zclear(3G)

NAME
     zclear - initializes the z-buffer of the current framebuffer

FORTRAN 77 SPECIFICATION
     subroutine zclear

PARAMETERS
     none

DESCRIPTION
     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 getgco(GCMZMA). Otherwise, it is equal
     to getgco(GCZMAX). 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 czclea instead of zclear.

     Because only the normal framebuffer includes a z buffer, zclear should be
     called only while draw mode is NORMDR.  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
     drawmo, getgco, scrmas, setpat, stenci, texbin, wmpack, writem, zbuffe,
     zwrite

NOTE
     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 z-
     buffer.  In situations where this is necessary, czclea should be used
     instead of zclear.  When using czclea, 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 zfunct(ZFALWA), 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.