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Gary Dunning

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Posts posted by Gary Dunning

  1. The purity of a colour can be computed by converting from RGB to HSV and then multiplying S by V (which is effectively what the Selective Colour adjustment does), where S and V are defined as follows:

    min = min(r,g,b)

    max = max(r,g,b)

    S = (max - min) / max

    V = max

    Therefore,

    S * V =  (max - min) * max / max = max - min

    That's why the same effect can be achieved with Affinity Photo's Apply Image filter.

    DR = max(SR,SG,SB) - min(SR,SG,SB)

    DG = max(SR,SG,SB) - min(SR,SG,SB) 

    DB = max(SR,SG,SB) - min(SR,SG,SB) 

  2. To create a saturation mask, use the selective colour adjustment as follows:

    1. Deselect relative.
    2. For the red, yellow, green, cyan, magenta and blue colours, set all the sliders to -100.
    3. For the whites, neutrals and blacks, set all the sliders to +100.

    N.B. This solution requires Photo 1.7.

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