HOW TO...
Improve your photo editing using layer masks
By Nik Rawlinson
What you need: GIMP Time required: One hour
The last time you painted a room, how did you stop your wall colour getting on the ceiling? Or the gloss of your door frames straying into the surrounding emulsion? You probably used tape to mask off those areas. Layer masks in image editors like GIMP, Photoshop and Affinity Photo work in much the same way, letting you control which areas are affected by a new layer of pixels. Here, we’ll explain how to use layers in the powerful free image editor GIMP (www.gimp.org). You can then easily transfer what you’ve learned to any other image editors you may use.
1 Make thumbnail image easier to see
Open GIMP then press Ctrl+O, select a photo and click Open. When the picture opens in the main window, you’ll see a matching thumbnail in the sidebar’s Layers panel. Start by making this larger so you can more easily see what’s happening when you apply a mask. To do this, click the configure button ( 1 in our screenshot below), then hover over Preview Size and click Huge on the menu that appears.
Click the uppermost of the two coloured squares 2 and change it to a bright colour (we’re using red) in the box that opens. Now add a new layer by clicking the Create button 3 . Make sure ‘Fill with’ is set to ‘Foreground colour’ then click OK. Your image will disappear, and you’ll now have two layers in the sidebar: the new red layer on top, and your original image below.
Click the Mask icon ( 1 in our screenshot below right) and when the menu appears, select ‘Black (full transparency)’ 2 and click Add 3 . Your red layer will disappear entirely in the main image window but, if you look in the layers panel, it’s still there, alongside a new black square. So, what happened?