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OpenGL High Frame Rates, Jittery Graphics

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Haptic    205
Hey everyone, I am using OpenGL (VBOs) and C++, at 1920x1080 resolution. My game has a 3D map you can move around and currently reaches 40-60 FPS. Even at this frame rate though, the surrounding world is not drawn 'fluidly' as it moves. This isn't to do with the motion of my character, as that is steady and smooth. I get subtle horizontal 'bands' that scroll down the page (not really screenshot-able), with and without V-Sync enabled. Because of these, I thought the frames might not be being finished before being displayed. At this framerate though, surely they are. I could implement triple buffering or something but other games have triple buffering disabled, and are still perfectly smooth at 20FPS. Objects that move around the screen are also very 'jittery'. Sorry about my vague adjectives! This is more of an issue (or maybe just more noticeable) after I upped the resolution from 1024x768 to 1920x1080. Sure, it's a big jump in resolution but its a standard choice in games nowadays so I expect this isn't the issue. Any ideas would be greatly appreciated. Thanks

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Akitsune    100
Banding suggests V-Sync issues - I would say leave that on while addressing the other problems, and see how that works.

It's probable that you update the object's position only 20 or so times a second, as it sounds like it works fine at 20 fps. Check to see if you do some sort of interpolation between those 20/sec intervals, if they exist. You could also be setting a position to as an integer when it should be a float, etc.

Hope this helps. If not, maybe a little more detail :)

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MarkS    180
Are you using an LCD monitor? I noticed the same thing recently with v-sync on and off. It then hit me that what I was seeing was ghosting that is common with LCDs. LCD displays do not automatically clear after each frame in the same way that a CRT does. There is no refresh with an LCD since there are no phosphors in the pixels to fade. As such, the current frame is always overwriting the previous. It almost appears as a high frequency single buffer animation.

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Haptic    205
Thanks for the replies.

Akitsune - Yes, sorry about my vagueness but I was not too sure which information was relevant.
The objects themselves are fixed in my world (trees, buildings etc) and so I translate them in the opposite direction that the character moves. The character movement is time-dependent. I do this every frame, before I render the scene. The system works fine on a whole (no float -> int conversions etc), but to get this fluid motion I need to attain a much higher framerate than I should.

Just as a sidenote, I've used my FPS calculation and a third-party FPS function and they both return identical results, so it's unlikely the FPS is inaccurate.

maspeir - Funny that you mention LCDs, the reason for my resolution increase is because I upgraded my ancient CRT to a new widescreen LCD. In truth this probably is when I started seeing the issue, but I'm having trouble duplicating my 'symptoms' in commercial games.

I've done some testing, and cutting down some in-game extras, I have VSync peaking at 60fps instead of 30fps. This is perfection. Exactly what I want. Shouldn't 30fps look more or less the same as 60fps to the human eye? I am incredibly surprised the difference is (extremely) obvious.

[Edited by - Haptic on March 10, 2010 4:18:33 AM]

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Original post by Haptic
Shouldn't 30fps look more or less the same as 60fps to the human eye? I am incredibly surprised the difference is (extremely) obvious.
That's something you cannot possibly tell. It might, in fact, but it might as well not.
The ability to see distinct frames very much depends on the lighting and the user's alertness and drug intake, as well as colour information and possibly other factors. Cones are more sensitive (to this) than rods, and the responsiveness of bipolar cells is moderated according to how much total light the brain gets to see. Also, motion blur (such as on a typical television set) can mask what would be disturbing on a LCD showing much higher quality computer graphics.

In a gloomy cinema like in the first half of the 20th century, 24 fps were totally enough. For watching TV in a typical mostly dark living room in the 1960s or 70s, 50 fps were considered ok. On a CRT computer screen, 60 fps are usually considered enough, however a LCD may as well require 80 or 100 fps, and it depends on how the user lights his working/gaming place, too. And, whether he has been up all night nor not, or whether he is on the 3rd can Red Bull.

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j83    101
Just for a point of reference,

I used to play PC games on a CRT at 120hz, as I loved the fluidity. However, with newer games that have very nice motion blur (object-based motion blur, etc.), framerate in the 20s-30s is still rather nice.

The problem is, on games that have no form of motion blur, there is no interpolation between frames, so things just "appear" in different positions depending on speed, camera, etc.

However, in movies/cinema, there is of course natural "motion blur" so 24-30 frames is nice, whereas higher framerate almost makes you loose that "cinematic" feel we experience and have experienced in the past.

Now, we have LCDs that take 30hz signals or 60 or whatever signal you give it, and interpolate up to 120hz or 240hz and that's nice (though your LCD is probably just the standard 60hz monitor)

And as for what the human eye can see, IIRC, there have been studies for pilots and other fields, and normally the human eye can see well above "90 frames" per second.

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