For a soccer game I want to add the chalk lines to the grass as a decal. Unity has a decal shader but it doesn't handle real-time shadows. I would rather keep the chalk and grass textures separate so would anyone have written a decal shader with real-time shadow support or has an idea how I could get shadows without combining textures? Btw, I can't use a transparent/cutoff shader as lines use alpha for fading/blur.
Render the chalk as different layer and composite them. If you don't want to render it as a different layer, I would recommend downloading the shader source code and tweaking the alpha blending formula. You can use the alpha blending to do much more then blend with transparency.
I made a whole bunch of shaders for doing decal type shading that account for lighting and underlying material using these techniques.