Basic Alpha Erosion
The key to basic erosion is the order of operations on the Subtract node and to clamp it afterwards. The texture you want to erode from should be in the A input of the Subtract node, the erosion amount should be in the B input. Below, we invert (OneMinus) the amount because of how we want to graph that in the Particle Editor. Additionally, we clamp the result in the 0-1 space so that errors cannot form.
Example: Subtract the Particle Color.alpha you want from the dissolve texture you are using → Multiply against a "Dissolve_Scale" parameter (default to 1) → Saturate → Depth Fade → Opacity.
Basic Alpha Erosion Method 2 (Lerp)
This method for alpha erosion can be used instead of subtraction. It is useful because it allows for extra steps that can be done, but is also generally cheaper than the subtraction method. Either are perfectly valid methods.
Advanced Erosion Techniques
The Max node keeps the center bright.
When I do translucent stuff, I run it through a Ceil node after the subtract to make sure I’m getting every bit of value out of my texture, then multiply the Ceil by the original Alpha/Mask/Etc.
Smoothstep: “Returns a smooth Hermite interpolation between 0 and 1, if x is in the range [ min , max ].” This, according to microsoft. Essentially it takes your value and remaps it as if it lay between min and max instead of 0 and 1. To better visualize that, here’s a graph for smoothstep(.2,.4, (x=y));
