Projection Angles
As the first portion of this module has shown, 2D video and especially 3D graphics can be used as valuable visual feedback and allow for an interactive review of the golf swing and help develop the path to improvement. However, there are limitations to what can be done.
You may have noticed that so far, we have not discussed making any actual measurements in 2D video and in the forms of 3D we have discussed. Because the golfer’s body and club move and rotate in all possible directions, performing quantitative assessment, is complex, and either should not be done when working with 2-dimensional views, or only with extreme care.
The most basic form of measurement that can be done with 2D video is to draw lines on a screen and measure the angle formed by the two lines. This type of angular measurement is called a “Projection Angle.” It gets that name because it is the angle between two segments as it is projected, and then measured, on a plane. It can also be described as the angle from a specific viewpoint. It is important to understand that projection angle measurements are not necessarily the same as making an anatomical measurement between two segments. Let's see for ourselves:
We're going to take a look at how camera view can impact how much rotation a golfer appears to have. Once again, we use the body lines and gears, and we're going to focus on the shoulder line that runs through the center of the right and left shoulder. This nearly overhead view, gives a great view of how much the golfer is turning his shoulders.
But in most lesson settings, this is not a realistic position to place the camera. So we'll add a line to mark how much shoulder turn he has. And then shift the camera down to a more traditional face on view. As you can see, as the cameras view changes, it appears that the golfer has a lot less shoulder turn. In fact, for many golfers a face on view will under estimate total shoulder turn. This is a problem we encounter when using projection angles or trying to measure movements that happen in three dimensions when we only have a single two dimensional view.
So it is easy to see how moving a camera changes the projection angle. There's another way this happens without the camera moving at all. In this case, we'd lower the camera relative to the golfer's shoulder plane, but the same thing can happen if we keep the camera angle the same but the golfer starts swinging with a flatter shoulder plane. Whether the camera moves down, or the shoulder plane gets flatter, the effect is the same, the shoulder plane gets higher relative to the camera. And the apparent amount of shoulder turn goes down.