Basically, a layer mask is a mask that you place on your paint layer. This will literally mask areas of the layer, so that the content underneath shows through. You can paint on it with greyscale colors: the more black the color, the less the layer under it will shine through, the more white, the less the layer under it will be shown. So complete white will let nothing through, complete black will let everything through. Basically, it is a bit like selecting a piece of your image, and then cutting it, so that the selected bits go away. So what is the use for a mask here? The big advantage is that it is non-destructive: if you decide that you masked out the wrong part of your layer, you can easily remove the mask and start anew, something a lot harder (not to say near impossible, especially in between sessions) with regular selection-cutting.
So, how to create a mask? There are 2 ways:
Start from scratch. ->->. The mask starts with everything being retained, that is, a complete white mask. Basically you will not see any changes as long as you do not paint on it.
Start from the current selection. ->->. The selectedness will be converted to whiteness. This means that fully selected area will be visible, fully unselected areas will be invisible, and the rest will be partially visible, depending on how much the area was selected.
First, make sure you are editing the mask, not the layer, by making sure ->-> is checked. (This is checked by default.) Then you can paint on the layer just like before, only now you are painting on the mask, instead of on the layer itself. To stop painting on the mask, you can uncheck the Edit Mask checkbox. There's also the option to show the mask, through checking ->->. (This is not checked by default). This option will render the entire layer as a visual representation of the mask in greyscale, instead of the actual layer. This can be handy to see where your mask is, but it might be not as handy when you want to edit it, since you cannot look at the actual layer.
Other actions: you can also remove the mask if you are not satisfied with it, and want to start over again, or just want to remove it, with ->->. You can also “apply” the mask, meaning that the mask will be made permanently. This means that the mask is removed, but that its effect of transparency will be committed to the layer.
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