[Development] Late API addition in QScreen for 5.0

Girish Ramakrishnan girish at forwardbias.in
Fri May 4 17:55:23 CEST 2012


On Fri, May 4, 2012 at 7:22 AM, Samuel Rødal <samuel.rodal at nokia.com> wrote:
> On 05/04/2012 04:03 PM, ext Christoph Feck wrote:
>> On Friday 04 May 2012 13:37:04 Samuel Rødal wrote:
>>> Hello,
>>> to be able to achieve smooth animations in QML 2, the animations
>>> should ideally use a fixed timestep, and not a timer which might
>>> have inaccuracies depending on the platform and won't give fully
>>> smooth results.
>> Does OpenGL 2 have API to drive animations by vertical blanking
>> interrupts instead of using a timer? From my experience with movie
>> players, you always get tearing or stuttering, if the frame rate of
>> the display is not a 100% exact multiple of that of the animation or
>> the video.
> No, there's not any API in OpenGL for that. It's all very windowing
> system and graphics driver dependent.
> I believe Mac OS X doesn't have any tearing, correct me if I'm wrong though.
> On Linux, it's all up to the graphics driver in my experience. With the
> binary Nvidia driver the only reliable way I've seen of enabling vsync
> has been to do "export __GL_SYNC_TO_VBLANK=1" before launching an
> application. AMD's Catalyst control panel now has an option called "Tear
> Free Desktop" which is supposed to also sync rendering, but when I've
> tried it I've still gotten tearing, just now it's in the same location
> each frame, which looks even worse.

The advice above needs to make it to the release notes since everyone
is going to face this problem. On X11 (with xcb), we were seeing high
cpu usage and poor performance with nvidia cards. After Samuel
enlightened us about the env variable, QML2 apps started working very
smoothly. (Donald, want to add more to this?)

Does it make sense to just set the environment variable in the xcb
plugin itself? Maybe it's required to be set before the gl library is
loaded, so then it won't work. What's the best way to advertise this,
given that most people won't read the release notes anyway :-) ?


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