**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* )
**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:
( )
| __**_**_**_____**_ |
| right-left 0 A 0 |
| |
| 0 __**_**_**_____**_ B 0 |
| top-bottom |
| 0 0 C D |
| |
| 0 0 -1 0 |
( )
**__________**
A = right-left
**__________**
B = top-bottom
**__________**
C = -zFar-zNear
**_**_**____**_**_____**
D = - 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 = _**____**
zNear
roughly log (r) bits of depth buffer precision are lost.
Because r a**p**proaches 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.
**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**
**SEE** **ALSO**
**glOrtho**, **glMatrixMode**, **glMultMatrix**, **glPushMatrix**,
**glViewport**