glFrustum

Name

glFrustumf , glFrustumx - multiply the current matrix by a perspective matrix

C Specification

void glFrustumf(GLfloat left,
    GLfloat right,
    GLfloat bottom,
    GLfloat top,
    GLfloat near,
    GLfloat far)

void glFrustumx(GLfixed left,
    GLfixed right,
    GLfixed bottom,
    GLfixed top,
    GLfixed near,
    GLfixed far)

Parameters

left, right

Specify the coordinates for the left and right vertical clipping planes.

bottom, top

Specify the coordinates for the bottom and top horizontal clipping planes.

near, far

Specify the distances to the near and far depth clipping planes. Both distances must be positive.

Description

　　glFrustum describes a perspective matrix that produces a perspective projection. The current matrix (see glMatrixMode) is multiplied by this matrix and the result replaces the current matrix, as if glMultMatrix were called with the following matrix as its argument:

 

(

2/(right - left)	0	A	0
0	2/(top - bottom)	B	0
0	0	C	D
0	0	-1	0

)

 

where

 

A = - ^{(right + left)}/_{(right - left)}

B = - ^{(top + bottom)}/_{(top - bottom)}

C = - ^{(far + near)}/_{(far - near)}

D = - ^2farnear/_{(far - near)}

 

　　Typically, the matrix mode is GL_PROJECTION, and (left, bottom, -near) and (right, top, -near) specify the points on the near clipping plane that are mapped to the lower left and upper right corners of the window, assuming that the eye is located at (0, 0, 0). -far specifies the location of the far clipping plane. Both nearand far must be positive.

Use glPushMatrix and glPopMatrix to save and restore the current matrix stack.

Notes

　　Depth buffer precision is affected by the values specified for near and far. The greater the ratio of far to near is, the less effective the depth buffer will be at distinguishing between surfaces that are near each other. If

 

r = ^far/_near

 

　　roughly log₂(r) bits of depth buffer precision are lost. Because r approaches infinity as near approaches 0, near must never be set to 0.

Errors

　　GL_INVALID_VALUE is generated if near or far is not positive, or if left = right, or bottom = top.