zclear(3G)							    zclear(3G)

NAME    [Toc]    [Back]

     zclear - initializes the z-buffer of the current framebuffer

C SPECIFICATION    [Toc]    [Back]

     void zclear(void)

PARAMETERS    [Toc]    [Back]

     none

DESCRIPTION    [Toc]    [Back]

     zclear sets the z-buffer in the area of the viewport to the largest
     positive z	value supported.  When multisample is enabled, and multisample
     buffers exist in the current framebuffer configuration in the current
     drawmode, this value is equal to getgconfig(GC_MS_ZMAX). Otherwise, it is
     equal to getgconfig(GC_ZMAX). Typically zclear is called prior to
     rendering each frame.  If you intend to clear the color bitplanes as well
     as	the z-buffer, or if you	require	control	of the value written to	the
     z-buffer, call czclear instead of zclear.

     Because only the normal framebuffer includes a z buffer, zclear should be
     called only while draw mode is NORMALDRAW.	 Also, the current z writemask
     controls which z-buffer bitplanes are modified during zclear execution,
     and screenmask, when it is	set to a subregion of the viewport, bounds the
     cleared region.  Other drawing modes, including polygon fill pattern,
     stenciling, texture mapping, writemask, and z buffering, have no effect
     on	the operation of zclear.

     After zclear executes, the	graphics position is undefined.

SEE ALSO    [Toc]    [Back]

     drawmode, getgconfig, scrmask, setpattern,	stencil, texbind, wmpack,
     writemask,	zbuffer, zwritemask

NOTE    [Toc]    [Back]

     On	the Personal Iris, for performance reasons, the	z-buffer is not
     physically	cleared	when zclear is called.	Z-buffering works as it
     normally would unless an attempt is made to read back data	from the zbuffer.
  In situations where this is necessary, czclear should be used
     instead of	zclear.	 When using czclear, the z-buffer will be physically
     cleared to	the specified value, if	the specified value is in the range
     -0x800001 to 0x7ffffe.

     For performance reasons, XS, XS24,	XZ, Elan and Extreme products do not
     physically	clear the z-buffer when	calling	zclear.	 On these systems, the
     LSB of the	z-buffer is used as a flag bit to signal that the z-buffer has
     been cleared.  The	23 remaining bits are used for the z comparison.  In
     order to physically clear the z-buffer, first call	zfunction(ZF_ALWAYS),
     then draw a polygon whose z coordinates map to the	desired	z value.  To
     avoid updating the	framebuffer, call wmpack(0) before drawing.  Restore
     the writemask and z function afterwards.


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