134 lines
3.6 KiB
Plaintext
134 lines
3.6 KiB
Plaintext
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'\" e
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'\"! eqn | mmdoc
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'\"macro stdmacro
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.ds Vn Version 1.2
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.ds Dt 24 September 1999
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.ds Re Release 1.2.1
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.ds Dp Jan 14 18:30
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.ds Dm 01 frustum.g
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.ds Xs 13962 6 frustum.gl
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.TH GLFRUSTUM 3G
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.SH NAME
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.B "glFrustum
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\- multiply the current matrix by a perspective matrix
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.SH C SPECIFICATION
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void \f3glFrustum\fP(
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GLdouble \fIleft\fP,
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.nf
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.ta \w'\f3void \fPglFrustum( 'u
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GLdouble \fIright\fP,
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GLdouble \fIbottom\fP,
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GLdouble \fItop\fP,
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GLdouble \fInear_val\fP,
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GLdouble \fIfar_val\fP )
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.fi
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.EQ
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delim $$
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.EN
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.SH PARAMETERS
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.TP \w'\f2left\fP\ \f2right\fP\ \ 'u
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\f2left\fP, \f2right\fP
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Specify the coordinates for the left and right vertical clipping planes.
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.TP
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\f2bottom\fP, \f2top\fP
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Specify the coordinates for the bottom and top horizontal clipping planes.
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.TP
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\f2near_val\fP, \f2far_val\fP
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Specify the distances to the near and far depth clipping planes.
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Both distances must be positive.
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.SH DESCRIPTION
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\%\f3glFrustum\fP describes a perspective matrix that produces a perspective projection.
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The current matrix (see \%\f3glMatrixMode\fP) is multiplied by this matrix
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and the result replaces the current matrix, as if
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\%\f3glMultMatrix\fP were called with the following matrix
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as its argument:
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.P
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.sp 5
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.ce
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.EQ
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down 150
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{left [ ~~~ {
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down 165 matrix {
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ccol { {{2 ~ "near_val"} over {"right" ~-~ "left"}} above 0 above 0 above 0 }
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ccol { 0 above {{2 ~ "near_val"} over {"top" ~-~ "bottom"}} ~ above 0 above 0 }
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ccol { A ~~~~ above B ~~~~ above C ~~~~ above -1 ~~~~}
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ccol { 0 above 0 above D above 0}
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}}
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~~~ right ]}
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.EN
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.sp
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.ce
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.EQ
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down 130
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{A ~=~ {"right" ~+~ "left"} over {"right" ~-~ "left"}}
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.EN
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.sp
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.ce
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.EQ
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down 130
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{B ~=~ {"top" ~+~ "bottom"} over {"top" ~-~ "bottom"}}
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.EN
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.sp
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.ce
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.EQ
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down 130
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{C ~=~ -{{"far_val" ~+~ "near_val"} over {"far_val" ~-~ "near_val"}}}
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.EN
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.sp
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.ce
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.EQ
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down 130
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{D ~=~ -^{{2 ~ "far_val" ~ "near_val"} over {"far_val" ~-~ "near_val"}}}
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.EN
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.sp 2
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.P
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Typically, the matrix mode is \%\f3GL_PROJECTION\fP, and
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(\f2left\fP, \f2bottom\fP, \-\f2near_val\fP) and (\f2right\fP, \f2top\fP, \-\f2near_val\fP)
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specify the points on the near clipping plane that are mapped
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to the lower left and upper right corners of the window,
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assuming that the eye is located at (0, 0, 0).
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\-\f2far_val\fP specifies the location of the far clipping plane.
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Both \f2near_val\fP and \f2far_val\fP must be positive.
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.P
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Use \%\f3glPushMatrix\fP and \%\f3glPopMatrix\fP to save and restore
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the current matrix stack.
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.SH NOTES
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Depth buffer precision is affected by the values specified for
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\f2near_val\fP and \f2far_val\fP.
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The greater the ratio of \f2far_val\fP to \f2near_val\fP is,
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the less effective the depth buffer will be at distinguishing between
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surfaces that are near each other.
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If
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.sp
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.ce
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$r ~=~ "far_val" over "near_val"$
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.sp
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roughly $log sub 2 (r)$ bits of depth buffer precision are lost.
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Because $r$ approaches infinity as \f2near_val\fP approaches 0,
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\f2near_val\fP must never be set to 0.
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.SH ERRORS
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\%\f3GL_INVALID_VALUE\fP is generated if \f2near_val\fP or \f2far_val\fP is not
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positive, or if \f2left\fP = \f2right\fP, or \f2bottom\fP = \f2top\fP.
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.P
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\%\f3GL_INVALID_OPERATION\fP is generated if \%\f3glFrustum\fP
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is executed between the execution of \%\f3glBegin\fP
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and the corresponding execution of \%\f3glEnd\fP.
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.SH ASSOCIATED GETS
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\%\f3glGet\fP with argument \%\f3GL_MATRIX_MODE\fP
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.br
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\%\f3glGet\fP with argument \%\f3GL_MODELVIEW_MATRIX\fP
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.br
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\%\f3glGet\fP with argument \%\f3GL_PROJECTION_MATRIX\fP
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.br
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\%\f3glGet\fP with argument \%\f3GL_TEXTURE_MATRIX\fP
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.br
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\%\f3glGet\fP with argument \%\f3GL_COLOR_MATRIX\fP
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.SH SEE ALSO
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\%\f3glOrtho(3G)\fP,
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\%\f3glMatrixMode(3G)\fP,
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\%\f3glMultMatrix(3G)\fP,
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\%\f3glPushMatrix(3G)\fP,
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\%\f3glViewport(3G)\fP
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