Jude Anthony (jude++at++p3.enzian.com)
Thu, 16 Jan 1997 19:28:44 EST
> I found this interesting comment in a document in Jim Blinn's home page
> out there on the web:-
[bogus double buffering explanation scrubbed]
> This seems very suprising - if it's true then it would appear to have
> rather severe implications for simulations running at 30Hz. If time
> somehow seems distorted in this way then vehicle operators (pilots
> especially) might get false speed cues with 30Hz simulations!
>
> Has anyone else out there heard this? Are there studies to show any
> kind of effect like that?
Mr. Blinn is incorrect. Double bufferring is the process of drawing to an
off-screen buffer while a different buffer is displayed. The whole thing
still goes fine at 30Hz, showing 30 frames every second. It's done to
prevent flickering; if you're drawing on a screen you're still showing, the
viewer could see pieces of what you're doing.
For completeness, consider two buffers, each with identical characteristics,
buffer1 and buffer2. Draw a scene a buffer1. Display buffer1. Change the
scene slightly, and draw it on buffer2 (which is not being displayed).
Display buffer2. Change the scene again, and draw it on buffer1 (which is
not being displayed). Display buffer1. Ad infinitum; you're double
bufferring. 30 potentially different, completely redrawn frames every
second.
Mr. Blinn's explanation is perhaps more along the lines of an animation
technique used for hand-drawn animation. To save animator time in many
movies and TV shows, some frames are displayed twice. You can get away with
some of that without the avaerage man seeing the slowdown. It's also used
when converting movies to TV; since TV is 30Hz, and movies 24Hz, one of every
four frames is displayed twice.
> Mr Blinn is presumably too busy to answer his email - I didn't get a
> reply when I asked him for backup info.
No wonder.
Later,
Jude Anthony
jude++at++p3.enzian.com
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