Preserving a Colour Range Within a Shift


You often need to preserve a colour range from within a broader map into another colour. Using the previous image I decided to preserve the sky to demonstrate the technique.


It is necessary to add quite a few control arrows to achieve the result. However if you identify the range to preserve using the medicine dropper, then start adding control arrows either side of this, you will find that they all map to the same point and it is difficult to separate the output points and move them to the desired destinations. Much easier to remove one of the control arrows that is concentrating the image into a single colour, and then add the required control arrows, then reinstate the deleted control arrow.



An intermediate transform window would look a little like this after re-instating the deleted arrow. The sky colour is the peak in the input histogram, immediately between the 4 anchors that I have added. The inner anchors will become the sky protectors and the outer anchors will map the tones next to the sky back to green. In fact I discovered after setting the transform to green that I needed extra anchors to control the move, so added a pair more anchors as well.



For this transform I have dispensed with the original, preferring to show an enlarged transform window. Note the two pairs of anchors around the sky. The outermost control arrows, almost coinciding with the outermost anchors, map to the green destination colour. The sky is back and so are the blue reflections in the bike. It is sometimes necessary to add more than 2 pairs of anchors to limit strong or radical shifts.


A small modification of this technique allows you to shift a range of colours to one destination and instead of preserving a contained range of colours, shift them to a different destination. However you may find the colour correction transform better for this, although it introduces an extra step in the manipulation and prevents you from fine tuning saturation and V at the same time.