3D, step 5: which way does the surface face?

You can now find the spot where each ray touches the sphere: pos = ro + rd * t. To light it we need the normal: the direction the surface faces at that spot.

For a sphere centred at the origin the answer is easy: the surface faces straight away from the centre, so the normal is just the position, normalized: n = normalize(pos).

Show it as a colour with the normal-colour trick: 0.5 + 0.5 * n. (You saw the same picture in the fake-3D sphere lesson, but this time it comes from a real ray hit.)

float3 col = float3(0.05, 0.06, 0.1);       // background
if (t < 12.0)                               // the ray hit something
{
    float3 pos = ro + rd * t;               // the spot it hit
    float3 n = normalize(pos);              // sphere at the origin: normal = direction from the centre
    col = 0.5 + 0.5 * n;
}

Next lesson adds the light and a function that works out the normal for any shape, not just spheres.