A Layer Mask is linked to a layer and hides part of the layer from the picture. What is painted black on the adjustment layer will not be visible in the final picture. What is grey will be more or less transparent depending on the shade of grey. As the layer mask can be both edited and moved around independently of both the background layer and the layer it applies to, it gives the user the ability to test a lot of different combinations of overlay.
[pP]>mature old man
This picture consists of a blue background and on top of that a layer of conifers cut using a layer-mask in the shape of a seagull.
[pP]>mature old man
The blue sky used as background layer.
[pP]>mature old man
The greenery used as top layer.
[pP]>mature old man
The layer mask that was applied to the top layer.[pP]>mature old man