Gary Dunning

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)

Saturation masks can also be created with the 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)

To remove white matte, use Apply Image with the following settings: DR = 1 - (1 - SR) / SA DG = 1 - (1 - SG) / SA DB = 1 - (1 - SB) / SA DA = SA

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.