I don't understand what a "wipe effect" or DIRECTIONAL fade in is "intellectually." To provide some context, here is my graphics environment:
Layer 3: if needed, I can use too (please note the runtime graphics environment only allows "simple addition" of the layers, that's all. I use GIMP to make images, the open source similar program to photoshop, but I don't care the tool, I'm tring to understand it intellectually).
Layer 2: this will be a smaller PNG image I want to do a "wipe effect" or DIRECTIONAL fade in, e.g. from left to right, over the full background below.
Layer 1: this will be whatever full background image I want.
This environment (the canvas in html) allows adjusting the opacity (or alpha) value of the entire layer, and I have the code to do this over time (that's easy). My problem is, I don't understand "intellectually" what I need to do, to give the appearance of something (a png) image fading in "left to right".
E.g. if I were to create a gradiated black to alpha layer 3, I can do that too, but I think unless I make it exactly match the outline of the graphic I'm seeking to fade in, it will equally "disturb" the background, right? So ... how is this done "intellectually", please? Do I even need 3 layers, or is 2 fine? Thanks.
Related Question: is it possible to produce a gradient, in a tool like GIMP, that only affects the alpha channel, and does NOT introduce a color? Because then if I have that, I could theoretically change that whole layer from being full opaque to full transparent over time.