Raphaël Quinet (quinet@xxxxxxxxxx) wrote:
Your example is fine, except for the last step using Noisify on the alpha channel. As Adam pointed out in his previous messages, the correct way to acheive the same effect would be to use Noisify on a layer mask, not on the alpha channel.
Be careful: A layer mask can *not* do everything you could do with manipulating the alpha channel directly. Especially it is impossible to increase the opacity of the layer with a layer mask.
Imagine a blurred circle on a layer and you want to make the transition to the transparency non-linear, e.g. more like a cosine. Basically this is impossible with a layer mask, because it cannot increase the opacity in some areas. Also - if we talk about more complicated shapes - it might be very tedious to manipulate the layer and the layer mask synchronously.
Of course this example can be done very conveniently with the curves tool on the alpha channel, but you have to make sure that accidental changes to total transparency do not throw away the color information until the tool is finished.
A layer mask is not a substitute for manipulating the alpha channel directly!
If you were to do something like this, where you wanted to have control of the full range of opacity in a layer mask, then the first mistake you made was to add alpha to the image when you should have added a layer mask.
In this situation it is best to remove all alpha from the image (or your roi), and use just a layer mask.
If you are going to disallow the editing of alpha, then you will probably want to have a way to extract alpha information into a layer mask (at the same time, removing alpha (or setting it all equal to 1.0) from the image) in case someone is handed a merged image. You should combine this operation with an edge detection scheme so that you can exclude removing alpha when alpha is actually being used to describe coverage instead of transparency.
-- Dan
