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

Jack jack at rybn.org
Tue Jul 17 23:09:03 CEST 2012

Le 17/07/2012 21:36, Cyrille Henry a écrit :
> hello,
> 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.
Hello Cyrille,

If you have Gem at 100 fps and you screen at 60 fps, you should
encounter a problem of sync at a certain moment, no ? If not, can you
explain more ?
Maybe the best is when you have Gem fps as a multiple of your screen fps ?
Thanx for the rest of explanation.


> 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.
> Cyrille
> 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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