MedicalVisualization

Clip Planes

← Iso Surface Examples | ● | Clip Plane Example →

Iso surfaces are nested (onion shells). To see the inner shells it is often necessary to remove tissue that is occluding the inner parts. We need to cut the onion! This is done via:

  • Preprocessing the volume
    • Segmentation techniques
  • On-the-fly
    • Clipping planes
    • Clipping volumes
      • Voxel-based mask
    • Semi-transparent iso-surfaces
      • Depth Peeling

A clip plane removes the geometry on one side of the plane.

With OpenGL at least 6 independent clipping planes can be specified for the hardware pipeline.

A single OpenGL clip plane is defined by the plane equation

$ ax+by+cz+d = 0 $

When evaluating the above formula for points (x,y,z), results less or greater than zero indicate points on one or the other side of the plane.

For a plane with pivot point $\vec{p}$ and a normal $\vec{n}$ (pointing into the half space that is to be shown) the plane coefficients are:

  • $a = \vec{n_x}$
  • $b = \vec{n_y}$
  • $c = \vec{n_z}$
  • $d = - \vec{n}\cdot\vec{p}$

The i-th OpenGL clip plane is specified via:

GLdouble equ[4];

equ[0]=a;
equ[1]=b;
equ[2]=c;
equ[3]=d;

glClipPlane(GL_CLIP_PLANE0+i,equ);
glEnable(GL_CLIP_PLANE0+i);
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← Iso Surface Examples | ● | Clip Plane Example →

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