Hi,
Just a quick update. I did a small test today and wanted to share the results.
I managed to create the effect directly in the material so that it appears along the edge of the first shadow layer. The color can be adjusted, and the thickness depends on how soft the shadow transition is. In other words, the more blurred the shadow edge is, the wider the effect becomes.
At the moment, I've only tested it in one area, but since it's working, the next step is to connect it to the rest of the shader parameters.
