hi, i had the same issue and i’ve figured out a way to do the-witness-style grass.
You need to point grass material* normal vector* to the sun (or just up). Thus it will always be uniformly lit (unless your light source goes under horizon). To do so you need to uncheck “tangent space normal” box (it will allow you to direct normals independently of actual surface angles) and to connect a constant vector (0,0,1) to the “normal” pin. (0,0,1) is blue color, which implies that your normal is pointed up along z, and since the “tangent space normal” box is false, that normal vector will be pointed along world coordinate z.
Nice thing is that the grass will still recieve shadows.
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