Display update

Support
1

Hello Chris.
I have realized that nextFrameTime() - now() gives about 0.2ms jitter. I assume this is about the error of next frame time estimate. Is there a function giving the actual time of updated display?
Thanks!

-Jing

2

Hi Jing,

The now function just returns the current time. It isn’t tied to the display refresh cycle, so there’s no reason to expect a constant delta between the values returned by it and nextFrameTime.

Are you trying to determine when the current frame was displayed? If so, the best available estimate would be

nextFrameTime() - 10e6 / refreshRate()

Cheers,
Chris

3

I try to copy a digital output while updating the stimulus on display (by put a DO channel to high before the action “update display”). I want to know exact time delay between the DO assigned and next frame shown. I found they are 15.5ms with 0.2ms jitter. I guess my question should be What’s the best way to have output synchronized with monitor refresh? we’d like to register/ trigger some events in physiology which requires ms resolution.

Thanks!

4

Hi Jing,

What’s the best way to have output synchronized with monitor refresh?

At present, I think your best option is the following:

  1. Invoke “update_stimulus_display”
  2. Store the current value returned by nextFrameTime
  3. Wait until now() >= stored_next_frame_time
  4. Set the DO line high

This isn’t ideal, because it’s possible (though not likely) that there will be sufficient delay between steps 1 and 2 that nextFrameTime will return the time of the frame after the one you want to synchronize with. To make this more robust, we could modify “update_stimulus_display” to store the correct next frame time in a variable.

Chris

5

Hi Jing & Evan,

To make this more robust, we could modify “update_stimulus_display” to store the correct next frame time in a variable.

This is done and in the MWorks nightly build. The “update_stimulus_display” action now accepts a new (optional) parameter, predicted_output_time. If the value of this parameter is set to the name of a variable, then “update_stimulus_display” will store in that variable the predicted output time of the frame in which the update will take effect.

Here’s my new recommended method for synchronizing a digital output with a display refresh:

  1. Invoke “update_stimulus_display” with predicted_output_time set to some_var
  2. Wait until now() >= some_var
  3. Set the DO line

Cheers,
Chris