Monday, August 16, 2010

Measuring Vertical Leap at a NBA Slam Dunk Contest

The Slam Dunk contest at the NBA All Star event is usually spectacular. Dwight Howard flying with his Superman cape is a great highlight. Slam dunks are all about getting up high and we want to know how high they jump?

Recently a reader of this website asked whether he could measure the vertical leap of winners of past slam dunk contests from NBA footage of the event, including Michael Jordan's leap from the free throw line in 1987. Unfortunately in a basketball broadcast the camera and usually cameras are often moving, panning or zooming to catch the best view of the player, particularly during a dunk. Analyzing the vertical leap from this footage can be tricky.

The answer to the readers question depends a lot on the video footage that is analyzed, but here are a few basics of video analysis and some ideas of how the vertical leap height may be measured from the footage.
  1. Camera angle and view needs to be good to make useful measures of heights (see Video Analysis 101 ). Most footage of the slam dunk contests used multiple cameras and they were often zoomed in at just the wrong time, making this difficult.
  2. We only need one image of the player at the peak of his jump to measure vertical leap, but we will also require some scaling so that we can convert the pixels in the image into a real measurement of height. To do this we need to have a single image that includes the player, the bottom of the backboard, the rim and the floor directly underneath the backboard. With all these in one frame of the footage we can make a measure.
  3. We will also want to know the height of the player so that we can use this height to estimate their vertical leap. We will measure the height of the jump and subtract the players standing height to get a measure of their jump.
  4. Even if all the information is available to us to make the measure, it will still be an estimate as the camera angle may be slightly off and this will make our measure less accurate.
Would this method in fact work?  We can try to analyze a slam dunk contest from 2000 and Vince Carter winning with some serious air. Check out the video below:

From this video we can grab a snapshot of the action at 1:44 in the video and do some measures. First we needed to know some standard basketball measures.
  1. Width of the backboard is 72 inches
  2. Height of the bottom of the backboard is 114 inches above the ground
  3. Vince Carter is listed at 6 foot 6 inches or 78 inches
  4. Hip height of a standard man is approximately 53% of his total height. Therefore Carter's hip height is about 41.3 inches.
Now we can make some measures of Carters vertical leap height. This footage has a graphic that measures his jump height at 37 inches. We will use this as a measure to see how well we can do at measuring the jump height using our estimates and video analysis. See the image below:

In the image we can see the following measures:
  1. We know that the width of the backboard is 72 inches. We have drawn a green line along the bottom margin of the backboard and used this known distance to scale all our other measures.
  2. Next we used the blue line to draw a vertical from the bottom of the backboard to the ground. We know this should be 114 inches. Unfortunately as our scale comes from the width of the back board (green line) we couldn't get it to measure exactly 114 inches with our estimates, but we are close at 114.19 inches.
  3. Finally we measure Vince Carter's hip height at the top of his jump and with the yellow line. Here we needed the line to go from the floor at the same height as the blue line to Carters hips. We estimate his hip height at 78.9 inches or 6.575 feet which is almost his head height.
  4. We can also notice that the camera is not perfectly horizontal with the backboard. We can see that the backboard is not perfectly horizontal accross the screen, although it is close. This angle may reduce the accuracy of our other measures.
Finally we can estimate Carter's maximum vertical leap from this video footage and for this particular dunk. We know his standing hip height is estimated at 41.3 inches and we estimated his hip height at the peak of the jump for the dunk at 78.9 inches. We subtract the standing hip height from the jump height and we get 37.6 inches.

If the measure shown during the broadcast at 37 inches is accurate then our estimate is pretty good and we  can probably try to do the same measures on other broadcast footage of slam dunk contests. All we need is one image with a view of the backboard, the player and the floor below the backboard as well as knowledge of the dimensions of the basketball backboard and height of the player.

We are always interested to hear about your video analysis projects or hear your comments. We are also available to consult to all our readers on their video analysis and biomechanics needs.  

No comments:

More Recommendations