# MirBSD manpage: glFrustum(3)

```
GLFRUSTUM(3G)       UNIX Programmer's Manual        GLFRUSTUM(3G)
```

## NAME

```     glFrustum - multiply the current matrix by a perspective
matrix
```

## C SPECIFICATION

```     void glFrustum( GLdouble left,
GLdouble right,
GLdouble bottom,
GLdouble top,
GLdouble near_val,
GLdouble far_val )
```

## 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.

near_val, far_val
Specify the distances to the near and far depth
clipping planes. Both distances must be posi-
tive.
```

## 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:

|
|
|
|
|   ____e__r___a___
|   right - left        0
|                                            |
|        0        ____e__r___a___   A           |
|        0        top - bottom               |
|        0              0        B       0   |
0        C           |
-1      0   |
D   |
0   |
|
|
_i__h___±__e__t_
A = t_o_p__t±_b__l_t_o_m_
B_=__t_v_a__-_±_o_e_a_r____a__l
C = -2__r_a_r___v__-__n_e_a_r___v_a_l_
D = -far_val - near_val

MirBSD #10-current     Printed 2021-12-07                       1

GLFRUSTUM(3G)       UNIX Programmer's Manual        GLFRUSTUM(3G)

Typically, the matrix mode is GL_PROJECTION, and (left, bot-
tom, -near_val) and (right, top,  -near_val) 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). -far_val specifies the
location of the far clipping plane. Both near_val and
far_val 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 near_val and far_val. The greater the ratio of far_val
to near_val is, the less effective the depth buffer will be
at distinguishing between surfaces that are near each other.
If
__a_____a__l
r = near_val
roughly log2(r) bits of depth buffer precision are lost.
Because r approaches infinity as near_val approaches 0,
near_val must never be set to 0.
```

## ERRORS

```     GL_INVALID_VALUE is generated if near_val or far_val 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 exe-
cution 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
```

```     glOrtho(3G), glMatrixMode(3G), glMultMatrix(3G),