Wsrtjtyk

I found a solution for the ray plane intersection code in Wikipedia, which works and where I simply solve a linear equation system.

Later I found some code for a point onto plane projection, which is obviously implemented differently and also yields different solutions under certain conditions.

However, I do not really get what is the difference between a projection of a point along a vector and the intersection of a ray (build by the point and vector). In both cases I would expect just to find the point where the ray intersects the plane?!

Is there anywhere a figure to illustrate the difference?

struct Plane {

  glm::vec3 _normal;

  glm::vec3 _point;

};



glm::vec3 intersection(const Plane &p, const glm::vec3 &point, const glm::vec3 &direction) {

  // See: https://en.wikipedia.org/wiki/Line%E2%80%93plane_intersection

  const float t = glm::dot(p._normal, point - p._point) / glm::dot(p._normal, -direction);

  return point + t * direction;

}



glm::vec3 orthogonalProjection(const Plane &p, const glm::vec3 &point, const glm::vec3 &direction) {

  // from https://stackoverflow.com/questions/9605556/how-to-project-a-point-onto-a-plane-in-3d

  const float t = -(glm::dot(point, direction) - glm::dot(p.getOrigin(), p.getNormal()));

  return point+ t * direction;

}

A ray is an infinite line, so it has a direction. Intersecting a ray with a plane means finding where the line passes through the plane.

A point is a dimensionless dot suspended somewhere in space. Projecting a point onto a plane means shooting a ray that passes through the point and is perpendicular to the plane (called the "normal"), and seeing where it lands.

The ray already has a direction, the point doesn't. The direction chosen to project the point is the one perpendicular the plane simply because that's how the projection is defined.

So you can have a special case where the ray's direction and the plane's normal coincide, in which case intersecting the ray with the plane and projecting a point that happens to lie somewhere on the ray lead to the same result, but that's just a special case.