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RGB model

Posted on 1998-03-17
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Last Modified: 2012-05-04
I wanted to know what relation betwen red, blue and green in the RGB model gives me all the variety (density) for the blue color (cyan, solid blue,....)
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Question by:ciri
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5 Comments
 
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Expert Comment

by:EbenBrooks
ID: 1112265
I'm not sure what you mean for "all the variety," because there are a great number of shades of blue between Cyan (R0,G255,B255) and Purple (R255,G0,B255). A pure blue would be R0,G0,B255), and a dark blue would be R0,G0,B127. What precisely are you looking for?
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Author Comment

by:ciri
ID: 1112266
I would like to get all the shades of blue (including the extremes: cyan purple, pure blue...). I hope it was possible using an equation that connect all the variables (red, blue and green). I have an application that loads an image. In this image I want to distinguish the portions in blue, but not a concrete blue (I want all the shades of blue). If I set in this variables, for example, R0, G255 and B255, I only see Cyan. My code is (in C) like the following:

  for(y=0;y<heigth(image);y++)
  {                                                     //two sentences "for" cover all the image
       for(x=0;x<width(image);x++)
      {
           if ((pLine[x*4+RGBA_RED] == 0)
              &&(pLine[x*4+RGBA_GREEN] ==255)
              &&(pLine[x*4+RGBA_BLUE] == 255)) //cyan
          {
                                       //show this pixel
                                      //with this code I only show cyan
                                     // i want a relation tha let me show all the shades of blue
                      }
      }
  }

Thanks.
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Author Comment

by:ciri
ID: 1112267
Adjusted points to 60
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Accepted Solution

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EbenBrooks earned 60 total points
ID: 1112268
Oh! I see what you mean.

For a color to fall into the range you specified, the value of Blue must always be equal to or greater than the values of Red and/or Green. If Blue is equal to the value of either Green or Red, then it CANNOT be equal to the other!

For instance: R254, G254, B255 is a shade of blue. It's very, very close to white, and may be indistinguishable by the human eye. At the same time, R254, G255, B255 is a shade of cyan of similar brightness.

If you want to make sure you're only including the values of blue that are distingushable as blue to the human eye, do this:

The value of Blue must be equal to or greater than the values of Red and/or Green.

If the value of Blue is equal to the value of either Red or Green, then the value of the other (Red or Green) must be 20-30 points below the value of Blue.

This should give you all the visible values of blue.

Hope this helps!

--Eben
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Author Comment

by:ciri
ID: 1112269
Your answer is excellent!!
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