Aspect Ratio and Calibration
Note: If you
are only interested in what to do about the aspect ratio problem,
you can skip this introductory material.
Since the early days of television, the American
television standard (NTSC) has specified an image that is about
525 lines high and 64 microseconds wide. Note the difference in
units! This is because TV images are defined to be scanned rather
than static. Part of the image is blanked out, to give the scanning
circuits time to prepare for scanning the next line. Of the remaining
image, part is expected to lie beyond the edges of the picture
screen, invisible to the viewer. What is left, the part called
the "clean aperture," is the only part that is supposed
to be seen by the viewer. This is the image that forms a picture
with an aspect ratio (i.e., ratio of width to height) of 4:3.
In other words, if a TV camera is properly aimed at a physical
object that is 4 meters wide and 3 meters high in the object plane,
and the camera lens is zoomed so the the image of that object
is 480 scan lines high, then the image of that object would precisely
fill the clean aperture.
To the computer industry, a computer monitor
is thought to consist of square picture elements (pixels), even
if it is actually a scanned cathode-ray tube. The height and width
of an image on a computer monitor is therefore specified in terms
of the number of pixels of height and width. The "pixel aspect
ratio" of the image is the ratio of width in pixels to the
height in pixels. If a photograph is digitized and displayed on
a computer monitor, then it will appear undistorted as long as
its pixel aspect ratio equals its original aspect ratio. With
this in mind, companies who design video capture cards make them
so the 52 microsecond line width of the clean aperture in a TV
signal is digitized into 640 pixels, giving a pixel aspect ratio
of 640:480, or 4:3. A video file captured with such a card can
be opened in VideoPoint,
calibrated with a known length in any direction and analyzed correctly.
Unfortunately, the television industry sees
things differently. The Society of Motion Picture and Television
Engineers (SMPTE) published a standard that defined a digitzed
television image to have a width of 720 pixels and a and height
of 486 pixels (the most important part of this standard for our
purposes is "Center, Aspect Ratio and Blanking of Video Images,
RP 187." SMPTE Journal, August 1995, page 570.). The
width of 720 was chosen to be compatible with both the American
NTSC standard and the European PAL standard. The height of 486
was chosen to be slightly larger than the clean aperture so there
would be room for digital edge artifacts to go unnoticed by the
viewer. For the same reason, the width of 720 was defined to include
slightly more than the clean aperture.
The DV consortium defined a standard that is
used in all consumer equipment carrying the DV logo. The DV standard
must differ somewhat from the SMPTE one, since it calls for a
height of 480 pixels rather than 486. However, the DV standard
was not published in any journal that that I could find on library
shelves, so I do not have a reference to the defined size of the
clean aperture. Nevertheless, by analogy with the SMPTE standard
and in view of the measurements described below, it is clear that
720x480 pixels is NOT the size of the clean aperture. According to an
interesting article by Chris Pirazzi, you should
convert to 654x480 pixels.
I used two different camcorders (a Canon Optura
and a Sony VX-700) to photograph a scene with several vertical
and several horizontal meter sticks. After transferring the DV
output of the camcorders to a computer, I opened each file in
using one meter stick and then measured the length of a perpendicular
meter stick. I did this for several sets of meter sticks and used
the average ratio of these measurements to see what factor was
needed to resize the horizontal dimension to yeild equal-length
meter sticks on the final image. For both camcorders, the result
was that a width of about 658 pixels worked. I resized each file
from 720x480 to 658x480 and found that calibration in either dimension
led to correct measurements in both dimensions.
However, recently some RIT students calibrated several newer Sony DV camcorders. They found that resizing the files from 720x480 to 640x480 yeilded correct calibrations in both dimensions.
Apparently Sony has changed the way it manufactures camcorders.
Whatto do before making measurements with DV files
If you are only making vertical measurements, use a vertical calibration object. If vertical measurements
are more important than horizontal ones (this could be the case for projectile motion if you need to measure g),
use a vertical calibration object.
If you need to make accurate measurements in both dimensions, check the calibration for your camera! To do this:
- Make a short video of two metersticks at right angles to each other. Set up the shot carefully
so the metersticks are in a plane perpendicular to the camera's optical axis, and the camera is aimed at the centers of the metersticks.
- If you are using VideoPoint Capture or LoggerPro, capture and save the movie with the default settings. Otherwise, capture the video in its native DV format, which has a pixel
aspect ratio of 720x480.
- When you save the file, recompress it
in another codec (H.264 or Sorenson 3, for example) and simultaneously
resize it to 640x480.
- Open the file in LoggerPro or VideoPoint. Use the horizontal meterstick to calibrate, and then measure the length of the vertical meterstick.
- If the result is between 0.995m and 1.005m, your videos are being properly captured.
- If the result is different, try resizing the video to 658x480
and check the calibration again.
- If the vertical measurement is still not close to 1 meter, you will need to find another size by trial and error that does work for your camera.