Peter Kerney (peterk++at++sydney.sgi.com)
Mon, 6 Feb 1995 16:55:01 -0500
You need to put the Extreme into single buffer mode (1x24bit) rather than
doublebuffer (2x12bit). This should not be a problem as you do not care what
it looks like while you are drawing it (I know the application), you are
only interested in the final output to be grabbed and output to the ACCOM.
Not sure how you are setting up your channel, but it should not be too
difficult to set the channel into 'singlebuffer(3G)' mode (don't forget
your 'gconfig(3G)'.
Your problem is that on low-end machines, you have a total of 24 bitplanes
for your graphics and when you are in doublebuffer you limit yourself
to 2x12bit framebuffers. The hardware is nice enough to dither this
down for you.
On the VGX, you have 2x32bit framebuffers so there is not a problem.
The ACCOM is "interlaced" as it provides a video signal and the system
is "non-interlaced". You should only see the shimmer on the interlaced
video monitor. This is due to the vertical retrace frequency and the
persistence of the phosphors on your video monitor. The "interlace flicker"
is an artifact of all interlaced video signals. It can be reduced by lowering
the contrast between adjacent scanlines, this is not possible on the Extreme
as the hardware is causing the artifact.
The reason that you see a difference in the intensity is that the Extreme
hardware dithers on a regular grid depending upon odd/even pixels and
depending upon which field you are looking at depends upon whether it is
dark or light.
Hope that this helps, PeterK.
> From: ben++at++vislab.su.edu.au
> Date: Mon, 6 Feb 95 12:30:13 +1100
> Subject: Dithered Output
> To: info-performer++at++sgi.sgi.com
> Reply-To: ben++at++vislab.su.edu.au
>
> DITHERED OUTPUT
> ---------------
>
> Hi Performer people,
>
> We're dumping our image frames from performer to an ACCOM
> video disk, and have found that we get different results in
> the image quality depending on whether we record the output
> from our crimson vgx, vs. our indigoII extreme.
>
> The Indigo2 extreme dithers its' output in a regular pattern
> that causes terrible shimmer in the non-interlaced display
> from the ACCOM. The crimson doesn't dither. The problem can be
> seen if we show seperate still-fields from the accom: one field
> seems brighter than the other, due to the regular dithering
> pattern. If you run "/usr/demos/bin/snoop" (or xmag) in the
> performer window you can see the dither pattern.
>
> Our target system will use an Indigo2. The display quality must
> be suitable for broadcast (PAL) tv. Is there a way to get
> non-dithered output from an extreme graphics board? We have
> reduced the colour range, but the shimmer remains because the
> fields are different in brightness. Are we stuck with it?
>
> Does performer always dither on an indigo2 extreme?
>
> regards,
> ben.
>
> _______________________________________________________
> Ben Simons Physics Building, A28,
> VisLab Systems Manager Sydney University. NSW.
> Phone +61-2-351-3005 AUSTRALIA. 2006.
>
--
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Peter Kerney. Silicon Graphics, Sydney, Australia. (peterk++at++sydney.sgi.com)
'Everybody wants a rock to wind a piece of string around.'
They Might Be Giants
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