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3.3 Capping Clipped Solids with the Stencil Buffer

When dealing with solid objects it is often useful to clip the object against a plane and observe the cross section. OpenGL's user-defined clipping planes allow an application to clip the scene by a plane. The stencil buffer provides an easy method for adding a ``cap'' to objects that are intersected by the clipping plane. A capping polygon is embedded in the clipping plane and the stencil buffer is used to trim the polygon to the interior of the solid.

For more information on the techniques using the stencil buffer, see Section 14.

If some care is taken when constructing the object, solids that have a depth complexity greater than 2 (concave or shelled objects) and less than the maximum value of the stencil buffer can be rendered. Object surface polygons must have their vertices ordered so that they face away from the interior for face culling purposes.

The stencil buffer, color buffer, and depth buffer are cleared, and color buffer writes are disabled. The capping polygon is rendered into the depth buffer, then depth buffer writes are disabled. The stencil operation is set to increment the stencil value where the depth test passes, and the model is drawn with glCullFaceGL_BACK(GL_BACK). The stencil operation is then set to decrement the stencil value where the depth test passes, and the model is drawn with glCullFaceGL_FRONT(GL_FRONT).

At this point, the stencil buffer is 1 wherever the clipping plane is enclosed by the frontfacing and backfacing surfaces of the object. The depth buffer is cleared, color buffer writes are enabled, and the polygon representing the clipping plane is now drawn using whatever material properties are desired, with the stencil function set to GL_EQUAL and the reference value set to 1. This draws the color and depth values of the cap into the framebuffer only where the stencil values equal 1.

Finally, stenciling is disabled, the OpenGL clipping plane is applied, and the clipped object is drawn with color and depth enabled.

next up previous contents
Next: 3.4 Constructive Solid Geometry Up: 3 Modelling Previous: 3.2 Decomposition and Tessellation