HOW TO | FIND YOUR way around Layer Masks in GIMP

1 Adding a Layer Mask

Layer Masks let you cut part of an image without damaging the layer. It creates an editable mask (represented by a black and white thumbnail in the layers palette) of the area you want to cut using an alpha channel. This is then editable at any time and doesn’t damage the layer or the one below. To apply a mask, you must first have a layer to apply it to (the background doesn’t count), so duplicate the background layer using the contextual menu in the layers palette. Select your new duplicate layer and click the contextual menu again, choosing Layers > Add a Layer Mask.

2 Creating and editing your mask

Once you have created a Layer Mask, you can choose which kind of Layer Mask you want. Create one that completely hides your layer (full opacity), or completely shows it (full transparency). You can also choose to make a Layer Mask based on a selection or from an existing alpha channel, which saves you having to make a new selection. We’re going to choose full opacity, which shows everything. If you look in the layer palette next to your highlighted layer, you see a white square appear. This is your Layer Mask. We will now hide the original layer for our editing.

3 Creating a transparent background

Edit your Layer Mask by painting in areas of black or white to denote the area you wish to keep. Your foreground and background colours default to black and white. We’re going to select our flower using the Intelligent Scissors tool, and once we have created a marquee of marching ants, we’ll invert the selection using Select > Invert. With the background selected, we’ll fill it with black using the Fill tool (the paint bucket in the Tools palette) to in order to reveal just the flower. If you look at the thumbnail of the mask, you see which areas are selected.

4 Fine-tuning your mask

Our mask still needs fine-tuning, though you can edit the mask with as much finesse as you require. Use any of the brushes to paint in your mask. When editing, make sure you have the mask selected in the Layers dialog and not the image, or else you’ll be painting black and white marks onto your image instead of your mask. The flower has now been cut out from the background without damaging the background or the new layer. We can now edit the background (turning it greyscale, or blurring it, for example) without affecting the top layer.