LAYER MASKS

INTENTION

layer masks are a fundamental tool in images manipulation. They allow you selectively modify the opacity (transparancy) of the layer the belong to. This difers from the use of the layer opacity slider ask masks has the ability to

  selectively modify the opacity of difrent areas acrosse a single layer.
this modification of a layer's transparancy through a masks is non -destructive to the layer it self.

This flexibility to define the opacity of different areas of a layer is the basis for more interesting image manipulation techniques such as selective coloring and luminosity masking.

• Adding a Mask To a Layer

Layer masks need to be added to a layer befor they can be used. The process the adding them is simple.

For this example I will use a simple image with only tow layers, as shown above. There is base image at the bottom of the stack,and a single layer of teal over it. The teal layer is the active layer(look for the white border) and the one which we will add a layer mask to.

Right -clik on the layer you want to add mask to (the " teal " layer in my example ) add the context menu with show and option to add layer mask..

 you can also add layer mask through the menu :
      
                     Layer -> Mask -> Add layer mask....

This will the bring up " add a mask to the  layer " dialog with some options :

There are many options for initializing the Layer Mask. Notice that the first option is to set the entire mask to White, which will result in full opacity on the layer (no transparency from the mask). The option to initialize to Black shows that the mask will make the entire layer fully transparent.
For the purposes of this tutorial, we will let the mask initialize to White (full opacity). You should notice a change in your Layers dialog now that shows the layer mask thumbnail to the right of the layer it applies to (in this case the “Teal” layer):

The layer mask has now been added to the" teal " layer. it is also active ( there is a white border around the thumbnail in the dialog , but is no visible due to the mask being white as well) and ready for modification.

• Modifying a Layers Transparancy With the Mask

At this poin any opreations performant on the canvas will apply to the mask 
and not to any layer them selves. To ilustration how masks can affect its layer transparancy , lets paint.
I am going to use the Rectangle Select tool to select roughly the top third of the image, and I’ll fill this selection with black.

Using the rectangle select tool, ill select roughly the top third of image:

I want to fill this selection with black , but before I do I need to make sure thet my foreground color is black. Click on the foreground color in the color area to bring up the "Change foreground color " dailog:

The Change " foreground" color " dialog allowsyou to set the foreground color. For this example set the color to black, RGB(0,0,0).

With the foreground color set you can now use the Bucket fill tool to fill in selection.

You can now click inside the selection to fill it with the foreground color (black). As soon as you do, you’ll be presented with a new view of your image on the canvas:

As you can see, filling the selected portion of the layer mask with black resulted in that area having 100% transparency, showing the layer below it.

If you Rectangle Select a different area of the mask, you can fill it in with a different shade of gray to produce a variable opacity. For example, I will select a few different regions of the mask, and fill it with different levels of gray:

 if you examine the layer mask ,you all see that there are different levels of gray being applied (black to white, from top to bottom), and their value is what determines the opacity of the layer.

Ok guys so I can say, thank you for visiting this blog and waut my next posting. See you......


Refrensi : https://www.gimp.org/tutorials/Layer_Masks/