Functions for getting the index and UV for a cube face struck by a direction vector originating inside the cube (i.e., cubemap coord to texture index and UV).

cubeHelpers.h.glsl

int GetCubeFaceIndex(vec3 dir)
{
  float x = abs(dir.x);
  float y = abs(dir.y);
  float z = abs(dir.z);
  if (x > y && x > z)
    return 0 + (dir.x > 0 ? 0 : 1);
  else if (y > z)
    return 2 + (dir.y > 0 ? 0 : 1);
  return 4 + (dir.z > 0 ? 0 : 1);
}

vec2 GetCubeUVFromDir(int faceIndex, vec3 dir)
{
  vec2 uv;
  switch (faceIndex)
  {
  case 0: uv = vec2(-dir.z, dir.y); break; // +X
  case 1: uv = vec2(dir.z, dir.y); break;  // -X
  case 2: uv = vec2(dir.x, -dir.z); break; // +Y
  case 3: uv = vec2(dir.x, dir.z); break;  // -Y
  case 4: uv = vec2(dir.x, dir.y); break;  // +Z
  default: uv = vec2(-dir.x, dir.y); break;// -Z
  }
  return uv * .5 + .5;
}