# GLFRUSTUM

Section: Misc. Reference Manual Pages (3G)
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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 near_val,
GLdouble far_val )
```

delim \$\$

## 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 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:

down 150 {left [ ~~~ { down 165 matrix {
ccol { {{2 ~ "near_val"} over {"right" ~-~ "left"}} above 0 above 0 above 0 }
ccol { 0 above {{2 ~ "near_val"} over {"top" ~-~ "bottom"}} ~ above 0 above 0 }
ccol { A ~~~~ above B ~~~~ above C ~~~~ above -1 ~~~~}
ccol { 0 above 0 above D above 0} }} ~~~ right ]}

down 130 {A ~=~ {"right" ~+~ "left"} over {"right" ~-~ "left"}}

down 130 {B ~=~ {"top" ~+~ "bottom"} over {"top" ~-~ "bottom"}}

down 130 {C ~=~ -{{"far_val" ~+~ "near_val"} over {"far_val" ~-~ "near_val"}}}

down 130 {D ~=~ -^{{2 ~ "far_val" ~ "near_val"} over {"far_val" ~-~ "near_val"}}}

Typically, the matrix mode is GL_PROJECTION, and (left, bottom, -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

\$r ~=~ "far_val" over "near_val"\$

roughly \$log sub 2 (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 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

glOrtho(3G), glMatrixMode(3G), glMultMatrix(3G), glPushMatrix(3G), glViewport(3G)

NAME
C SPECIFICATION
PARAMETERS
DESCRIPTION
NOTES
ERRORS
ASSOCIATED GETS