OpenGL man pages
## glFrustum.3gl

## Name

glFrustum - multiply the current matrix by a perspective matrix

## C Specification

void glFrustum( GLdouble *left*,
GLdouble *right*,
GLdouble *bottom*,
GLdouble *top*,
GLdouble *near*,
GLdouble *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. (*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, respectively, assuming that the eye is
located at (0, 0, 0). -*far* specifies the location of the far clipping
plane. Both *near* and *far* must be positive. The corresponding matrix is
| |
| 2·near |
| ---------- |
| right-left 0 A 0 |
| |
| 0 2·near B 0 |
| ---------- |
| top-bottom |
| 0 0 C D |
| |
| 0 0 -1 0 |
right+left
A = ----------
right-left
top+bottom
B = ----------
top-bottom
far+near
C = - --------
far-near
2·far·near
D = - ----------
far-near
The current matrix is multiplied by this matrix with the result replacing
the current matrix. That is, if M is the current matrix and F is the
frustum perspective matrix, then M is replaced with M·F.
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
far
r = ----
near
roughly log r bits of depth buffer precision are lost. Because r
2
approaches infinity as *near* approaches zero, *near* must never be set to
zero.

## Errors

GL_INVALID_VALUE is generated if *near* or *far* is not positive.
GL_INVALID_OPERATION is generated if glFrustum is executed between the
execution of glBegin and the corresponding execution of glEnd.

## Associated Gets

glGet with argument GL_MATRIX_MODE
glGet with argument GL_MODELVIEW_MATRIX
glGet with argument GL_PROJECTION_MATRIX
glGet with argument GL_TEXTURE_MATRIX

## See Also

glOrtho, glMatrixMode, glMultMatrix, glPushMatrix, glViewport

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Last Edited: *
Fri Dec 6 11:18:03 EST 1996
*
by
*AFV*

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