Is there an easy way to invert the transparency of a layer - that is, lock the content so that only the transparent areas can be painted? This seems much more useful to me than the preserve transparency function.
Possibly, but I am not sure of the steps...What I tend to do (which I_think_provides the same result) is this alternate approach:
1. Ctrl/Cmd+Click on the layer thumbnail in the Layers panel to select layer content
2. Ctrl/Cmd+I to invert the selection
3. Paint will be in the transparent areas of the layer
Not sure if that helps but I thought I'd share...
thanks for this. I was hoping that there would be a more direct way of doing this.