Knowledge Base

Dropped Frames Correction

Aim

To interpolate data points when a video frame is not available at the required sample time.

Note: This information does not apply if you use analog cameras, GigE cameras and USB cameras supported with EthoVision XT. Normally those cameras do not skip a significant number of video frames, provided that you set the camera properly, especially the resolution, the frame rate and the exposure time. For digital cameras, see Configure the Digital Camera.

Important: This correction only deals with missing video frames, not when EthoVision XT does not detect the subject in a valid video frame, for example because of wrong detection settings. If you want to interpolate the points when the subject was not found, do this in the Track Editor. See Interpolate Points.

Background Information

Some low-end cameras like webcams and IP cameras are not capable of achieving a fixed video frame rate. With those cameras, the time between two consecutive video frames is not constant enough, so that EthoVision XT cannot assign a video frame to a specific sampled time.

EthoVision XT accepts video frames originated from a camera or a frame grabber, and assigns them to the expected sample time t, t+1, t+2, etc. according to the specified sample rate.

In the figure below, frames 1 and 3 arrive at the expected times. Frame 2 arrives a bit late but it is assigned anyway to its expected time. In all those cases EthoVision XT produces a valid sample.

However, if video frames do not arrive for significant time, no frame is assigned for one or more sample times. The result is a missing sample at time 3. Note that, because a frame was missing, frame 3 is assigned to time 4.

Whenever missing samples occur, EthoVision XT can fill the gaps with interpolation, using the adjacent valid samples.

Procedure

1. In the Detection Settings, under Advanced, open Smoothing.
2. Next to Dropped Frames Correction, do one of the following:
   • Select On if you want to apply the dropped frames correction. The missing samples will be replaced with valid samples obtained with linear interpolation.
   • Select Off (default) to leave the missing samples. If you select Off, you can always interpolate the missing samples after acquisition, in the Track Editor. See Interpolate Points.

Notes

• Dropped frames correction occurs during tracking. Interpolation is performed only for missing samples due to dropped frames. EthoVision XT interpolates between the last valid sample before a missing sample and the first valid sample after the last missing sample, for a maximum of 10 consecutive dropped frames. If more than 10 dropped frames are found, correction is not done, and the subject remains missing for those samples.
• Dropped frames correction does not change the video time of the valid video frames. For example if the frame rate is not exactly 30 fps, the time between subsequent frames will not be exactly 1/30 s = 0.03333 s. Small differences in the time between subsequent frames remain.
• Interpolation is only applied to the body points, and to the subject area. The center point is interpolated linearly using the center points in the adjacent samples. The nose- and tail-base point are obtained by doing interpolation on the angles formed by the segment joining the nose-point, the center point and the tail-base point. See How the Nose-Point and Tail-Base Point Are Interpolated.
• Interpolation is not applied to: body elongation, head direction, area change from the previous sample (used to calculate mobility), and activity. Those values remain missing for the interpolated sample.
• To know the percentage of video frames actually corrected, see View the Detection Statistics.

See Also

• Advanced Detection Settings: Smoothing