## 2 Materials and Methods
### Experimental Protocols  
To evaluate the methods, three experimental protocols were performed: (I) recording a set of static markers in known positions; (II) recording a moving rod with known dimensions and; (III) a front crawl swimming trial. The first two protocols provided information about the method's accuracy and precision, while the third one allowed for comparing the methods in a more realistic situation.  
First, 15 static markers were positioned in the calibrated plane on pre-defined real coordinates, as presented in Figure 2. This trial was also simultaneously recorded using both cameras; the moving camera started the video at the position of approximately \(-2500\) mm, and finished at approximately 2500 mm. The reconstruction errors of each camera were evaluated by the Euclidean distance from the real coordinates. As mentioned before, extrapolations beyond the calibrated area also increase the reconstruction error; therefore, the data from the moving camera were evaluated, considering the full field of view (F_FOV), and also subdivided into the coordinates evaluated at the horizontal center of the field of view (C_FOV) and the coordinates evaluated out of this zone based on a \(\pm 500\) mm threshold from the center of the frame. Correlation between the estimated errors and the absolute horizontal pixel coordinate of the point were also evaluated. The F_FOV data from the moving camera were also compared with those the obtained using the static camera.  
In the second protocol, one evaluator moves a 600 mm rod across the calibrated plane at 0.462 \(\pm\) 0.048 m/s, while rotating it in the calibrated plane within the range of \(\pm 30^{\circ}\); the center of the rod was kept about 500 mm under the surface in such way that the rod was always completely underwater. This trial was simultaneously recorded by both cameras, and the trolley operator was instructed to try keeping the camera and rod always aligned. The video was analyzed when the rod was in the space within \(\pm 1500\) mm, the edges of the rod were digitized, and its coordinates at each frame as well as the evaluated rod length in each frame for each camera were compared. In this protocol, the effect of the rod position in the static camera field of view was evaluated by subdividing the coordinates evaluated at the full field of view (F_FOV), the horizontal center of the field of view (C_FOV), and coordinates evaluated out of this zone. The correlation between the estimated length errorsand the absolute horizontal pixel coordinate of the point was also evaluated. The data from the moving camera were also compared with the F_FOV data from the static camera.  
The third protocol consisted of a 25 m front crawl swim. The trial was simultaneously recorded by both cameras, and the trolley operator was instructed to try keeping the camera and the swimmer's hip always aligned. Markers were fixed on the right iliac crest (hip), the right wrist, and the right foot of the swimmer. These coordinates were extracted from each camera, and the iliac crest marker was also used to calculate the swimmer's velocity on each frame. The most central stroke cycle was analyzed, and the difference between the vertical coordinates of the wrist and foot points and the swimmer's hip position and velocity were calculated between the cameras. The 0.95 and 0.93 intraclass correlation coefficients (fixed-effect, 2-way ANOVA model) demonstrate the high reliability of coordinate digitalization for the static and moving cameras, respectively.  
In summary, the first protocol evaluates the accuracy and precision of the models in measuring the absolute position of the markers in the swimming pool reference based on a set of static markers; the second protocol evaluates the accuracy and precision of the models in measuring the relative positions of two points based on the rod length; and the third protocol exemplifies the practical application of the methods.