How to calculate a ray plane intersection

Question

How do I calculate the intersection between a ray and a plane?

The following code produces the wrong results.

float denom = normal.dot(ray.direction);

if (denom > 0)
{
    float t = -((center - ray.origin).dot(normal)) / denom;

    if (t >= 0)
    {
        rec.tHit = t;
        rec.anyHit = true;
        computeSurfaceHitFields(ray, rec);
        return true;
    }
}

The input variables are described as follows:

• ray represents the ray object.
• ray.direction is the direction vector.
• ray.origin is the origin vector.
• rec represents the result object.
• rec.tHit is the value of the hit.
• rec.anyHit is a boolean.

My function has access to the plane:

• center and normal defines the plane

Answer 1

As wonce commented, you want to also allow the denominator to be negative, otherwise you will miss intersections with the front face of your plane. However, you still want a test to avoid a division by zero, which would indicate the ray being parallel to the plane. You also have a superfluous negation in your computation of t. Overall, it should look like this:

float denom = normal.dot(ray.direction);
if (abs(denom) > 0.0001f) // your favorite epsilon
{
    float t = (center - ray.origin).dot(normal) / denom;
    if (t >= 0) return true; // you might want to allow an epsilon here too
}
return false;