Psychtoolbox>Screen.{mex*} subfunction

[fps starttime] = Screen(‘StartVideoCapture’, capturePtr [, captureRateFPS=25] [, dropframes=0] [, startAt]);

Start video capture device specified by ‘capturePtr’. If ‘captureRateFPS’ is
provided, the device is requested to capture at that rate. Otherwise it is
requested to operate at 25 Hz. The real capture rate may differ from the
requested one, depending on the capabilities of your capture device. For
professional firewire cameras when used with the IIDC-1394 videocapture engine,
PTB tries to select the closest supported framerate that is at least as high as
the requested one. For other videocapture engines, e.g., [GStreamer](GStreamer), the result
is camera dependent. It may choose the closest supported rate, or the maximum
supported framerate. The real capture rate is returned as return argument ‘fps’.
If you pass the special value realmax, then PTB will try to select the highest
framerate that your capture device supports at the given video resolution and
other settings.
Some capture devices may only support a small set of framerates and may fail to
start completely if you or PTB on your behalf request a unsupported
‘captureRateFPS’. Some devices ignore the requested framerate completely and run
at whatever rate they like, e.g., Apples iSight cameras adjust their framerate
depending on lighting conditions. You may need to tinker a bit here, the
auto-selection of framerates is not fool-proof.
If ‘dropframes’ is provided and set to 1, the device is requested to always
deliver the most recently acquired frame, dropping previously captured but not
delivered frames if necessary. The default is to queue up as many frames as
possible. If you want to do video recordings, you want to have the default of
zero. If you want to do interactive realtime processing of video data (e.g,
video feedback for action-perception studies or build your own low-cost
eyetracker), then you want to use dropframes=1 for lowest possible latency.
‘startAt’ This optional argument provides a requested start time in seconds
system time, e.g., values returned by GetSecs() or Screen(‘Flip’), when capture
should really start. Psychtoolbox will wait until that point in time has elapsed
before really triggering start of capture. This provides a means of
soft-synchronizing start of capture with external events. The optional return
value ‘starttime’ contains the best estimate (in system time) of when video
capture was really started. Accuracy of ‘startAt’ and ‘starttime’ is highly
dependent on operating system and capture device. It is assumed to be very
accurate on Linux with the dc1394 capture engine and professional class IIDC
1394 firewire cameras. No such guarantees are possible on other operating
systems or with other hardware. Specifically standard consumer DV cameras or USB
based webcams or built in cameras in Laptops (iSight et al.) may have rather
unpredictable and variable latency and start timing. Choose a pro operating
system and pro equipment if you really need timing precision!

###See also: CloseVideoCapture StartVideoCapture StopVideoCapture GetCapturedImage