How accurate are angle and distance measurements made on video with Dartfish? That’s a question we are often asked and we hope that this study can go some way towards an answer.
Moataz Eltoukhy, Shihab Asfour, Craig Thompson & Loren Latta in their paper Evaluation of the Performance of Digital Video Analysis of Human Motion: Dartfish Tracking System published in the International Journal of Scientific & Engineering Research March 2012, set out to compare the accuracy of motion capture in three dimensions (Vicon) and two dimensions (Dartfish). Their conclusion was “…this method has serious potential. The moment calculations were rather close to the 3D motion capturing system measured values.”
Tips for getting best results
- Starting with the basics: In any 2-D measurement system the camera must be perpendicular to the plane of movement and facing directly at the object being measured. Where this is impractical or impossible, measurements made will be relative rather than absolute i.e. they are comparable with other measurements made under the same conditions but cannot be stated to be the actual position, distance or angle with any degree of accuracy.
- The study uses Basler scientific cameras which have CCD rather than CMOS sensors. A CCD sensor behaves in the same way as photographic paper – the entire image is exposed simultaneously. A CMOS sensor exposes the image in the same way as a photocopier, in a rolling action (only much faster). Fast moving objects can be slightly distorted on video captured by a CMOS sensor.
- Dartfish automatic tracking targets a group of pixels and identifies where they appear in one frame to the next. Anything that can be done to make those pixels stand out will aid automatic tracking. Use body markers; use an exposure setting which eliminates motion blur; use additional lighting as required to achieve a colorful, well defined image.
- Automatic tracking is actually semi-automatic. In situations where automatic tracking does not work, step through the video frame by frame and when the tracking is lost, manually drag the marker or measurement tool back into position. Dartfish ‘learns’ the correct position and will ‘remember’ it when the video is replayed.
You do not have to track the object throughout the video: Tracking starts at the frame where it is switched on and can be suspended when no longer required. - Measurement units can be changed from the default (meters). See the Dartfish options (Tools menu > Options > Analysis).
- Zoom in to accurately place markers or make measurements. The easiest way to zoom in and out is to use the mouse: Click the video then use the scroll wheel of the mouse; on older laptops, stroking the right edge of the touch-pad would achieve the scroll/zoom effect; newer touch-pads have the same pinch-to-zoom gesture that would be used on a smart phone. Its worth taking a moment to discover how to do this.
- Use the ‘0 px’ line thickness of drawings to aid accurate placement.
- Learn more about measurements, tracking and data in the Extracting Data from Video tutorial
