If you are able to use UV mapped textures, you could also try SDF's generated in Substance Designer (if you have that as well).I think you would get best results if you set Distance Node to "Manhatten Distance". It's not 100% perfect but this is extremely fast for iteration since the process is completely automated and you…
BTW, even though many solutions have been proposed ... It's still not too late to show a practical example of what you are *actually* trying to do with this mask. Also we don't know if you are limited to a texture only, if you have access to edit the geo, and so on. This actually sounds a lot like one of the many tricks to…
Vertex color (either in a 3D software, or the equivalent as 2D vector) is pretty much exactly what you describe - by assigning pure white to the verts around the border, and pure black to the verts of the middle line :
Something to consider with @supaclueless and @"Eric Chadwick" approaches are that you get a constant 'speed' towards the outer edge. I've stepped through a similar distance field here as an example of how it might look when animated Pior's approach will give you a constant 'time' to reach the edge (ie. speed will vary with…
You guys are the best! pior - Thanks very much for the idea and for taking the time! That's kinda what I had in mind when I mentioned geometry, more or less. I'd rather not pursue that method because it's a bit labor intensive if I want this effect to be on a character, let's say, which has complex geo that I don't want to…