Clipping and Projection

  • Next we perform clipping of the scene's objects againist a three dimensional viewing volume called a viewing frustum

  • This step totally eliminates any objects (and pieces of objects) that are not visible in the image

  • Next the objects are projected into two-dimensions

  • Transformation from eye space to screen space




View Frustum

view frustum is a semi-infinite pyramid, although the base is usually rectangular rather than square because the screen isn't square. In practice, however, it is normal to chop off the frustum near the point and somewhere further down towards the base. The view frustum defines what parts of the 3D world can be seen from the camera position when looking through a window (usually the screen). Here's a little piccie to help clarify things a bit :You can define a frustum using the view rectangle, the camera origin and the near- and far-plane distances from the camera origin. This information can then be used to make the top, bottom, left, right, near and far planes. All the planes have their normals facing into the frustum (by convention, doesn't matter so long as they either all point inwards or all point outwards).
Once you have these 6 planes it is easy to determine whether any given point is inside the frustum or not - you just make sure it is above every plane. You just test it against each plane in turn, rejecting the point if it fails at any point in the test.