title: How is a "wipe effect" intellectually accomplished, animated DIRECTIONAL fade-in from left to right
body: <p>I don't understand what a "wipe effect" or DIRECTIONAL fade-in is "intellectually", what's happening when we see someone "do it."  To provide some context, here is my graphics environment:</p>
  
  <p>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).</p>
  
  <p>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 (or right to left), over the full background below. </p>
  
  <p>Layer 1: this will be whatever full background image I want.</p>
  
  <p>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".</p>
  
  <p>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.</p>
  
  <p>So, I can "write the code", I'm not looking for any "handouts" there; I'm struggling to intellectually see what I need to do, to produce this rather common type of animation. </p>