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:
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{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 ]}
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{A ~=~ {"right" ~+~ "left"} over {"right" ~-~ "left"}}
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{B ~=~ {"top" ~+~ "bottom"} over {"top" ~-~ "bottom"}}
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{C ~=~ -{{"far_val" ~+~ "near_val"} over {"far_val" ~-~ "near_val"}}}
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{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
