03 Switch to the Mask tool
Now, with that top layer selected, I can click the Mask module at the top right of the screen. The left toolbar now displays a selection of masking tools, and I can use the ‘-‘ (minus) brush icon near the top to paint a squiggle over the blank sky. I don’t need to go right up to the edges of the aircraft because Perfect Mask will do that for me, based on the tones I’ve selected.
04 Check the edges
Well that was that easy! In fairness, this was the perfect candidate for a cutout – a clearly-defined object against a plain, contrasting background. But even though the results may look good from a distance, you always need to zoom in to 100% to check the outline of the mask.
Here, you can see that the mask has left a purple-blue outline around the aircraft. It’s not the software’s fault – these edge ‘halos’ are a fact of life with most digital images and they’re caused by a combination of chromatic aberration and image processing/sharpening artefacts. The software can’t select them because they’re a completely different tone to the background.
But that’s where the Adjustment panel comes in – you can see it here, and you activate it with the button in the Adjust Mask panel on the right…
05 Using the Grow/Shrink slider
The solution is really simple. I can use the Grow/Shrink slider to shift the edge of the mask a few pixels further inwards. 4 pixels is perfect here – it gets rid of the coloured fringe around the wings without removing too much of the fine detail like the tail wheel and radio aerial, and I’m adding a Feather of 1 pixel just so that the edge of the mask isn’t too hard – it needs to have roughly the same degree of softness as the detail in the aircraft.