# glFrustum

## Name

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.