Clipping FAQ

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How do I tell if a vertex has been clipped or not?

You can use the OpenGL Feedback feature to determine if a vertex will be clipped or not. After you're in Feedback mode, simply send the vertex in question as a GL_POINTS primitive. Then switch back to GL_RENDER mode and check the size of the Feedback buffer. A size of zero indicates a clipped vertex.

Typically, OpenGL implementations don't provide a fast feedback mechanism. It might be faster to perform the clip test manually. To do so, construct six plane equations corresponding to the clip-coordinate view volume and transform them into object space by the current ModelView matrix. A point is clipped if it violates any of the six plane equations.

Here's a GLUT example that shows how to calculate the object-space view-volume planes and clip test bounding boxes against them.

Here is a tutorial titled Frustum Culling in OpenGL.

When an OpenGL primitive moves placing one vertex outside the window, suddenly the color or texture mapping is incorrect. What's going on?

There are two potential causes for this.

When a primitive lies partially outside the window, it often crosses the view volume boundary. OpenGL must clip any primitive that crosses the view volume boundary. To clip a primitive, OpenGL must interpolate the color values, so they're correct at the new clip vertex. This interpolation is perspective correct. However, when a primitive is rasterized, the color values are often generated using linear interpolation in window space, which isn't perspective correct. The difference in generated color values means that for any given barycentric coordinate location on a filled primitive, the color values may be different depending on whether the primitive is clipped. If the color values generated during rasterization were perspective correct, this problem wouldn't exist.

For some OpenGL implementations, texture coordinates generated during rasterization aren't perspective correct. However, you can usually make them perspective correct by calling glHint(GL_PERSPECTIVE_CORRECTION_HINT,GL_NICEST);. Colors generated at the rasterization stage aren't perspective correct in almost every OpenGL implementation, and can't be made so. For this reason, you're more likely to encounter this problem with colors than texture coordinates.

A second reason the color or texture mapping might be incorrect for a clipped primitive is because the color values or texture coordinates are nonplanar. Color values are nonplanar when the three color components at each vertex don't lie in a plane in 3D color space. 2D texture coordinates are always planar. However, in this context, the term nonplanar is used for texture coordinates that look up a texel area that isn't congruent in shape to the primitive being textured.

Nonplanar colors or texture coordinates aren't a problem for triangular primitives, but the problem may occur with GL_QUADS, GL_QUAD_STRIP and GL_POLYGON primitives. When using nonplanar color values or texture coordinates, there isn't a correct way to generate new values associated with clipped vertices. Even perspective-correct interpolation can create differences between clipped and nonclipped primitives. The solution to this problem is to not use nonplanar color values and texture coordinates.