[visionegg] Re: What happens during a dropped frame?
- From: Andrew Straw <astraw@xxxxxxxxxxx>
- To: visionegg@xxxxxxxxxxxxx
- Date: Mon, 18 Jan 2010 11:08:27 -0800
Jens Kremkow wrote:
> Dear all,
>
> I am using noise stimuli in which the exact content of each frame is
> important for the analysis at a later stage.
> My, I hope simple, question is: what happens on the screen when a frame
> is dropped?
>
> Does it work like this:
>
> frame[0] ok -> frame[0] on screen
> frame[1] dropped -> frame[0] on screen
> frame[2] ok -> frame[2] on screen
>
> Or is it more complex?
Hi Jens -- it depends on how you're generating the frames. If you're
doing a simple playback like:
for i in range(10):
draw_image(i)
screen.swap_buffers()
then you will get the following space-time diagram for an X moving to
the right at constant speed (view with monospaced font):
space ---->
t __X______
i ___X_____
m ____X____
e ____X____
| _____X___
| ______X__
| _______X_
v ________X
In other words, the draw_image() function doesn't know a frame was
skipped, and so the diagonal line in space-time gets a constant time offset.
On the other hand code like:
for i in range(10):
draw_image_for_time( get_current_time() )
screen.swap_buffers()
Will do this:
space ---->
t __X______
i ___X_____
m ____X____
e ____X____
| ______X__
| _______X_
| ________X
v _________
In this case, the overall diagonal line in space-time is maintained, but
there is a momentary glitch at the frame skip. Most of the VE stimuli,
e.g. the sinusoidal gratings, are of the second type.
I hope that helps.
-Andrew
--
Andrew D. Straw, Ph.D.
California Institute of Technology
http://www.its.caltech.edu/~astraw/
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