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Grokking The Gimp
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4.2.2 Manipulating the Layer Mask

As was first described in Section  2.1.1, the active layer in the Layers dialog is highlighted in blue. When the active layer consists of an image layer and a layer mask, only one of these two components can be active at a time. Operations performed in the image window affect the active component of the layer, either the image or the layer mask. The layer mask   is made active by clicking on the layer mask thumbnail. The active focus is returned to the image by clicking on the image thumbnail.

You can determine whether the image or the layer mask is active by a white border drawn around the appropriate thumbnail in the layer strip. However, if the layer mask is white, it may be difficult to tell that there is an additional white border around it. Carefully examine Figure  4.12(b). Here it can be seen that the image thumbnail does not have a white border and that the layer mask thumbnail looks a little taller than the image thumbnail. This is a clue that the layer mask is active. Changing the active focus from the layer mask to the image by clicking on the image thumbnail is shown in Figure  4.13.

  
Figure 4.13: Toggling from an Active Layer Mask to an Active Layer Image
Figure 4.13

It can be seen that the image thumbnail now has the white border.

A layer mask is a direct way of editing an image layer's alpha channel.  Thus, performing edits on the layer mask changes the opacity of the corresponding image layer. This is illustrated in Figure  4.14.

  
Figure 4.14: Editing a Layer Mask Edits the Image Layer's Alpha Channel 
Figure 4.14

Figure  4.14(b) shows that the image consists of a single layer with a layer mask. The layer mask has been constructed so that the white part of the mask corresponds to the bird shown in Figure  4.12(a) and the black part corresponds to the rest of the image.

Figure  4.14(a) shows the effect of the layer mask; it makes the region around the bird's body completely transparent. It is important to note that the background part of the image has not been destroyed; it just can't be seen due to the modified values in the alpha channel. Section  4.3 describes in detail how a layer mask can be edited to produce a result like the one shown in Figure  4.14. However, before doing that, several additional features of layer masks must be described.

It is often useful to be able to examine the layer mask without the visual interference of the image layer. This can be accomplished by Alt-clicking  on the thumbnail of the layer mask. Doing this toggles off the visibility of the image layer in the image window, leaving just the layer mask. In addition, the outline of the layer mask thumbnail in the Layers dialog becomes green. This is illustrated in Figure  4.15.

  
Figure 4.15: Viewing Only the Mask in the Image Window
Figure 4.15

The mask seen in the image window is shown in Figure  4.15(a), and the green border around the layer mask thumbnail is shown in Figure  4.15(b). The effect can be toggled off by Alt-clicking on the thumbnail a second time.

Similarly, it can be useful to toggle off the visual effect the layer mask   has on the image. This is done by Control-clicking on the layer mask thumbnail. When this is done, the effect of the layer mask on the image is toggled off, and the border of the layer mask thumbnail becomes red. Figure  4.16

  
Figure 4.16: Suppressing the Effect of the Layer Mask in the Image Window
Figure 4.16

illustrates this. The full image can now be seen in Figure  4.16(a) and a red border can be seen around the layer mask thumbnail in Figure  4.16(b).

A layer mask can be converted to the image layer's alpha channel by choosing the Apply Layer Mask   function from the Layers menu. This is not normally necessary, though, because the action of the mask is the same whether it remains a mask or is converted to the alpha channel. However, the Apply Layer Mask dialog also permits the layer mask to be discarded. This is the only way to remove a layer mask. Applying a layer mask by converting it to an alpha channel destroys the mask and removes it from the Layers dialog. It can be recovered, though, by choosing Add Layer Mask from the Layers menu and choosing Layer's Alpha Channel  as the mask option.

Finally, a layer mask can be converted directly to a selection. This is done by choosing Mask To Selection   from the Layers menu. This operation creates a selection by mapping the white pixels in the layer mask to selected pixels in the image window, black pixels in the layer mask to unselected pixels, and gray pixels in the layer mask to partially selected pixels. Note that applying the mask as a selection does not destroy the mask.




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