Sometimes you want to blend 2 different textures using the mask which is a black and white image (dirt maps or texture masks) *see B. The way I usually did is to select one of the channel which has better contrast, drag it to "create new channel" to create the alpha, and use that alpha (ctrl+click the alpha thumbnail) to create the selection (indicated by marching ants), and then apply that selection to A. Well this method has disadvantage if you want to use texture which has color information for example. I don't know if this method I'm gonna show is the right one, but it achieve the result I wanted.
- Click the B. , ctrl+a to select all, ctrl+v to copy
- Go to A. , create the "add layer mask", click the white area *it must be selected otherwise you won't see alpha channel
- Go to channel Tab, click on the alpha layer and make it visible.
- Make sure the layer is selected, and the eye icon is on, ctrl+v to paste
- Done, now you need to do is delete/hide the original mask which we copied earlier
Many thanks to Peter my classmate for his help
Update: My GF pointed me out an easier way.
Click on the B/w layer (B), in channel, click the RGB instead of creating an alpha from one of the channel, so basically it will select everything, then apply mask on the (A). As simple as that, how stupid am I.