Detecting dropped frames

Occasionally you will drop frames if you:

  • try to do too much drawing

  • do it in an inefficient manner (write poor code)

  • have a poor computer/graphics card

Things to avoid:

  • recreating textures for stimuli

  • building new stimuli from scratch (create them once at the top of your script and then change them using stim.setOri(ori)(), stim.setPos([x,y]…)

Turn on frame time recording

The key sometimes is knowing if you are dropping frames. PsychoPy® can help with that by keeping track of frame durations. By default, frame time tracking is turned off because many people don’t need it, but it can be turned on any time after Window creation:

from psychopy import visual
win = visual.Window([800,600])
win.recordFrameIntervals = True

Since there are often dropped frames just after the system is initialised, it makes sense to start off with a fixation period, or a ready message and don’t start recording frame times until that has ended. Obviously if you aren’t refreshing the window at some point (e.g. waiting for a key press with an unchanging screen) then you should turn off the recording of frame times or it will give spurious results.

Warn me if I drop a frame

The simplest way to check if a frame has been dropped is to get PsychoPy® to report a warning if it thinks a frame was dropped:

from psychopy import visual, logging
win = visual.Window([800,600])

win.recordFrameIntervals = True

# By default, the threshold is set to 120% of the estimated refresh
# duration, but arbitrary values can be set.
# I've got 85Hz monitor and want to allow 4 ms tolerance; any refresh that
# takes longer than the specified period will be considered a "dropped"
# frame and increase the count of win.nDroppedFrames.
win.refreshThreshold = 1/85 + 0.004

# Set the log module to report warnings to the standard output window
# (default is errors only).

print('Overall, %i frames were dropped.' % win.nDroppedFrames)

Show me all the frame times that I recorded

While recording frame times, these are simply appended, every frame to win.frameIntervals (a list). You can simply plot these at the end of your script using matplotlib:

import matplotlib.pyplot as plt

Or you could save them to disk. A convenience function is provided for this:

win.saveFrameIntervals(fileName=None, clear=True)

The above will save the currently stored frame intervals (using the default filename, ‘lastFrameIntervals.log’) and then clears the data. The saved file is a simple text file.

At any time you can also retrieve the time of the /last/ frame flip using win.lastFrameT (the time is synchronised with logging.defaultClock so it will match any logging commands that your script uses).

‘Blocking’ on the VBI

As of version 1.62 PsychoPy® ‘blocks’ on the vertical blank interval meaning that, once Window.flip() has been called, no code will be executed until that flip actually takes place. The timestamp for the above frame interval measurements is taken immediately after the flip occurs. Run the timeByFrames demo in Coder to see the precision of these measurements on your system. They should be within 1ms of your mean frame interval.

Note that Intel integrated graphics chips (e.g. GMA 945) under win32 do not sync to the screen at all and so blocking on those machines is not possible.

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