# A New Framework of Human Interaction Recognition Based on Multiple Stage Probability Fusion
## 5 Algorithm Verification and Results Analysis
### Algorithm Tested in UT-Interaction Dataset
#### 5.1.1 HMM Number of Hidden Nodes Test in Different Stages  
The number of hidden states in the single atom action or interaction of two persons at different stages of interactive behavior has a great influence on the stability of the HMM and the recognition result. The HMM of each stage is trained and tested, and the results are shown as follows:  
(1) HMM in the start stage of interactive behavior:  
When the ROI is on the left side, the test results of the HMMs' hidden node is shown in Figure 8a. The average recognition rate of the left side of human behavior in the start stage of interactive behavior has reached the highest value when the number of hidden nodes is set to 5. When the ROI is on the right side, the test result of the HMMs' hidden node is shown in Figure 8b. The average recognition rate of the right side human behavior in the start stage of interactive behavior has reached the highest value when the number of hidden nodes is set to 3, 5 and 10. As there is a mutual relationship between the two sides of the interactive behavior, the hidden state of the left side of the human atom behavior in the start stage of the interactive behavior should be corresponding to the hidden state of the right side of human atom behavior, so the hidden node number of the HMM of the right side of human atom behavior should be consistent with the left side. The hidden node number of the HMM of the right side human atom behavior is set to 5 in the final test system.  
Figure 7: Exemplar frames from UT-interaction dataset.  
HMM in the middle stage of interactive behavior:  
In the middle stage of interactive behavior, the interaction feature is extracted as a whole. The interaction in this stage is more complex than the other stages, so the hidden state of the interactive behavior is more so than in other stages. The test results of the HMM's hidden node in this stage are shown in Figure 9a. The confusion matrix with the optimal number of nodes is shown in Figure 9b.  
The information of the interactive behavior in this stage is always less than the other stages, as is the case with the number of hidden states of the HMM in this stage. The test result of the HMM's hidden node number of the left side of the interaction in the end stage is shown as Figure 10a. The test result of the HMM's hidden node number of the right side of the interaction in the end stage is shown as Figure 10b. As the end stage contains less information, the number of hidden states is less than other stages, which is what was predicted in this paper. Combined with the test results in Figure 10b, the hidden node number of the HMM of the left and right side human atom behavior is set to 2 in the final test system.  
Figure 8: The relationship between the average recognition rate and the number of the HMM’s hidden nodes in the start stage of the interaction behavior. (**a**) the left side; (**b**) the right side.  
Figure 9: (**a**) The relationship between the number of hidden nodes of the HMM and the average recognition rate, (**b**) the confusion matrix of the optimal results in the middle stage HMM in the end stage of interactive behavior.