adding texture to apha channel

[Ofnuts]

To edit the alpha channel of a layer:

• Layer>Mask>Add layer mask and initialize to Transfer layer's alpha channel.
  
• In the Layers list you will see two thumbnails for the layer, one is the layer (without alpha channel) and the other one is the mask
  
• Click on either to designate the targets of your edits.
  
• The status bar at the bottom of the image window tells you if you are editing the layer (layer_name) or the mask (layer_name mask)
  
• If the mask is the active one when you paste, what you paste will be merged in the mask when you "anchor" the paste.