# Skeleton Tracking Accuracy and Precision Evaluation of Kinect V1, Kinect V2, and the Azure Kinect
## 3 Experiments  
For general accuracy and precision measurements, we mounted a white reflective plate to the end effector of a robotic manipulator--ABB IRB4600--moved the plate along  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Kinect v1** & **Kinect v2** & **Azure Kinect** \\ \hline Color camera & 1280 \(\times\) 720 px @ 12 fps & 1920 \(\times\) 1080 px @ 30 fps & 3840 \(\times\) 2160 px @ 630 fps \\ resolution & 640 \(\times\) 480 px @ 30 fps & & NFOV unbinned—640 \(\times\) 576 @ 30 fps \\ Depth camera & 320 \(\times\) 240 px @ 30 fps & 512 \(\times\) 424 px @ 30 fps & & NFOV binned—320 \(\times\) 288 @ 30 fps \\ resolution & 320 \(\times\) 240 px @ 30 fps & 512 \(\times\) 424 px @ 30 fps & & NFOV unbinned—1024 \(\times\) 1024 @ 15 fps \\ Depth sensing & Structured light–pattern & & & \\ technology & projection & & & \\ Field of view & 57\({}^{\circ}\) H, 43\({}^{\circ}\) V & 70\({}^{\circ}\) H, 60\({}^{\circ}\) V & NFOV binned—75\({}^{\circ}\)\(\times\) 65\({}^{\circ}\) \\ (depth image) & alt. 58.5\({}^{\circ}\) H, 46.6\({}^{\circ}\) & alt. 70.6\({}^{\circ}\) H, 60\({}^{\circ}\) & WFOV unbinned—120\({}^{\circ}\)\(\times\) 120\({}^{\circ}\) \\  & & & & \\  & & & & \\ Specified measuring & 0.4–4 m & 0.5–4.5 m & & NFOV binned—0.5–4.6 m \\ distance & 430 g (without cables and & 610 g (without cables and & & WFOV binned—0.25–2.88 m \\ Weight & power supply); 750 g (with cables and power supply); 1390 g (with cables and power supply) & 440 g (without cables); 520 g (with cables, & power supply is not necessary) \\  & and power supply) & cables and power supply) & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparison of the three Kinect versions [29].  
Figure 2: Schematic of the Azure Kinect.  
the depth axis of each sensor, and performed depth measurements in 7-8 discrete positions (Figure 3), where the distance between adjacent positions was 400 mm. The absolute positioning accuracy of the robot end effector was within 0.02 mm according to the ABB IRB4600 datasheet. Therefore, it was suitable for all our experiments. For all experiments in all positions, we performed 500 measurements which were then used for further statistical evaluation.  
We aligned the depth \(Z\) axis of each sensor with the robot axis thusly. We aligned the center of the plate with the center of the depth image in the approximate distance of 1500 mm. Then, we moved the plate to an approximate distance of 3000 mm and repeated the procedure; these two points formed a line that corresponded to the depth axis of the depth sensor. We further tested this alignment by moving the plate along the designated line and verified that the depth axis was identified correctly. This procedure is thoroughly covered in [29]. For all experiments and sensors, we used default camera calibrations.  
Each sensor was warmed up for one hour, as previous research showed this is necessary to get the best depth results [29].  
After aligning the coordinate systems of individual sensors with the coordinate system of the robot, the first experiment we carried out was a comparison of their precision and accuracy. We use these terms in a standard scientific way: by accuracy, we mean how close the measurements are to the true value, and by precision (repeatability), we mean how close the measurements are to each other. As stated before, the alignment did not solve the origins of the individual coordinate systems; therefore, we are not discussing absolute accuracy, but relative accuracy. By relative accuracy, we therefore mean accuracy relative to the first measured position. The core of all our accuracy measurements was measuring points in positions 400 mm away from each other.