Datalogging with video Using motion capture apps

About the Tracker Desktop App Tracker is a free Desktop Application. You load a video file that you take of a moving object into the Tracker software. It will then enable you to analyse and graph the movement of the object over time. You need to use the ruler calibration tool in the software to mark a known real world distance. Then you use Shift and Click to mark where the object is in each frame. The tracker software then uses the time between video frames - the framerate (e.g. 30 frames/second) to calculate how much the object has moved between each frame to produce a variety of distance and time graphs. NOTE: Make sure your camera is STILL when taking video to use for this task.

About Vernier Video Physics Vernier Video Physics is an app for mobile devices. You can capture a video of a moving object, such as a rocket car, in the app and then analyse how it moves. You need to use the Origin and Scale tools in the app to mark a known real world distance. Then you use tap and drag to mark where the object is in each frame using the Points option. The software then uses the time between video frames, the framerate, to calculate how much the object has moved between each frame to produce a variety of distance and time graphs. You can also export the data. NOTE: Make sure your mobile device is STILL when taking video to use for this task.

Step 1: Pick a good position Find a good location a safe distance from the object you want to record. Make sure you are perpendicular to the direction the moving object is taking as this helps make for accurate recordings. In this example, the camera/tablet has been set up to the side of the timing gates and it will record the rocket cars as they travel through the gates. A tripod is useful as it prevents the camera moving which would also effect the data. You could also analyse the cars acceleration away from the start line or move further back to create complete graphs of the cars moving from the start to the end of the track which could be helpful to explore the effects of mass and aerodynamics at all the different stages of a run. You could also use more than one camera! Start / Firing line

Step 2: Record a run Listen for the countdown to start and then start to record. Stop the recording when the car or object has passed. If you are using Vernier Video Physics, the video camera is built into the app itself and you can add a new video by pressing the ‘+’ icon. Note: If you record too much video, you can always edit away the parts that do not include the moving object. TIP: Rocket cars will be fast moving blurs in the video. It can help to say the name or number of the car or team so that is recorded with the clip. Individual cars may be hard to identify later when you have recorded several.

Step 3: Set the scale & origin Once you have recorded a video clip of a moving object you are ready to analyse it. You need to set the scale so that the real world distances can be calculated. You can also set the origin which is where the 0 points on a graph would be. In this case the distance between the sensors on a gate is known to be exactly 1m and the scale markers have been dragged over those points and the distance of 1.0 meters that it represents added to the scale. NOTE: The distance you use as a scale need to be in the same plane and distance away as the object you are analysing.

Step 4: Add the points The software allows you to mark a point that represents where an object is in each frame. In this case, the nose of the rocket car has been chosen as the point that will be marked. The point marker tool can be dragged and then when it is in the right place, a tap will add the mark for that frame. The video will then advance to the next frame automatically and you can add the new position using drag and tap. Continue repeating this until the car has been marked in every frame where it is visible. You have now captured the data and can click the graph icon to see it.

Step 5: View the data With the data now captured and the scales set, the app can now produce a variety of graphs. These graphs show that the car moved just over two metres in the recorded section and its velocity reduced from around 17.9 meters/second to just under 17 metres/second over that distance. Air resistance and friction may be two of the reasons why this car slowed at this rate. By analysing the performance graphs of several cars over the same track, patterns may be seen that might help suggest which aerodynamic designs and rocket car masses perform best. NOTE: You can export the data to use in a spreadsheet or other application to explore the data in more detail.

Going further You can use applications like these to analyse other types of movement. How does a pendulum move? How does a ball thrown through the air move? The technique of tracking motion in video is also used commercially to do Motion Capture (MoCap) for video games and animations. Many points attached to a body are tracked and turned into streams of data that can be used to animate 3D models the same way as the original person. 2D and 3D cameras and analysis of the images are also key to how autonomous vehicles can sense and react to objects moving in the world around them.