worth it.

               

              SarahOwens25 wrote:

               

               

               

               

              Why is the difference in the process of color formation of major concern with the use of computers in the preparation of materials for 4-color process color printing?

               

               

               

               

              This is the one question where you are steering towards color management. Perhaps you are working on a print design project?