HP j6700 OpenGL Implementation Guide for HP-UX 11.x - Page 61

programming hints OpenGL performance hints • Single buffered applications • Alpha planes • Depth 8 visuals • Stereo • Gradient backgrounds rescaling normals When normal rescaling is enabled, a new operation is added to the transformation of the normal vector into eye coordinates. The normal vector is rescaled after it is multiplied by the inverse modelview matrix and before it is normalized. The rescale factor is chosen so that in many cases, normal vectors with unit length in object coordinates will not need to be normalized as they are transformed into eye coordinates. As of Release 1.05 of HP's implementation of OpenGL 1.1, the GL_RESCALE_NORMAL_EXT token is supported. It is accepted by the parameter of glEnable, glDisable, and glIsEnabled, and by the parameter of glGetBooleanv, glGetIntegerv, glGetFloatv, and glGetDoublev. Normals that have unit length when sent to the GL, have their length changed by the inverse of the scaling factor after transformation by the model-view inverse matrix when the model-view matrix represents a uniform scale. If rescaling is enabled, then normals specified with the Normal command are rescaled after transformation by the Modelview Inverse. Normals sent to the GL may or may not have unit length. In addition, the length of the normals after transformation might be altered due to transformation by the model-view inverse matrix. If normalization is enabled, then normals specified with the glNormal3 command are normalized after transformation by the model-view inverse matrix and after rescaling if rescaling is enabled. Normalization and rescaling are controlled with glEnable and glDisable with the target equal to NORMALIZE or RESCALE_NORMAL. This requires two bits of state. The initial state is for normals not to be normalized or rescaled. Chapter 5 59