single refresh display duration - help?

[Paul Groot]

Hi Anne-Wil,

You will get the best onset performance if you:

1) set the pre-release time of the objects before both slides to the
maximum possible value (i.e. same as duration of that object). In
other words: if the 13ms slide is displayed immediately after the
other slide, you must set the pre-release time of the first slide to
496 (or 500 if you apply point 3 below).

2) Use bitmaps instead of JPEG and make sure the color depth is the
same as used in your exp. This might help because decoding jpeg files
takes a lot of CPU time (although the smaller jpeg files will load
faster from disk.)

3) use durations which end in the middle of two screen refreshes I.e,
if the display is using a refresh rate of 75Hz, you will set the fast
stimulus to 0.5*1000/75=7ms, and the longer stimulus to
37.5*1000/75=500). Not sure if this is still required with the new EP2
release, though...


Depending on the hardware performance and screen resolution, this
still might be insufficient to get single frame stimuli. If this is
the case, you might want to prepare the stimuli using off-screen
canvases at the start of the sequence and copy to the display canvas
using a fast copy at the vertical blanking interval. This should
always work, unless the graphics card is crappy.

best,
paul

2012/3/5 liwenna <liwenna at gmail.com>:
> Hi all,
>
> I have something and I hope someone may have experienced a similar
> thing that may help me out on this one.
>
> I have an experiment in which a sequence of displays occurs with the
> two most important display having durations of 496 and 13 ms
> respectively (give or take max 2 ms). Back then, it took me quite some
> work but in the end I got the timing right (yay).
>
> Now... I am trying to build a second, similar but less complex, task.
> This new is less complex since it shows only a single. rather than
> multiple, image at the 496 ms display, and has a far less complex list
> structure due to less different procedures. However: I can not get the
> timing right at all..  the display that follows the 13 ms display just
> keeps being delayed by one refresh (13 ms).
>
> I have tried pretty much everything for hours and hours:
> - building the new task from scratch, imitating all (or at least I
> believe so?) settings from the old (correctly timed) task (and then
> tweaking all the settings too)
> - modifying the correctly timed task into the new task  (and then
> tweaking all the settings)
> - using off-screen canvases rather than slides (but it seems that the
> 13ms 'canvas' is simply not shown in about 25% of the trials... which
> is not good enough either)
>
> I believe I have covered pretty much everything that could be done
> about it, but I honestly fail to see why the old task would be timed
> correctly (still is!) and any attempt at the more simple task just
> fails in terms of timing.
>
> I so very much hope that someone here is able to identify the one
> thing that I just keep overlooking? >.<
>
> Best,
>
> Anne-Wil
>
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