If you have VideoPoint, software which extracts numerical data from digitized
movie files, then you may be interested in my simplified instruction for it.
Feel free to give it to students. Otherwise delete this long message NOW. It
describes a simple way of using the software. I wishthey had a version of the
program which is limted to essentials. Advanced features (such as plotting
graphs, calculating distances between moving points, calculating locations of
the center of mass, viewing motion from an arbitrary non-inertial frame of
reference, etc.) add to the complexity, size and cost.
Also watching how everything is done quickly on the screen is not the best
way of learning. Mastering the tool would be less demanding without the
"advanced" stuff. I was cursing a lot while trying to make sense out of the
User's Guide. The "simple way" described below is likely to be appropriate
for an introductory physics lab. The CD-ROM, on which the software is sold,
includes over 250 digitized movie clips of good physics experiments. That
is great. Each is a short file; you can use another program (instead of
VideoPoint) to work on these files, if you prefer.
Ludwik Kowalski
The price was $160 for the first copy, including the printed manual, and
$50 for each additional CD-ROM. MAC and WINDOW versions from PASCO. For
information refer to hhtp://www.lsw.com/videopint or videopoint@sw.com
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You are going to extract data from still frames of a digitized movie file.
Consecutive frames are still pictures taken every 1/30 seconds. About ten
to twenty frames will usually provide enough data for a single activity.
Your goal it to determine coordinates of one or more moving objects. The
coordinates will always be with respect to the lower left corner in each
frame.
VideoPoint was designed for many tasks; what is described below will be
sufficient for our purposes. The software has already been installed and
you must proceed as follows:
1) Start the application by double-clicking on its icon (located in the
VideoPoint folder). Then click anywhere in the "About VideoPoint"
window. The "Startup" window will be displayed on you screen.
2) Use this window to specify the movie file you want. To do this click
on the OPEN MOVIE button and enter the name of the file (and folder)
into the dialog box which appears. Click OPEN and wait for the first
movie frame to be displayed in the dialog box. Accept 1 by pressing
the OK button (because you want to study one moving object).
3) The first frame of your movie reappears in the new window. You know
it is the first frame because t=0 is near the lower left corner. The
frame number and the total number of frames in your movie are shown
in the upper right corner. Click the little triangle (next to t=0; it
is called PLAY/PAUSE button) and frames will parade one after another.
Each frame is identified by the time t, relative to the first frame.
Observe the SLIDER advancing from left to right over the time control
bar. The showing stops on the last frame or when the slide button is
clicked again. Move this button manually, by dragging it left or right
with the mouse. This is one way to select quickly a particular frame.
The two traingles on the right of the control bar (called STEP buttons)
can be used for going from one frame to the next, either forward or
backward. This is another way to going to a selected a frame.
4) In addition to the frame viewing window (described above) you should
see the Table window and the Coordinate window. Ignore the Coordinate
window; it is not needed unless the advanced features are used. The
table, in the Table window, has three columns. The first column is
already filled with values of times in seconds, for consecutive frames.
The next two columns are reserved for x and y of the moving object you
want to study. It will be one object each time. Enlarge the Table window
and move it (by the title bar) to hide the unused Coordinat window.
5) You already know how to navigate from one frame to another and see
where a moving object is. Now you will learn how to get its coodrinate
data. Click on the displayed Movie window (to activate it) and see what
happens when the mouse pointer is moved from one place to another. The
x and y coordinates of the pointer are displayed near the lower left
corner of the window. Unfortunatery they are expressed in arbitrary units
called pixels. You want them to be in real world units, such as meters
or centimeters. Therefore you must calibrate frames. This is possible
when you know the size of at least one object on the screen. Suppose
that the object is a box facing the camera and that its size is 50 cm.
6) From the MOVIE menu select the SCALE MOVIE command. The dialog box
appears. Enter "50" for the known length and select "cm" for the
units. Then press the CONTINUE button. Click on each side of the box.
A line appears across the box. This is the end of calibration; all
coordinates will now by in centimeters.
7) You can examine as many locations as you wish; just position the
pointer over them and read x,y. This can be done in each frame. As you
can see, placing a pointer over any screen position does not record
anything into the table. But it shows the coordinates of that location.
You can, if you wish, prepare a table by hand and write the data into
it, for as many points as you wish. Then go to other frames and record
new positions of moving points. This way of collecting data can called
manual; nothing is automatically recorded in the Table window.
8) To enter the data into the table you must click on a point, move to
the next frame and click on it again (most likely in a new location),
move to the next frame, etc. To facilitate this process the frame
advancing takes place automatically after each click. What is entered
into your table is the pair of coordinates of points at which you
click; you must be careful to click in rigtht places. And you are not
allowed to click more than once in each frame. The way of gathering
data "by clicking" is more rigid than the "manually" but you do not
have to write anything, the table is created for you and the data are
entered into it automatically at each click.
9) After collecting the data choose the SAVE AS command from the FILE
menu and assign the name for what you are saving. Make sure you are
NOT choosing a name of an existing file. You can now end the session
by choosing the QUIT command from the same menu. Note that by saving
the result of your work you do not create another version of a movie
file; you are saving the table and information about where the movie
is. If you open the file saved in a previous session (it is in the
same folder as the VideoPoint application) you see the table with the
previousely created (t,x,y) data. You can continue working on it by
adding more data points. Eventually you will delete the file which
contains your data. This will not delete the original movie file,
only the file you created to store the data.
10) Start analysing the data, as specified by the instructor. Keep in
mind that the table with data can be printed with the PRINT WINDOW
command in the FILE menu. It can also be pasted into a text document,
for example, into your lab report.