WSU places two markers on the plate to identify the y direction (toward pitcher) and the plate tip as in Figure 5. If the plate is not level, these markers are shimmed to make level with respect to the upward (z) vector. The marker on the tip of the plate is directly used in motion capture software to assign the global origin of the coordinate system. The calibration reference images are shown in Figure 3.

Figure 6. Calibration panel reference images from each camera for the same scene.

Once the initial calibration is complete, a pneumatic cannon is used to launch pitches over the plate at 95 mph through the calibrated volume as in Figure 7. These pitches are filmed by each camera until around 100 pitches have been captured.

Figure 7. Pitches are launched with a pneumatic cannon through the calibrated volume at high speeds

After collecting the high-speed videos from a ground-truth test, ball position is meticulously tracked in each frame by a technician in the lab. The technician uses software tools to optimize the images, and identify the center point of the ball as shown in Figure 8. Seven frames are tracked from each camera while the ball is crossing over the leading edge of the plate.

Figure 8. Still frames tracked in the lab showing pixel boundary identification in a synchronized pair of frames, one from each camera.

Since we would be very lucky to have a frame timed exactly as the ball crosses the front face of the zone, the ball position at y=17 inches is interpolated from trajectory data before and after the ball crosses into the zone. Here, the side (x) and height (z) vectors are regressed to the y direction (toward pitcher) and a linear equation is used to interpolate the side and height position when directly over the leading edge, at y = 17 inches. Pitch location at the front face of the strike zone (y=17” plane) is the central data product of each pitch during a ground truth test.

Pitch Release

Using a similar method to the strike-zone ground truth, we can also precisely track ball position at pitch release. In the case of a pitch-release ground truth test, a larger calibration board is elevated to calibrate the volume surrounding a pitcher’s release point as in Figure 9.

Figure 9. Elevated calibration board for a pitch-release ground truth test.

In a pitch-release ground-truth test, the ball is tracked as it passes through the calibrated space and the release point can be identified down to the frame-rate resolution of the cameras used. Figure 10 shows the frame-to-frame ball movement near the pitch-release point. Here, the ball position in subsequent frames is represented by red dots.