Quite satisfied! I’m almost sure I won’t be able to achieve PSSM texture shadows with hundreds/thousands of trees on a single scene. I decided to add here some ground shadow using vertex normals CPU computation. Result is a GPU free shadow on the whole map. I still have some issues precalculating the exact position of future trees, and shadows are nothing but blured spots… but it should fit quite enough for distant shadows, and for low-end GPU cards.
At last! Since the beginning I had a crappy diffuse with terrain normals – which induce a kind of face pixelization at far distance. Today I decided to completely cleanup my face normal process in order to add smoother transitions. That’s nothing but a normal interpolation (yep, I still do love interpolation), but the real stuff was to calculate vectors average on tile borders when further faces don’t exist yet