[visionegg] Re: What happens during a dropped frame?

  • From: Jens Kremkow <kremkow@xxxxxxxxxxxxxxxxxxxxxxxx>
  • To: visionegg@xxxxxxxxxxxxx
  • Date: Tue, 26 Jan 2010 07:16:17 +0100

Dear Andrew,


On 18.01.2010, at 20:08, Andrew Straw wrote:

> 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.
> 

It indeed helped my understanding. Thanks a lot for responding so quickly.

Regards,
Jens

> -Andrew
> -- 
> Andrew D. Straw, Ph.D.
> California Institute of Technology
> http://www.its.caltech.edu/~astraw/
> ======================================
> The Vision Egg mailing list
> Archives: //www.freelists.org/archives/visionegg
> Website: http://www.visionegg.org/mailinglist.html


=====================================The Vision Egg mailing list
Archives: //www.freelists.org/archives/visionegg
Website: http://www.visionegg.org/mailinglist.html

Other related posts: