dither(3G) dither(3G)
NAME
dither - controls the dithering of pixels
C SPECIFICATION
void dither(mode)
long mode;
PARAMETERS
mode expects one of two values:
DTOFF disables dithering.
DTON enables dithering (default).
DESCRIPTION
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
color, colorf, getgdesc
NOTES
Some IRIS-4D models do not support dithering. Use getgdesc(GDDITHER) to
determine whether dithering is supported. Some machines support
dithering only in RGB mode. If getgdesc(GDCIFRACT) 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
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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
REENABLEDITHERCMD.
On all Personal Iris models, dithering can be enabled only when drawing
in one of the following modes: shademodel(GOURAUD) or depthcue(TRUE).
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