glFrustum man page on DigitalUNIX

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glFrustum(3G)							 glFrustum(3G)

       glFrustum - multiply the current matrix by a perspective matrix

       void glFrustum(
	       GLdouble left,
	       GLdouble right,
	       GLdouble bottom,
	       GLdouble top,
	       GLdouble zNear,
	       GLdouble zFar );

       Specify	the  coordinates  for  the  left  and  right vertical clipping
       planes.	Specify the coordinates for  the  bottom  and  top  horizontal
       clipping planes.	 Specify the distances to the near and far depth clip‐
       ping planes.  Both distances must be positive.

       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 glMultMa‐
       trix() were called with the following matrix as its argument: {2 zNear}
       / {right - left}		0				 A  0
	 0			  {2 zNear} / {top -  bottom}		     B
	 0				      0				     C
	 0				      0				    -1
				  A = {right + left} / {right - left}
				  B = {top + bottom} / {top - bottom}
				  C = -{zFar + zNear} / {zFar - zNear}
				  D = -{2 zFar zNear} / {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 win‐
       dow, 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 posi‐

       Use glPushMatrix() and glPopMatrix() to save and	 restore  the  current
       matrix stack.

       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 / zNear

       roughly log[2] (r) bits of depth buffer precision are lost.  Because  r
       approaches  infinity  as zNear approaches 0, zNear must never be set to

       GL_INVALID_VALUE is generated if zNear or zFar 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().

       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(3), glMatrixMode(3), glMultMatrix(3), glPushMatrix(3),  glView‐


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