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dither(3G)							    dither(3G)

NAME    [Toc]    [Back]

     dither - controls the dithering of	pixels

C SPECIFICATION    [Toc]    [Back]

     void dither(mode)
     long mode;

PARAMETERS    [Toc]    [Back]

     mode   expects one	of two values:

	    DT_OFF disables dithering.

	    DT_ON enables dithering (default).

DESCRIPTION    [Toc]    [Back]

     Dithering is a pixel operation that attempts to convey as accurately as
     possible the color	of an image when the framebuffer stores	fewer bits of
     color than	are being computed or iterated.	 This is done by creating a
     pattern of	carefully chosen colors	that, when viewed from a distance of a
     few feet or more, give the	illusion of an accurately shaded image.	 For
     example, if only 4, 8, or 12 bitplanes are	available in the framebuffer,
     and color is being	iterated with 24 bits (8 bits for each of red, green,
     and blue),	dithering can be used to modulate the drawn colors to better
     approximate the 24	bit image.

     Dithering works in	both RGB and color index modes.	 In RGB	mode,
     dithering is performed independently for red, green, blue,	and alpha.
     When blending is enabled, blending	occurs before dithering	and uses the
     full precision of the iterated colors.  Dithering works on	all drawing
     primitives:  points, lines, text, polygons, and pixel writes and copies.
     However, dithering	has no effect if the framebuffer stores	all the
     computed bits of color.  For example, pixel copies	from framebuffer to
     framebuffer are not dithered.

     Dithering is enabled by default.  When drawing pre-dithered images, or
     when drawing single-width RGB lines on a 4	or 8-bit framebuffer, it may
     be	best to	turn dithering off.

SEE ALSO    [Toc]    [Back]

     color, colorf, getgdesc

NOTES    [Toc]    [Back]

     Some IRIS-4D models do not	support	dithering. Use getgdesc(GD_DITHER) to
     determine whether dithering is supported.	Some machines support
     dithering only in RGB mode.  If getgdesc(GD_CIFRACT) is FALSE then
     dithering is not supported	in colorindex mode.

     On	all RealityEngine models, there	is hardware support for	video
     dithering enabled by default.  This feature differs from render dithering
     because it	only affects the pixels	on the way out to the video.  On
     RealityEngine this	dithering takes	the 12-bit components and dithers them

									Page 1

dither(3G)							    dither(3G)

     to	10 bits	before they are	sent to	the gamma lookup (which	then outputs 8
     bits per component).  On releases prior to	Irix 5.3, this feature was
     enabled through this call.	 Video dithering is no longer affected by this
     call by default.  To re-enable this call you'll need to setenv

     On	all Personal Iris models, dithering can	be enabled only	when drawing
     in	one of the following modes: shademodel(GOURAUD)	or depthcue(TRUE).

									PPPPaaaaggggeeee 2222
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