Updated: a day ago
Masks are easy. The most difficult thing about them is finding decent images to use them with! Fortunately, I’ve got a couple of examples that work pretty well.
These two ‘beach’ pictures were taken within seconds of each other. As you can see, the top right half of image one is pretty bland; so is the bottom right half of image two. Using a Layer Mask, they can be blended together.
The first thing to do is to put both images in the same frame. Select the Move tool, right at the top of the toolbox. Hold down the shift key and click and drag image two into the frame of image one. The Move tool allows it to happen, and the shift key means the incoming image is centered in the frame of the target image. And, as they’re both exactly the same size, they fit the frame perfectly.
Select the top layer, layer 2, and click on the ‘layer mask’ icon at the foot of the Layers window. That puts a white layer mask to the right of the image, on the same layer.
A white layer mask means it’s 100% opaque, so you can’t see the image on the layer beneath it. But, if you painted on it with black or grey, those areas would either be completely or partially, tranpsarent. So, if you paint it with a gradient going from black to white, it will allow the two images to ‘fade’ together.
Choose the Gradient tool, with a linear gradient. Then press the letter ‘D’ on the keyboard, which sets black and white as the ‘foreground’ and ‘background’ colours. Click and drag from one end of the arrow to the other. If black is the colour on the left of the gradient, like mine, the image under bottom of the arrow will become transparent, fading to solid at the other end. If you don’t like the result, just click and drag again. There’s no need to ‘undo’, as you’re re-painting the entire mask every time you click and drag.