glFrustum
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## Name

glFrustum — multiply the current matrix by a perspective matrix

## C Specification

`void `glFrustum`` | `(` | `GLdouble `left`, GLdouble `right`, GLdouble `bottom`, GLdouble `top`, GLdouble `zNear`, GLdouble `zFar`);`

## Python Specification

``glFrustum`` | `(` | ``left`, `right`, `bottom`, `top`, `zNear`, `zFar`) → `None``

## 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.
`zNear`, `zFar`
  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 zNear right - left 0 A 0 0 2 zNear top - bottom B 0 0 0 C D 0 0 -1 0 ]
A = right + left right - left
B = top + bottom top - bottom
C = - zFar + zNear zFar - zNear
D = - 2 zFar zNear zFar - zNear

Typically, the matrix mode is `GL_PROJECTION`, and (`left`, `bottom`, -`zNear`) and (`right`, `top`, -`zNear`) 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). -`zFar` specifies the location of the far clipping plane. Both `zNear` and `zFar` 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 `zNear` and `zFar`. The greater the ratio of `zFar` to `zNear` is, the less effective the depth buffer will be at distinguishing between surfaces that are near each other. If r = zFar zNear roughly log 2 ( r ) bits of depth buffer precision are lost. Because r approaches infinity as `zNear` approaches 0, `zNear` must never be set to 0.

## Errors

`GL_INVALID_VALUE` is generated if `zNear` or `zFar` is not positive, or if `left` = `right`, or `bottom` = `top`.

`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

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