[GEM-dev] Problem with glitching on OS X at high frame rates

Cyrille Henry ch at chnry.net
Tue Jul 17 21:36:17 CEST 2012


osX synchronize openGL rendering to the screen. i.e if your external screen is at 59.9 fps, Gem will not be able to render more than 59.9 fps.
since Gem rendering is connected to pd timing, it is connected to audio processing. i;e : if gem is only able to compute 59.9 fps when 60 is asked, then pd time will not be synchronize with real world time. so you have click on the sound.

(desincronize openGL render to screen is usually not an option, since result are ugly with fast moving images)

moreover, Gem rendering are computed during pd data processing, i.e. between every audio block. by default, pd use 64 sample buffer. so pd compute audio buffer every 1.5ms. You have to expect a bit of jitter on the 16.666ms needed between 2 images rendered, accentuating previous effect. (if gem image is rendered 17ms after the previous, one have to wait an extra 16 ms...)

Using faster computer will not solve this problem. using faster screen will offer a solution, but that's not a good solution for me, and probably not for you.

If you really need frame accurate render, easiest solution is to desynchronizes sound and image processing. pd~ is not an option since the 2 process will have the same timer.
So, you have to use 2 pd, one for the sound, one for the image, synchronised using netsend / netreceive (on localhost). The best is probably to ask 100fps to Gem, so that it will sync to the screen frequency, whatever the screen is). Then, you should use the screen frequency as the time base of your performance.

using this solution is not to hard to have sound synchronize on screen rendering.
hope that help, don't hesitate to ask more precision if i'm not clear.


Le 16/07/2012 19:31, Theo Burt a écrit :
> Hi, me and a friend of mine do a lot of live audio-video performance, professionally, all using pd/gem as an environment. We both use Macbook Pros (different models), and for the last couple of years we've been having a serious problem which we now suspect is something to do with GEM/PD, rather than, for example, the graphics cards or drivers.
> The problem occurs when running at higher frame rates - it is highly preferable for us to run at 60 frames per second, because (1) it syncs with the refresh rates of most projectors and seems to produce much smoother/more regular movement, and (2) often we are using very precise, high frequency flashing, for optical effects, that we need to be able to sync exactly with screen refreshes. We are generally running the gem window in a second screen (such as a monitor or projector), but it happens when running on one screen too.
> As soon as we run at 60 frames per second (or near it), glitching starts to occur frequently in the audio, and the audio goes out of sync with GEM. While the glitching is occurring, processes in PD and GEM jerk along, pausing frequently. To resolve it (temporarily), the audio has to be reset in PD (by reselecting the audio device). The glitching then goes away and everything runs smoothly, but it returns spontaneously after a short time. The glitching is also exacerbated by moving windows around on the screen. It might be worth noting that the same patch running on the very same machine in Windows XP (using bootcamp) does not have this problem.
> We have different GPUs, mine is a Nvidia 9600m GT, and his was an ATI of some sort. We are using very simple, geometric graphics, and are not approaching the processing limits of the GPUs at all (if I do push the graphics card very hard, I notice that frames are skipped rather than glitching occurring). I also note that it happens equally when there is low CPU activity, so it's not related to that.
> My friend has just purchased a new Macbook Pro, yesterday with a 60% more powerful Nvidia GPU, and unfortunately the glitching is still occurring. It occurs in OS X Lion, and also was happening in Snow Leopard. We've tried disabling power management on the GPU, and this doesn't help either.
> I've looked through the gem code in an attempt to try and understand what is happening, but I am afraid it is beyond me!
> Does anyone have any idea at all what the problem could be? It would make such a huge difference to us if we could resolve it. Thanks very much for any help in advance (and for developing GEM in the first place)
> All the best, Theo
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