# glFrustum

multiply the current matrix by a perspective matrix

## Signature

glFrustum( GLdouble ( left ) , GLdouble ( right ) , GLdouble ( bottom ) , GLdouble ( top ) , GLdouble ( nearVal ) , GLdouble ( farVal ) )-> void
glFrustum( left , top , )

## Parameters

VariablesDescription
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:
$\left[\begin{array}{cccc}\frac{2\mathit{nearVal}}{\mathit{right}-\mathit{left}}& 0& \mathit{A}& 0\\ 0& \frac{2\mathit{nearVal}}{\mathit{top}-\mathit{bottom}}& \mathit{B}& 0\\ 0& 0& \mathit{C}& \mathit{D}\\ 0& 0& -1& 0\end{array}\right]$
$\mathit{A}=\frac{\mathit{right}+\mathit{left}}{\mathit{right}-\mathit{left}}$
$\mathit{B}=\frac{\mathit{top}+\mathit{bottom}}{\mathit{top}-\mathit{bottom}}$
$\mathit{C}=-\frac{\mathit{farVal}+\mathit{nearVal}}{\mathit{farVal}-\mathit{nearVal}}$
$\mathit{D}=-\frac{2\mathit{farVal}\mathit{nearVal}}{\mathit{farVal}-\mathit{nearVal}}$
Typically, the matrix mode is GL_PROJECTION , and $\left(\mathit{left},\mathit{bottom},-\mathit{nearVal}\right)$ and $\left(\mathit{right},\mathit{top},-\mathit{nearVal}\right)$ 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). $-\mathit{farVal}$ 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
$\mathit{r}=\frac{\mathit{farVal}}{\mathit{nearVal}}$
roughly ${\mathit{log}}_{2}\left(\mathit{r}\right)$ bits of depth buffer precision are lost. Because $\mathit{r}$ 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

## Sample Code References

The following code samples have been found which appear to reference the functions described here. Take care that the code may be old, broken or not even use PyOpenGL.

glFrustum
OpenGLContext tests/boundingvolume.py Lines: 76, 81
OpenGL-Demo PyOpenGL-Demo/redbook/cube.py Lines: 80
OpenGL-Demo PyOpenGL-Demo/GLUT/glutplane.py Lines: 241
OpenGL-Demo PyOpenGL-Demo/GLUT/gears.py Lines: 209
OpenGL-Demo PyOpenGL-Demo/GLE/maintest.py Lines: 53
{LGPL} VisionEgg VisionEgg/Core.py Lines: 1086
{LGPL} PyMT pymt/ui/window/__init__.py Lines: 19, 456
{GPL} GLChess src/lib/scene/opengl/opengl.py Lines: 63