I'm generating terrain in Opengl geometry shader and am having trouble calculating normals for lighting. I'm generating the terrain dynamically each frame with a perlin noise function implemented in the geometry shader. Because of this, I need an efficient way to calculate normals per-vertex based on the noise function (no texture or anything). I am able to take cross product of 2 side to get face normals, but they are generated dynamically with the geometry so I cannot then go back and smooth the face normals for vertex normals. How can I get vertex normals on the fly just using the noise function that generates the height of my terrain in the y plane (therefore height being between 1 and -1). I believe I have to sample the noise function 4 times for each vertex, but I tried something like the following and it didn't work...
vec3 xP1 = vertex + vec3(1.0, 0.0, 0.0);
vec3 xN1 = vertex + vec3(-1.0, 0.0, 0.0);
vec3 zP1 = vertex + vec3(0.0, 0.0, 1.0);
vec3 zN1 = vertex + vec3(0.0, 0.0, -1.0);
float sx = snoise(xP1) - snoise(xN1);
float sz = snoise(zP1) - snoise(zN1);
vec3 n = vec3(-sx, 1.0, sz);
normalize(n);
return n;
The above actually generated lighting that moved around like perlin noise! So any advice for how I can get the per-vertex normals correctly?