Video-Based Motion Analysis

Transcription

1 Video-Based Motion Analysis Bruce McKay Saint Ignatius College Introduction This session will cover the basics of recording and producing video clips for analysis. Video analysis provides the opportunity to make accurate measurement of motion outside the Physics lab as well as inside. Analyse everyday motion, not just whatever is available in a physics lab. Challenge students to produce imaginative projects or come up with your own creative uses. Participants in this session, working in small groups, will use a digital video camera to record their own video clips, edit and export them as QuickTime Movies and then analyse their video. Two different Video Analysis Applications will be used, each available for Mac and PC: Tracker which is free and Java based, and VideoPoint: Physics Fundamentals which is the recently released commercial replacement for VideoPoint. Why Video? You probably will not be prepared to even admit to knowing anything of the days of physics pracs being performed with a Polaroid camera and strobe light to analyse motion and collisions on an airtrack, or of the Project Physics flim loops. But some of us do remember the dim dark ages of the 1970s and 80s. Video cameras did not exist then. Imagine what it would have been like in those days to say Here is digital video camera, take it and film.. then we will transfer the video to computer for a frame by frame analysis! Perhaps you are a fan of data loggers and motion sensors but the choice and scope offered by a video camera opens up a whole new range of activities. I like students to analyse real everyday motion rather than be restricted to activities in a laboratory. I started using VideoPoint about seven years ago. A digital video camera was well over $2000 but they have dropped considerably in price, I think you can find something now for about $600. If you want to put up with lower quality, you can even use an isight camera or a web camera. I used to use VideoPoint to analyse movies but became frustrated as it was not updated beyond version for Mac or version 2.1 for Windows. It required QuickTime 3 so that gives an idea of its true vintage. It continued to run on Mac Classic and probably on Windows XP but my frustration as a Mac user was that it would not run in Mac OS-X. I then heard of Tracker a Java based application written by Doug Brown at Cabrillo College I started using Tracker and then found VideoPoint: Physics Fundamentals in Tracker is free and is regularly updated. VideoPoint: Physics Fundamentals is available as a 30 day trial version but is not cheap. It can be purchased on line: Single user US$180, 5 user US$375, 10 user US$675 and 30 user US$1375.

2 I am happy with Tracker but some other physics teachers prefer the interface and ease of use of VP Fundamentals. Videoing the motion There are a few simple precautions that are necessary. The camera should be mounted on a tripod at a sufficient distance from the action to enable the complete motion to be viewed in the frame. (One projectile motion project was a javelin being thrown and the camera had to be a long way away to keep the motion in view.) The motion should be perpendicular to the camera view. An object should be measured or a distance marked out in such a way as to enable the scale to be determined. (This will be input into the analysis application.) It is helpful if the motion goes from left to right. That will help with measuring and graphing the motion. Transferring from Camera to Computer, Editing and Exporting as a QuickTime Movie As a Mac user, I import the video to computer using imovie, but I am told that a similar method is available using Windows XP. (VideoPoint: Physics Fundamentals comes with the VideoPoint Capture application which works on Mac or PC but it is not required on a Mac.) The remainder of this section will describe how this is done in imovie on a Mac. The camera is connected using a firewire cable and imovie should open automatically. 2

3 You now have control of the camera from the computer using the Play, Fast Forward and Rewind buttons. When you have found the section of video you want, click Import and then Stop when you reach the end. Now that you have imported the video, it is a matter of selecting the small amount that you want and then exporting it. The bottom triangle markers have been moved to the start and finish of the motion and with the triangular playhead in this selection, it can be copied or cut and then pasted into the timeline of imovie. This can now be exported as a QuickTime movie and the best option is Full Quality DV. This QuickTime movie is now available for analysis in either Tracker or VP Fundamentals. 3

4 Analysis using Tracker Introduction Tracker is currently at version It requires Java and QuickTime 7 and of course it runs in OS-X. It was developed by Doug Brown at Cabrillo College on a PC and there were initially some small difficulties running on a Mac, but Doug made it known that he wanted feedback and wanted to get it running properly on a Mac. I did contact him about a small problem I was having and he replied very quickly. I sent an at night and a reply was waiting in the morning. Tracker has excellent help included and comes with video samples for analysis but I much prefer students to produce their own videos. Tracker enables students to perform a frame-by-frame analysis of a QuickTime movie. Various data can be calculated and graphed. A full window is shown below but the various parts of the window can be opened or closed. Downloading and Installing Tracker After you visit the and download Tracker.zip, it is a simple matter to unzip it by double clicking. The latest version of the actual application Tracker.jar may be later than that in the unzipped file. You may have to download it separately. 4

5 Using Tracker Locate the file Tracker.jar and double click to open Tracker. Select IMPORT from the FILE menu and navigate to the QuickTime movie you wish to analyse. (It must have the.mov extension.) (OPEN should work just as well IMPORT with a QuickTime movie.) When the movie is opened the image will probably look terrible. From the VIDEO menu, Select FILTERS, NEW and DEINTERLACE. Select odd or even, it does not matter which and you should now see a high quality image. Click the play button to preview the video in tracker. Now by either selecting the axes icon in the top left, or by selecting AXES from the TRACK menu, show the axes, drag the origin to your preferred place and then LOCK the axes (from the TRACK menu). You will see other options that are available. Now similarly, show the tape measure, drag the ends to your measured length, double click on the number and enter your value and then lock the tape measure from the TRACK menu. You can choose either to show or hide the tape measure. Next, from the TRACK menu select NEW and choose POINT MASS, or do likewise from the Track icon. You should see in yellow Shift-click to mark Mass A. Move the cursor to the part of the object you are going to track and shift-click (or right-click). The Apple Mighty Mouse works fine with tracker. The point is marked and the movie advances automatically to the next frame. Continue until you have the data points you desire. To view the data and to graph it, look for the small faint triangles at the bottom right of the main window. If you click on the correct triangle, you will bring in Plot and Data windows on the right. (You can also split the main window into two, one above the other.) You can change the size of these windows. If you click on the icon on the left hand top of the Data Table, you will see that you can change it to World View or Plot View and similarly the icon at the top left of the Plot View can be changed to World View or Datatable View. Clicking on Data on the top right of the Data View, should show the quantities that can be included in the table. (Sometimes there are no quantities visible in this window but a slight change in size drag the bottom right hand corner of the window, brings things right.) The graph can be changed by moving the cursor over the quantity on either axis. The cursor changes to a hand and clicking brings up all the quantities you can graph. 5

6 After viewing the graphs in Tracker, I prefer to copy and paste data to Excel. To copy data, highlight the data required and with the data selected, CONTROL CLICK or Right Click to bring up a pop-up menu that gives Copy as an option. I use WPtools in Excel to analyse and fit data and to produce graphs. The latest version of Tracker, 1.5.1, now gives Analyse as another option. I have not yet tried this but as it provides statistics and also curve fitting, it may no longer be necessary to use Excel. Information on WPtools is available from WPtools can be downloaded free from While it does not have the most elegant interface, I have found that WPtools still works with the latest versions of Excel and it filled a gap that Tracker formerly failed to fill. VideoPoint: Physics Fundamentals VP Fundamentals leads you through the whole process as the following images indicate. VP Fundamentals presents the option of capturing a movie, of opening the resource library (of videos provided with VP Fundamentals) or of opening a QuickTime movie. The last option is not as obvious but is achieved from the OPEN menu which sits above the window shown above. After clicking on Open, you use the usual navigation to find and open your movie 6

7 The text in the images is self explanatory, but in case it is not clearly visible, it is reproduced below each image. Preview: Preview the movie before moving to the next step. Calibrate Movie: In the movie frame, select an object of known length. Drag the ends of the yellow meter stick to either end of this object. In the field below the movie, indicate the length (in chosen units) that the selected object represents in the real scene. Click OK before moving to the next scene. 7

8 Set Up Analysis: The position of the origin, the rotation of the axes and the frame which corresponds to zero time can be set. Collect Data. Clicking on an object marks its position and advances to the next frame. It is possible to magnify that part of the image (as shown). 8

9 Plot Data. After the data has been plotted, it is possible to play the video and watch the circle move along the graph as it moves through the video. 9

10 The data has been plotted and a curve fit can be done. A Manual Fit is available but to do an Auto Fit it is necessary to open Preferences and select Allow auto fit. The auto fit has been performed. 10

11 Discussion VideoPoint: Physics Fundamentals has a nice new interface and now runs well in Mac OS-X and in Windows XP. It is easy to use and students are led through the whole process. Tracker does something similar, with a different interface which in some cases is not quite as intuitive for a Macintosh user. VideoPoint: Physics Fundamentals is supplied with a user manual as a.pdf file. The manual is hardly necessary but might be helpful if problems arise. Tracker has a Help Menu which provides excellent information. I am happy to use Tracker and it is free. VideoPoint: Physics Fundamentals is moderately expensive but perhaps it is worth it. If you are interested in video-based motion analysis, you can get started for a low outlay. You have to make your own decision as to where your money is best spent. Useful Websites Tracker Home: Tracker Download: Tracker Webstart: Videos from Cabrillo College: VideoPoint: VideoPoint: Physics Fundamentals: WPtools: WPtools Download: 11