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 ⁢ nearVal right - left 0 A 0 0 2 ⁢ 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 ⁢ farVal ⁢ 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 ⁡ 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/.