GLFRUSTUM() MachTen Programmer’s Manual GLFRUSTUM()

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 )

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.

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 glMatrix-
Mode) 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 130 {left ( ~~ matrix {
ccol { {{2 ~ "zNear"} over {"right" - "left"}} above 0
above 0 above 0 }
ccol { 0 above {{2 ~ "zNear"} 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 ~=~ -{{"zFar" + "zNear"} over {"zFar" - "zNear"}}}

down 130
{D ~=~ -{{2 ~ "zFar" ~ "zNear"} over {"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

$r ~=~ "zFar" over "zNear"$

roughly $log sub 2 (r)$ 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 pos-
itive.

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, glView-
port

MachTen 2