The Advantages of Masks and How They Work
When a mask is applied to a layer, the mask makes parts of the layer transparent so that any layers below show through. This can be an effective way to combine two or more photos to produce a final image that combines elements of each of them. However, it can also open up the ability to edit areas of a single image in different ways to produce a final image that looks much more striking than if the same image adjustments had been applied universally to the whole picture. For example, in landscape photos, you could use this technique to darken a sky at sunset, so that the warm colors don’t burn out while lightening the foreground. You could achieve similar results of combined layers by deleting parts of the upper layer rather than using a mask to make areas transparent. However, once part of a layer has been deleted, it cannot be undeleted, but you can edit a layer mask to make the transparent area visible again.
Using Layer Masks in GIMP
The technique demonstrated in this tutorial uses the free GIMP image editor and is well suited for a range of subjects, particularly where the lighting varies significantly across a scene. It shows how to use layer masks in a landscape image to combine two different versions of the same image.
Prepare a GIMP Document
The first step is to prepare a GIMP document that you can use to edit specific areas of an image. Using a photo of a landscape or similar that has a very obvious horizon line will make it easy to edit the top and bottom portions of the image so that you can see how this technique works. When you are comfortable with the concept, you might try applying it to more complex subjects. If you’re not too confident with GIMP’s adjustment tools, use the Channel Mixer mono conversion technique to prepare a similar GIMP document.
Apply a Layer Mask
We want to hide the sky in the top layer so that the dark sky in the lower layer shows through.
Fine Tune the Join
It may be the case that the top layer is a little brighter than the bottom layer, but the mask has obscured it. This can be adjusted by painting the image mask using white as the foreground color. Select the Brush Tool, and in the Tool Options, select a soft brush in the Brush setting. Use the Scale slider to adjust the size as required. Try reducing the value of the Opacity slider also, as this makes it easier to produce more natural results. Before painting onto the layer mask, select the small double-headed arrow icon next to the foreground and background colors to make the foreground color white.Select the Layer Mask icon in the Layers palette to ensure that it is selected and that you can paint onto the image in the areas where you want to make transparent parts visible again. As you paint, you will see the Layer Mask icon change to reflect the brush strokes that you are applying, and you should see the image changing visibly as transparent areas become opaque again.
