1991-2006
Silicon Graphics, Inc.
glFrustum
3G

# glFrustum

multiply the current matrix by a perspective matrix

## C Specification

                    void glFrustum
                    GLdouble left
                    GLdouble right
                    GLdouble bottom
                    GLdouble top
                    GLdouble nearVal
                    GLdouble farVal

## 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.

`nearVal`
`farVal`

  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 &it; nearVal right - left 0 A 0 0 2 &it; nearVal top - bottom B 0 0 0 C D 0 0 -1 0

A = right + left right - left

B = top + bottom top - bottom

C = - farVal + nearVal farVal - nearVal

D = - 2 &it; farVal &it; nearVal farVal - nearVal

Typically, the matrix mode is `GL_PROJECTION`, and left bottom - nearVal and right top - nearVal 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). - farVal specifies the location of the far clipping plane. Both `nearVal` and `farVal` 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 `nearVal` and `farVal`. The greater the ratio of `farVal` to `nearVal` is, the less effective the depth buffer will be at distinguishing between surfaces that are near each other. If

r = farVal nearVal

roughly log 2 &af; r bits of depth buffer precision are lost. Because r approaches infinity as `nearVal` approaches 0, `nearVal` must never be set to 0.

## Errors

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

`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`

glGet with argument `GL_COLOR_MATRIX`

## See Also

glOrtho, glMatrixMode, glMultMatrix, glPushMatrix, glViewport

## Copyright

Copyright 1991-2006 Silicon Graphics, Inc. This document is licensed under the SGI Free Software B License. For details, see http://oss.sgi.com/projects/FreeB/.