In this paper, we investigate the influences of network delay on QoE (Quality of Experience) such as the operability of haptic interface device and the fairness between players for soft objects in a networked real-time game subjectively and objectively. We handle a networked balloon bursting game in which two players burst balloons (i.e., soft objects) in a 3D virtual space by using haptic interface devices, and the players compete for the number of burst balloons. As a result, we find that the operability depends on the network delay from the local terminal to the other terminal, and the fairness is mainly dependent on the difference in network delay between the players’ terminals. We confirm that there exists a trade-off relationship between the operability and the fairness. We also see that the contribution of the fairness is larger than that of the operability to the comprehensive quality (i.e., the weighted sum of the operability and fairness). Assessment results further show that the output timing of terminals should be adjusted to the terminal which has the latest output timing to maintain the fairness when the difference in network delay between the terminals is large. In this way, the comprehensive quality at each terminal can be maintained as high as possible.
A number of researchers have been paying their attention to networked real-time games with haptic sense [
To keep the consistency high at the terminals, we can employ the local lag control [
To maintain the fairness high, we can use the adaptive ∆-causality control [
In previous works, for example [
In [
In this paper, we investigate the influences of network delay on the operability of the haptic interface device and the fairness between players by carrying out QoE assessments subjectively in a balloon bursting game, where two players burst balloons (i.e., soft objects) in a 3D virtual space. We also perform objective QoE assessment at the same time as the subjective assessment. We further investigate the relationship between subjective and objective assessment results.
The system configuration of the balloon bursting game is shown in
1 and 2) bursts balloons with his/her respective stylus in a 3D virtual space, and the two players compete with eachother for the number of burst balloons. The system consists of two terminals (terminals 1 and 2), each of which has a PC, a haptic interface device (Geometric Touch [
In addition, we enhance the balloon bursting game by adding a function handling the olfactory sense to carry out an investigation as in [
In this paper, as shown in
The reaction force applied to the haptic interface device is generated by the haptic rendering engine [
In order to get the reaction force against a player via the haptic interface device, we use the following function of the Open Haptics HLAPI (Haptic Library Application Programing Interface) [
hlGetDoublev (HL_REACTION_FORCE, r_force),
where hlGetDoublev() is a function used for getting the reaction force (HL_REACTION_FORCE) from the haptic rendering engine, and r_force is a user-defined parameter which receives a returned value of HL_REAC- TION_FORCE. The value of the force applied to a balloon when a player pushes the balloon with a stylus is received from the parameter HL_CURRENT_FORCE. In a preliminary experiment, we confirmed that the value of HL_CURRENT_FORCE is almost equal to that of HL_REACTION_FORCE; that is, a player feels almost the same reaction force to the force pushed by him/her. The player feels larger reaction force as the penetration depth of the stylus becomes larger. The penetration depth of the stylus is the distance from the surface of the balloon to the tip of the stylus. When the balloon is distorted by the stylus, its volume is varied. We can obtain the volume from the haptic rendering engine.
In order to clarify the relations among the reaction force, the penetration depth, and the volume of a balloon, we here explain four cases (called the standard balloon case, small balloon case, hard balloon case, and soft balloon case). In the standard balloon case, the radii of three dimensional axes (x, y, and z) of the balloon are 1.1, 1.5 and 1.1, respectively (see
There may be several methods of judgment of bursting a balloon in a virtual space. For example, a balloon can be burst according to the volume of the balloon and/or the force pushed with the stylus. In the former method, the balloon is burst when the volume of the balloon is smaller than a threshold value. In a preliminary experiment where we used the former method, we set the threshold value to 90% of the initial volume of the balloon.
In the latter method, when the force applied to the balloon from the cursor of the haptic interface device is larger than another threshold value, the balloon is burst. We set the threshold value to 0.4 N in our preliminary experiment.
As the other method, if either of the conditions of the above methods is met, the balloon can be burst. In our assessment in this paper, we use a method in which a balloon is burst when the volume of a balloon reaches a threshold value for simplicity.
In our assessment system, the two terminals are connected to each other via a network emulator (NIST Net [
delay for each packet transmitted between the terminals. We can here take account of network delay jitter as in [
In this assessment, we investigated the influence of network delay on only the operability of haptic interface device. We did not investigate the fairness between players because the fairness is perfect when the values of network delay are the same between the terminals [
In each assessment, we changed the delays in random order from 0 ms to 500 ms at intervals of 50 ms for each subject. The order of assessments was also selected in random order for each subject. The subject burst the balloons with the stylus continuously for 30 seconds. After each stimulus, he/she was asked to base his/her judgment about the easiness of bursting based on the five-grade impairment scales (5: Imperceptible, 4: Perceptible, but not annoying, 3: Slightly annoying, 2: Annoying, 1: Very annoying) [
In this case, we carried out QoE assessment to clarify the influences of network delay on the operability, fairness, and comprehensive quality. In the assessment, two subjects played the balloon bursting game together. Before the assessment, each pair of subjects played the balloon bursting game for three times to get used to the game on the condition that delays 1 and 2 are set to 0 ms; that is, the same condition is provided to the pair. By practicing, each subject knows how to burst a balloon by using a haptic interface device. In this case, we carried out the assessment by setting delay 1 to 0 ms or 200 ms, and setting delay 2 to 50 ms, 100 ms, 200 ms, 300 ms, or 500 ms. We carried out QoE assessment with 16 subjects (males and females) whose ages were between 20 and 30.
Assessments in which delay 1 was set to 0 ms and 200 ms are referred to as assessments 1 and 2, respectively. In order to know the suitable local lag values at the two terminals to maintain both operability and fairness, we carried out the assessments by setting local lags 1 and 2 to different values. Local lag 1 was changed from 0 ms to 500 ms at intervals of 50 ms, and local lag 2 was set to the same value as delay 2. In each assessment, the order of combinations of delays and the local lags were changed in random order for the pair. It took 30 seconds for each stimulus. After each stimulus, each subject was asked to base his/her judgment about the operability, fairness, and comprehensive quality in terms of wording used to define the five-grade impairment scale (5: Imperceptible, 4: Perceptible, but not annoying, 3: Slightly annoying, 2: Annoying, 1: Very annoying) [
We show the MOS values of operability versus delay 1 for four assessments in
In
We show the MOS values of operability at terminals 1 and 2 as a function of local lag 1 in
In
In
By comparing
To clarify the optimum values of local lag at both terminals, we have calculated the average MOS values of two terminals, which are shown in
We show the MOS values of comprehensive quality at terminals 1 and 2 in
MOS values of comprehensive quality for all the values of delay 2 at terminal 1 hardly depend on local lag 1. In
From
We show the average numbers of burst balloons at terminals 1 and 2 in
In
As described in Section 4.2.2, by comparing the objective results to subjective results, we found that they are related to each other. We should clarify how they are related to each other in detail, and examine whether the subjective results can be estimated from the objective results or not. To investigate the relationship between the average number of burst balloons (or the local lag) and the MOS value of operability, we carried out the regression analysis [
Equation | R2 |
---|---|
OMOS = 0.228Nburst ? 0.448 | 0.968 |
OMOS = ?0.007∆ + 4.634 | 0.968 |
burst balloons, ∆ is the local lag, and R2 is the contribution rate adjusted for degrees of freedom [
We also carried out regression analysis to investigate the relationship between the MOS value of fairness and the absolute value of average difference in the number of burst balloons, and that between the MOS value of fairness and the absolute value of local lag difference. As a result, we obtained equations shown in
In addition, we examined the relationship between the average MOS values of two terminals and the objective results. The estimated equations for average MOS value of operability are shown in
In this paper, we investigated the influences of network delay on QoE such as the operability and fairness for a balloon bursting game in a networked virtual environment with haptic sense by carrying out subjective and objective QoE assessments. As a result, we found that the operability of haptic interface device strongly depended on the local lag. On the other hand, we can say that the operability depends on the network delay from the local terminal to the other terminal. We also observed that the soft objects became harder and more slippery as the
Equation | R2 |
---|---|
0.891 | |
0.936 |
Equation | R2 |
---|---|
0.931 | |
0.928 |
Equation | R2 |
---|---|
0.977 | |
0.978 |
Equation | R2 |
---|---|
0.966 | |
0.956 |
network delay increased. Also, we examined that the fairness between players depended on the difference in network delay between the terminals or the local lag difference between the terminals. We further confirmed that there existed a trade-off relationship between the operability and fairness. Moreover, we clarified how the local lags should be set at each terminal to be able to keep both operability and fairness as follows. The local lag at each terminal can be set to the same value as the network delay from the local terminal to the other terminal when the absolute difference in network delay between the terminals is smaller than or equal to about 50 ms. When the difference is larger than about 50 ms, we can set the local lags to the larger value of network delay between the terminals.
Additionally, we examined the relationships between subjective and objective assessment results. We found that the MOS value of operability could be estimated from the average number of burst balloons or local lag with a high degree of accuracy. We also illustrated that the MOS value of fairness could roughly be estimated from the average difference in number of burst balloons or the absolute value of local lag difference. Furthermore, we noted that the MOS value of comprehensive quality could be estimated from the average number of burst balloons and the absolute value of average difference in number of burst balloons, or from the local lag and the absolute value of local lag difference to a large extent.
As our future work, we will carry out QoE assessments with other judgments of bursting balloons in the balloon bursting game; for example, a balloon can be burst when the force applied to the balloon reaches a threshold. We will also confirm the trade-off relationship of the operability and fairness in other networked real-time games. We need to confirm whether the MOS value of comprehensive quality can be kept high or not at both terminals by setting the local lags according to the difference in network delay between the terminals as described in Section 4.2.1. Furthermore, it is important to carry out the confirmation in networked real-time games over the Internet.
This work was partly supported by JSPS KAKENHI Grant Number 26420354.
Mya Sithu,Yutaka Ishibashi,Pingguo Huang,Norishige Fukushima, (2015) Influences of Network Delay on Quality of Experience for Soft Objects in Networked Real-Time Game with Haptic Sense. International Journal of Communications, Network and System Sciences,08,440-445. doi: 10.4236/ijcns.2015.811040