Re: cyclical frame rates

Forums > Jitter > cyclical frame rates

Aug 13 2012 | 11:34 am


I've been running into this issue again and spent a good while testing. This time I wanted to expand my setup with a 4th projector.

Original setup:
– working display on HDMI: 1920×1080 @ 60Hz
– 3 projectors connected to a triplehead2go on DVI: 2400×600 @ 85Hz

One OpenGL rendering context & window covers the triplehead output. Qmetro set at 85Hz, sync 0. This runs fine at 80+ fps.

Now when I connect the 4th projector to a mini-DP port (with a VGA dongle) and stretch the render window across it so it becomes 3200×600 the fps starts cycling between 80+ and 50-something fps.

If I leave the original 2400×600 window as is and add a second render context and window for the 4th projector instead: same thing, fps cycling.

But if I put that same second window on the HDMI display instead of the projector there's no cycling. I have a feeling that has to do with this display running at 60Hz instead of 85 like the projector. If I put the qmetro at 60Hz there's no cycling. But that's reducing potential performance by 25% to fix the issue and I would like to get the extra fps in my project…

Next thing I tried is taking out the HDMI display @ 60Hz out of the loop. Then there's only 2 displays both with a refresh rate of 85Hz. Cycling is much more moderate now between 80+ and 70 fps. Setting qmetro to 70 Hz shows no cycling. That's not ideal though acceptable but now I don't have a working/patching display… Possibly getting a display that works at 85Hz as well can resolve that.

So after testing all this an typing it here I tried something that 's a total long shot: change the main display to another than the HDMI screen. So I put OSX's menu bar (in display preferences) to the projector on the mini-DP port. Well what do you know: 80+fps steady! If I make the triplehead output the main display it doesn't work and I get fps cycling…

Attached is a screenshot of the working display arrangement. Left is the triplehead display on DVI, middle the 800×600 projector on mini-DP and right the 1920×1080 display on HDMI.

So my main conclusion is that there's a lot of voodoo going on behind the scenes at OS or hardware level and that we have little control over these things. There seems to be a big factor of luck involved in whether our particular setups are going to work optimally.

Any thoughts or experiences to share on the subject?

System: OSX 10.7.4 (hackintosh), Max 6.0.5, AMD Radeon HD6870 graphics card, i7 2600k cpu, 8GB 1600MHz ram

NB: I'm taking care of reinitializing the render contexts by toggling window floating after positioning etc. Putting the displays to the right or let of the desktop (positive or negative window coordinates) has no effect.

This is my test patch:

-- Pasted Max Patch, click to expand. --


  1. ScreenShot20120813at1.28.51PM.png


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