Question about the timing of Screen Flip

1
I tried a simple code including the following lines. So the screen was flickering at 15 Hz, with 60 Hz of monitor refresh rate.

...
waitframe = 1;
ifi = Screen('GetFlipInterval',w);
tvbl = Screen('Flip',w);
tvbls = [];
k = 1;
while ~KbCheck
Screen('FillRect',w,128-127*(-1)^k);
tvbl = Screen('Flip',w,tvbl+(waitframe-0.5)*ifi);
tvbls = [tvbls tvbl];
k = 3-k;
end
...


I though the result of diff(tvbls) would be around .033 (i.e. 2*ifi). But I found they were around .0267. But if waitframe was set to zero, i.e. the screen flip at every vertical retrace, diff(tvbls) became closely around .016. Why?
2
That is funny. What operating system are you running (version, etc all the details) and what graphics card? You are using a fullscreen window, right?
Best,
Dee


On Thu, Apr 18, 2013 at 11:16 PM, ptbmin <ptbmin@...> wrote:

I tried a simple code including the following lines. So the screen was flickering at 15 Hz, with 60 Hz of monitor refresh rate.

...
waitframe = 1;
ifi = Screen('GetFlipInterval',w);
tvbl = Screen('Flip',w);
tvbls = [];
k = 1;
while ~KbCheck
Screen('FillRect',w,128-127*(-1)^k);
tvbl = Screen('Flip',w,tvbl+(waitframe-0.5)*ifi);
tvbls = [tvbls tvbl];
k = 3-k;
end
...

I though the result of diff(tvbls) would be around .033 (i.e. 2*ifi). But I found they were around .0267. But if waitframe was set to zero, i.e. the screen flip at every vertical retrace, diff(tvbls) became closely around .016. Why?


3
I tried the same code on another computer, and the results looked OK. Both computers are installed with Windows 7. The difference is the one I tried yesterday has two graphics cards with the nVidia Optimus technique. It seems the Optimus technique messed up everything.

--- In psychtoolbox@yahoogroups.com, "Diederick C. Niehorster" <dcnieho@...> wrote:
>
> That is funny. What operating system are you running (version, etc all the
> details) and what graphics card? You are using a fullscreen window, right?
>
> Best,
> Dee
>
>
> On Thu, Apr 18, 2013 at 11:16 PM, ptbmin <ptbmin@...> wrote:
>
> > **
> >
> >
> > I tried a simple code including the following lines. So the screen was
> > flickering at 15 Hz, with 60 Hz of monitor refresh rate.
> >
> > ...
> > waitframe = 1;
> > ifi = Screen('GetFlipInterval',w);
> > tvbl = Screen('Flip',w);
> > tvbls = [];
> > k = 1;
> > while ~KbCheck
> > Screen('FillRect',w,128-127*(-1)^k);
> > tvbl = Screen('Flip',w,tvbl+(waitframe-0.5)*ifi);
> > tvbls = [tvbls tvbl];
> > k = 3-k;
> > end
> > ...
> >
> > I though the result of diff(tvbls) would be around .033 (i.e. 2*ifi). But
> > I found they were around .0267. But if waitframe was set to zero, i.e. the
> > screen flip at every vertical retrace, diff(tvbls) became closely around
> > .016. Why?
> >
> >
> >
>