Hi all, first post. This website is a great resource for me since I''m a novice OpenGL programmer. On to my post... Can someone tell me what the phenomena is that I''m seeing when I translate or rotate my polygons very quickly around the screen? I see multiple images of my object, much like a trail. If I translate my flat shaded poly back and forth very quickly, I can count up to 5 ghost polys trailing the real one. I''m guessing it has something to do with monitor refresh rate and my app frame rate... Any insight would be much appreciated. Thanks! -geoff

you would be curect with that asumshen,
what that is, is you draw one, they eye sees it but the computer has alrady drawn the next and the next... so they eye sees all 1..5 of them at one time.
the eye can only "refresh" ~55 times a sec, so if your app is running at 100 FPS and you draw somthing moveing vary fast ( like what your doing ) the eye will see more then one at a time becouse to it there all in the same refresh.
to see this you can take a screen shot of your app and view it in paint ( or somtheing else ). and there sould only be one poly, if there is more then one then you arnt doing somthing right in your GL porgramme, if thats the case i would bet your not clearing the back/zbuffer evry frame.

hope that helps



-VBLimits
Sorry about the Spelling..

The eye is slow to translate images; that's about the only reason why you see multiple images.

You can also see this effect if you take something you can either spin or move quickly such as a pencil.

Take the pencil by one end and move it up and down very quickly. It will appear that the pencil is taking up the entire circumfrence of the semi-circle you made whereas in fact the pencil can only be in one place at one time, unless you look into some more advanced physics, but that's way beyond the scope of this question.

Also, if you have ever seen something rotate REALLY REALLY fast then it will appear that it doesn't rotate at all. Your eyes sure are strange little anomalies, huh?

Hope I helped

Edited by - Floppy on December 19, 2001 10:36:28 PM

a little clarification...

while your app may be drawing 100 FPS, your monitor will only display 60 FPS (or whatever its set to).

If the effect is any different than the ghosts you see when moving the mouse back and forth very quickly, it maybe be your code, but I doubt that''s the case.

I think some of this comes also from the after-burn of the phosphors in the monitor. Flatscreen monitors tend to have a longer after-burn (tho not as bad as they used to).

If you work in a really dark area, like I do, it probably is the afterburn on the monitor, no matter which you have. Sometimes, the brightness for the monitor might be too high. It happens with the mouse too. Get a black background, get it really dark in the room, and after your eyes adjust, you can see as if you had the mouse trails feature turned on. Sometimes it actually gets that bad.

Thanks for all the information. What I had thought pretty much jives with what you''ve all stated.

Maybe a follow up is in order...

Is there anything that can be done to minimize this phenomena? Decrease brightness on the monitor? Increase the vertical refresh rate? Lock app framerate? I''d imagine setting vertical refresh to 60 Hz and locking framerate to 30 FPS would help some...

Thanks again, all.

-geoff

I''d say check the thing out ona friend''s computer and monitor - and maybe another friend''s comp.

And also do the other tests described below.

Not only doing them will help you figure out what''s wrong, you''ll learns something new as well...

Dæmin
(Dominik Grabiec)
dominik.grabiec@student.adelaide.edu.au

Ok forget what all these guys are saying - either it''s windows or your computer is on drugs or you''ve took so many drugs and you can''t remember you''re on them!

Soz - I''m on my linux box again... Puts me on this big ego-trip every time!!! *g*

linux fag