Lighting II: specular reflection, generalized lighting models

CS 248 - Introduction to Computer Graphics
Autumn Quarter, 2006
Marc Levoy
Lecture notes for Thursday, November 30 (finished on Tuesday, December 5)

To save time, I did not cover the issue of local versus infinite L and V in class. I'm leaving it in the notes in case you're interested. Infinite L, i.e. infinitely distant lighting, is invoked in OpenGL by setting the homogeneous coordinate W = 0.0 when specifying the position of the light source in (for example) glLightFv(x,y,z,W). Infinite V, i.e. infinitely distant viewing, is invoked in OpenGL by setting GL_LIGHT_MODEL_LOCAL_VIEWER = GL_FALSE in glLightModeli(). Note that moving the viewer to infinity in this way is only for the purpose of lighting, not viewing, i.e. it changes the intensities computed for surfaces, but it does not change the geometry of the image you see.

Note: If vertex normals are passed into the rendering pipeline along with vertex coordinates, and they can be assumed to be pre-normalized, then the rendering pipeline doesn't need to perform any square roots at all.

OpenGL natively supports only Gouraud (i.e. intensity) interpolation. Phong (i.e. normal vector) interpolation is not supported natively, but it can be implemented (during rasterization) using the flexible shading architecture available in most modern hardware accelerators. Angle interpolation would be harder to implement. However, assuming modest changes in normal vector direction from vertex to vertex, the visual difference between normal and angle interpolation is small. OpenGL does not specify whether Gouraud interpolation should be linear or rational linear. Modern graphics boards implement it using rational linear interpolation.

levoy@cs.stanford.edu

Lighting II: specular reflection, generalized lighting models

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