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Separating Colour Components

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  • Separating Colour Components

    Hi Richard,

    I have read the first 5 chapters of HPPE3 and am now going back to read them again and hopefully this time take in more than I did the first time. Right away I found a question regarding the separation of colour components on page 46. In step 5, you state that we can turn the red light into a channel by adding equal parts of blue and green. I am having a problem trying to get into my head why it is necessary to add the blue and green. I thought a red channel would have the red information from each pixel shown as a grayscale tone (from 0 to 255). Is it necessary to have the green and blue to make it a gray colour? Why is it necessary to have equal amounts of green and blue? Is that just the way it works?

    I know this is very basic but your book is written in a manner that if I do not understand a concept, there is really no point in going on because everything just builds on previous concepts.

    Thanks in advance for your help.

    Art Cartmell

  • #2
    Well, yes, it is pretty good to understand the concepts before moving on...maybe not imperative, but important. The better you get earlier concepts, the more likely you will grasp the latter ones.

    There is more than one way to do this conversion to grayscale. The idea of the change is that you want to get the grayscale results -- minus the color. At one point in the book, RGB light theory is mentioned...white is the culmination of mixing red, green and blue lights. The way that I thought made the most sense in showing this conversion, and for working with other concepts in the book is that grayscale is represented in RGB by pixels with equal intensity of red, green and blue...0,0,0, is black/grayscale, 34,34,34 is dark grayscale, 128,128,128 is medium gray. So duplicating the component, and changing the color of it (not the tone), and adding the three components renders the component content. Even amounts of red, green and blue light in every pixel renders a grayscale result.

    Another way to do the same thing is this:
    1. filter out the Red component
    2. Open Hue/Saturation
    3. Choose Red from the drop list and move the Lightness slider all the way to the right.

    You are just taking the red areas of the component and lightening them (actually adding in more green and blue). It might be easier to do, but I don't know that it will conceptially make more sense.

    As far as taking the component and trying to do another luminosity and color separation...that won't work. You'll end up with something much darker than the needed result--a faithful rendering of the component as it would appear as a full-color image.

    That is part of the can't look at the component like a full-color image. The component is like seeing only one-third of the puzzle: wrapping red cellophane around your head so that everything around you is filtered. There is no white in the filtered world, just a purer red. The only way to remove the filteringis to add back the blue and green that the filter leaves behind.

    Does that help any?


    • #3
      Thanks, Richard, for your patience as a newbie tries to make sense of this whole thing. I understand now that gray is represented by equal amounts of Red, Green and Blue, all the way from 1,1,1 to 254,254,254 and an individual pixel with components of say, 123,45,78 would be represented in the red channel as 123,123,123, in the green channel as 45,45,45 and in the blue channel as 78,78,78, a gray tone in each case but defined by the individual colour component. If we just have the red filter applied, then the pixel would be seen as 123,0,0 which would be a red colour, not a gray tone.

      I'm not sure if I have expressed things properly but it seems to make sense to me now. Your example of just wrapping red celophane around my head seemed to help me grasp the concept. I did not get my head around the idea of using Hue/Saturation to lighten the image and thus adding the blue and green back in but I am not worried about that at the moment. I will mull that over and one day it will be clear.

      I look forward to the day when all the pieces of the puzzle fit together and it will all seem so obvious.



      • #4
        That puzzle will probably be one you will work on for a good long time. with probably 35 years of photography (most of it for fun) and 15 or so full-time editing (probably 5 more with serious computer use and graphics involvement), i still find new stuff daily to think about. You get better and know more...I don't know if you ever know it all.


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