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programming hints

OpenGL performance hints

Chapter 5 59

• 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 <cap>

parameter of glEnable, glDisable, and glIsEnabled, and by the

<pname> 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.

OGLImp.book Page 59 Tuesday, February 14, 2006 9:15 AM

HP j6700 Supplementary Guide

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