Legacy ImageMagick Discussions Archive

• c1 = 0
  c2 = c1 + 17
  c3 = c2 + 17
  ...
  c13 = 255

convert -size 13x1 xc: -draw '
      fill #FFFFFF rectangle    0,0 13,1
      fill #6EA1FF rectangle    1,0 13,1
      fill #3770DF rectangle    2,0 13,1
      fill #0040BF rectangle    3,0 13,1
      fill #FC85F8 rectangle    4,0 13,1
      fill #FD42F7 rectangle    5,0 13,1
      fill #FF00F7 rectangle    6,0 13,1
      fill #FA8484 rectangle    7,0 13,1
      fill #FC4242 rectangle    8,0 13,1
      fill #FF0000 rectangle    9,0 13,1
      fill #FAAF84 rectangle    10,0 13,1
      fill #FC9142 rectangle    11,0 13,1
      fill #FF7300 rectangle    12,0 13,1' grad.png

convert 01.png grad.png -interpolate Integer -clut 01Clut.png;

All the examples for the testing CLUT handling is shown in http://www.imagemagick.org/Usage/color_mods/#clut

This shows how the range of values is scale and then used as a diagonal position across the CLUT image for the lookup. For a single row or column of pixels that naturally equates to being equivalent to the longest dimension.

For example 13 colors -> image that is 13 long with values going from 0 to 12

A color value of 50% grey -> 0.5 whcih is then scaled by 13 -> 7.5 or the very middle of the image in 'image coordinates' (as opposed to pixel or drawing coordinates).

The position for lookup in the CLUT image is naturally a floating point number, but colors only exist in integer units., which is why the -interpolate setting is used to determine that actual color used.

Integer mean always round down the position to the lower integer value, so 7.5 -. 7.0 or the pixel color at pixel number 7 (the 8th color in the list of colors creating the image)

Nearest neighbour rounds the value to the nearest integer. which for 7.5 would be 8, and result in the color of the 8th pixel in the clut image or 9th color in the color table used to create that image.

Note interpolated lookup is also the lookup used by -fx equations, and in fact -clut lookup were originally developed from -fx equations.

That is the color in source image used to lookup colors from the second image by position within that image.


WARNING: Interpolated color lookup can also involve virtual pixels! The default setting of virtual pixels (the pixels that surround the image proper when doing actual lookups) defaults to 'edge', which basically removes its effect for more interpolation methods, though it does have an effect for a 'cubic' or 'spline' interpolation, in the gradient curves generated.

Setting -virtual-pixel to something other than 'edge' however can have very strange effects on colors looked up from a clut image, especially single row and column clut images! For example I theorize that using -virtual-pixel white, using one of the larger 16 pixel interpolation methods, will 'tint' the color lookup at both ends of the CLUT image, specifically single row-column CLUT's.

Also as the color lookup is restricted to a diagonal across the CLUT image (which allows IM to use either a row or column CLUT image without change), means in a image that isn't using a single row or column of colors, you can get non-uniform color lookups as the diagonal crosses different pixel areas. This probably can be seen best using integer lookups.


See Interpolation
http://www.imagemagick.org/Usage/misc/#interpolate

And Virtual Pixels
http://www.imagemagick.org/Usage/misc/#virtual-pixel