The Effects of Network Latency on PlayerGaming ExperienceAn Interactive Qualifying Project ReportSubmitted to the Facultyof theWORCESTER POLYTECHNIC INSTITUTEIn partial fulfillment of the requirements for theDegree of Bachelor of ScienceByMeixintong ZhaYing ZhangSubmitted toProfessor Mark ClaypoolWorcester Polytechnic Institute
1AbstractHumans spend 3 billion hours a week playing video games . While playing, network latenciescan cause interaction delay between the client and the server and affect players’ gamingexperiences. While there is research about the effects of delay on whole network game systems,there is little research on the effects of delays on fundamental player actions. We built two gamesthat isolate two fundamental game actions (shooting and movement) to evaluate how players areaffected by network latency. Game statistics were used to evaluate performance, and emotiondetection software and a heart rate monitor were used to evaluate players’ stress level duringgameplay. Our results from a 36-person user study show that players’ performance decreases aslatency increases and players’ stress level increases as latency increases.
2ContentThe Effects of Network Latency on Player Gaming Experience0Abstract1Section 1 Introduction3Section 2 Background and Related Works2.1 Sources of Delay2.2 QoE2.3 Stress Measurement66782.4 Game Types/Actions9Section 3 Methodology3.1 Game Design3.1.1 Design Choice3.2 Sushi Shooter: Game Balance3.2.1 Movement Speed3.2.2 Maximum Score and Target Score3.2.3 Beta Testing and Modification3.3 Player Performance Output3.4 Stress Measurement Design3.4.1 Emotional Detection3.4.2 Heart Rate Measurement3.5 Testing Environment3.6 Procedure10101113141415161616182020Section 4 Analysis4.1 Demographics4.2 Data Parsing4.3 User Performance Analysis4.4 Heart Rate Variability Data Analysis4.5 Emotion Data Analysis222223233032Section 5 Conclusions and Future Study41References43
3Section 1 IntroductionImagine playing a ranked game in League of Legends (Riot Games, 2009), with yourmost skilled champion and a commitment to winning this match. Suddenly your network actsup—every time you cast spells, especially those that are direction-targeted, you can feel a delaybetween when you click and the spell triggers and as a result, you miss everything. How muchpatience do you have before quitting the game? How angry are you before trying to smashsomething?Like highway traffic congestion in real life, latency because of network congestion can bereally annoying in the virtual world. A network delay of 500 additional milliseconds can causeonline game players to leave the game, and the abandonment rate can reach 87 percent with a2-second delay in load time . In order to keep players playing and to provide a better gamingexperience, game developers and researchers are seeking to improve delay compensationtechniques for games.For finding the best delay compensation techniques, we need to understand how latencyaffects the player’s performance in the game. Player’s actions vary according to different typesof games. For example, for a MOBA (Multiplayer Online Battle Arena) game like League ofLegends (Riot Games, 2009), players sometimes are required to aim in a certain direction or areato hit other champions instead of targeting other champions directly like some actions in FirstPerson Shooters (FPS) games . While there are many studies about how the player isimpacted by the delay from local systems, networks and servers, little research had been done on
4delay impact on the fundamental player actions. Experiment on fundamental player actions likemovement and shooting at moving targets under different amounts of delay can be extrapolatedto different types of actual games and used in improving delay compensation techniques.Different genres of real-time games possess different network requirements in order toprovide a smooth gameplay experience for users. However, online gameplays are alwaysaccompanied by some amount of delay. Sometimes, the delays are not high enough to benoticeable to players. Studies show that while Warcraft III (Blizzard, 2002) is played inreal-time, reaction time plays a small role compared to understanding the game . The effectsof typical network delay(less than a second) do not impact the overall outcome. RTS (Real TimeStrategy) and RTT (Real Time Tactic) games have delay requirements mostly similar to that ofWeb browsing games (on the order of seconds) . Actions that require precise (precision), rapidresponses (short deadline) are greatly impacted by degradations in frame rates and delay ; FPS(First Person Shooter) and MOBA (Multiplayer Online Battle Arena) are two typical types ofgame which typically lower less tolerance to latencies.To understand how lag affects a player’s experience, we need to understand the factors ofQuality of Experiences (QoE) such as stress and performance. Online gameplay involvesmultiple motivations, such as achievement, social interaction, and immersion. Researchers foundthat openness and agreeableness are positively related to more hours of online gameplay, whilegamer conscientiousness and emotional stabilities are negatively correlated to more onlinegameplay . Latency affects user emotion when their performances are degraded; this createsfrustrating and stressful gameplay situations for users, maybe causing them to leave the game
5. High latencies can exasperate the cycle of stress and poor performance. It may be helpful forgame developers and network providers to know how latency affects the player.Our goal for this project is to conduct a user study with an atomic action based game forevaluating how the player’s gaming experience is affected by network latency. We break downthe measurement of experience into two subsets— performance and stress. In our experiments,we build two games that isolated two atomic actions commonly used in most games; clicking toshoot at a target in Sushi Shooter , and clicking to dodge the falling object in Square Dodger . Weapply the "Method of Limits" to determine what range of latency is acceptable versusunacceptable by gradually increasing the intensity of latency in discrete steps and analyzingplayers’ performance while measuring stress through emotion detection software and a heart ratemonitor .Results of a 36-person user study show that, in both games, players’ performancedecreases as latency increases. In other words, as the added latency increases, players are morelikely to miss the target in Sushi Shooter and more likely to get hit by the falling squares in theSquare Dodger . Results from the emotion detection data show that players’ positive emotiondecreases as latency increases, and players’ negative emotions increase with the added latency.This suggests that players’ emotion is negatively affected by network latency during gameplay,which indicates an increase in players’ stress as latency increases.The rest of this report is organized as follows: section 2 describes the background andrelated work; section 3 presents our methodology for game design and experiment design;section 4 analyzes the experiment results, including data-visualized graphs; and section 5summarizes the report and discusses possible future works.
6Section 2 Background and Related WorksFor online video games, network latency is one type of lag that affects the ping of theonline game. Players’ gaming performance and stress usually degrade due to network latency;the stress level of players will be affected. This chapter proving background knowledge for ourexperiments on the effects of network latency on the players gaming experience, includingsources of delay, Quality of Experience, stress measurement, and game types/actions.2.1 Sources of DelayUnlike single-player games which operate on the local machine, an online-game runs ona central server to maintain consistency between individual clients. The clients send changerequests to the server and update the local game state by receiving updates from the server. Thisdelay in communication between clients and server is the fundamental source of lag. Othersources of lag can be categorized to 1) hardware deficient at the client-server, and 2) a poornetwork connection between the client and server .The hardware problems tie to the game architecture. Games consist of a loop of frames,and accept user input and perform calculations during each frame. The game updates the stateand produces output. The frequency of generating frames is referred to as the frame rate. Foronline games, the updates are sent to the central server from the client and back to the client tocomplete the update. A low frame rate makes the game less responsive to updates and may forceit to skip outdated data.
7Network delays are caused by bandwidth, congestion, the physical distance between theend-systems and the wireless network . Wireless networks tend to cause higher latency thanwired networks because of electromagnetic interference come from other devices such asmicrowaves.2.2 QoEQuality of Experience (QoE) can be impacted by network latency during gameplay. QoEis a measure of the delight or annoyance of a customer's experiences with a service [ 11] , whichin our study refers to online games. A variety of methods can be used for QoE measurement witha common method being human rating, Claypool studied about how computer games playeractions are impacted by the delay from the local system, networks and servers ; othersubjective tests are done by Rahul Amin and his team. They developed a Mean Opinion Score(MOS) metric to determine each gamers’ QoE, based on four post-survey questions: GameplaySatisfaction, Gameplay Frustration, Impact of Lag on Gameplay, and Likeliness to changenetwork service providers .The method of measuring user subjective QoE in our paper is by conducting a survey,which asks users to rate the quality of responsiveness from 1 (low) to 5 (high) for eachcombination of delay and speed.In addition to subjective tests for measuring user QoE, objective tests, which are moretechnology-centered, can provide quality results faster . Boyan et al, measured players’ QoEby detecting their emotional states. They first measured the level of presence of basic emotionslike happiness, sadness, engagement, anger, and fear. Then detected physiological data like skin
8conductivity and/or heart rate variability. They found statistically significant correlationsbetween attention and presence, eye closure and presence, eye closure and flow, and engagementand surprise . Similarly, Drachen et al, studied heart rate and electrodermal activitycorrelations with subjective gameplay experience testing the feasibility of these measures incommercial game development contexts. Their results indicate a significant correlation (p 0.01) between psychophysiological arousal and self-reported gameplay experience .2.3 Stress MeasurementStress is an important factor to measure in many fields related to human psychology andis an aspect of our study for finding out how player’s stress levels are influenced by networklatency. Many methods have been developed for stress measurement. For example, stressassessment tests can indicate people’s level of stress by asking them questions; Biodots that canindicate people’s level of stress by different colors when stuck on a person’s skin. Unfortunately,the Biodots does not work for our project, because the color change is based on skin temperature,assuming that people’s skin temperature decrease when they are stressful. While the situationcould be really different for people who are playing an exciting game, their skin temperaturecould increase when their stress levels increase.Automatic facial behavior analysis software can provide an effective measurement ofstress by detecting and tracking human faces. According to Deshmukh et al , there are threestages that are most commonly used in facial emotion recognition: face detection and tracking,where the software finds the face without human intervention; feature extraction, where thesoftware extracts the information from the facial expressions; and expression classification,
9where the software recognized the emotion based on the given information . There are manyeffective emotion detection applications that have free use by the public. The software we used iscalled the EmotionDetectionAsset, which is developed by the RAGE project of the OpenUniversity of the Netherlands etectionAsset].2.4 Game Types/ActionsAn online game is a video game that is partially or primarily played through the Internet.Online games can be from many genres, including First-person shooter (FPS), Real-timeStrategy (RTS), Multiplayer Online Battle Arena (MOBA), Massively Multiplayer Online(MMO), Role-Playing Game (RPG), Action, or Sport . Each genre has different gameactions. RTS games have command issuing (e.g attack, defend, end turn) game actions, whileFPS games have character movement, shooting, and base capture game actions. RPG and MOBAgames design a collection of characters with unique appearance and skills for different gameaction outputs. Generally, character movements and projectile skill shots are two common gameactions for Action, FPS, and MOBA games. We study the effects on latency on both in theproject.
10Section 3 MethodologyThis chapter contains the game design choice, player performance output, stress measurement,testing environment, and procedure.3.1 Game DesignTwo fundamental atomic actions are represented presented in our games: i) shooting ii) dodging.Game design principles that we incorporated are:1. Control the amount of latency between the player input and the resulting game action.2. Game actions are atomic and controlled by mouse clicks.3. Game difficulty—balanced between gamers as well as users who are not familiar withcomputer games4. Reward system that motivates players to win and influences their heart rates or emotions5. Record user and game event for performance measurementAccording to these principles, we designed two games with each game corresponding to oneatomic action. The two games were designed with different color schemes and characterappearances to reduce visual fatigue for the players. The shooting game, named Sushi Shooter,was designed for testing the shooting action and its hit rate under different input latency. Thedodging game, named Square Dodger , was designed for testing the horizontal movement actionand the success rate of dodging under different input latency.
113.1.1 Design ChoiceFigure 3.1.1aOur goal was to design a simple shooting game where the player clicks the mouse toshoot a bullet from the bottom of the window toward the enemy located at the top of the window.We built Sushi Shooter w ith Python using Pygame [https://www.pygame.org/docs/]. ThePygame library has the display, sound, event, draw, and time modules to develop a game; it ishighly portable and can run on nearly every platform and operating system. The screen size ofSushi Shooter is 400 x 400 pixels with the game played in fullscreen mode to reduce backgrounddistraction. The player-controlled character is an octopus and the target is sushi. We use an oceanbackground image. The purpose of the light blue color and simple character appearance is toreduce any visual distraction and make the player focus on the controls and scoring.Python has the “pygame.time” module for monitoring time: pygame.time.set timer( USEREVENT , millisecond ). The “set timer” function takes the user-event ID and the time inmilliseconds to activate an event in the corresponding time. We attached the shooting event withlatency after the mouse click action by using this function.
12Figure 3.1.1bThe second fundamental action we wanted to test is movement - specifically horizontalmovement by mouse clicks. During our prototype designing stage, skillshot dodging andmaze-based dodging designs were considered because these game types reduce distractions. Ourfinal design is to dodge falling squares with different speeds. The dodging game was built inGameMaker Studio, which is a cross-platform game engine developed by YoYo Games[https://www.yoyogames.com/]. As a popular tool among amateur and freeware developers,GameMaker is flexible and supports fast game development, especially for our project. Thein-built alarm function allows simulated lags by adjusting the response time of the mouse click.The original size of the game is 510 x 768 pixels and played in fullscreen during testing.The background was black, the speed of falling squares was randomly assigned from 5-20 pixelsper second; the color of falling squares are randomly assigned from the color palette, the size ofthe falling squares is 8x8 pixels while the size of the player controlled red box is 10 x 10 pixels.
133.1.2 Player Controls and FeaturesIn Sushi Shooter , the player clicks the left mouse button to shoot a bullet vertically up.We also ask the player to press spacebar whenever the player feels the latency. This functionadds a subjective measurement to the player quality of experience. Each time as the player pressspace bar, the octopus will blink once. There is a countdown clock on the left side and a scoretracker on the right-hand side.For Square Dodger, the player can only move horizontally with left and right mouseclicks. Participants were asked to press spacebar whenever they feeling the latency, and there isan indication at the upper left corner when the spacebar was pressed (Figure 3.1.2). Every timethe player is hit, the health at upper right corner decreases by 5. The maximum health was set to1000 so that it is impossible for a player to fall to negative health before the game ends.Figure 184.108.40.206 Sushi Shooter: Game BalanceThe steady and repetitive movement of the target sushi and the cooldown of the bulletmake Sushi Shooter a performance capped game. We adjust the speed of the target and bullet so
14that the game is challenging for most players while it is still not too difficult for the player to hitthe target.3.2.1 Movement SpeedIn Sushi Shooter , the three variables that decide the difficulty of the game are the bulletspeed, target sushi movement speed, and added latency. We initially determined the bullet speedand sushi movement speed based on the average projectile skill shot speed and charactermovement speed in League of Legends to ensure playability. The average character movementspeed in L eague of Legends is 450 units per second. The skill shot speed in Legends ofLegends ranges from 600 to 2000 units per second. The ratio of skill shot speed to charactermovement speed is ranged from 1.3 to 4.4. We tested the shooting with different speeds andfinalize the two variable: 3 pixels per millisecond for the sushi movement speed and 7 pixels permillisecond for the bullet speed, results in a 2.3 ratio.3.2.2 Maximum Score and Target ScoreFigure 3.2.2
15We used a stopwatch and found that the time for the sushi to traverse from one side toanother is 1.7 seconds. Accounting for the bullet cooldown time and the increasing lag, it ispossible to get one score each time the sushi traverses the screen. The maximum possible scorefor a 200-second game is200sec1.7sec 118 . Featuring scores and feedback is one component of theflow of games that serve as an intrinsic motivator in brand use and selection. Sushi Shooteris a straightforward game with one atomic action. We want the user to try throughout the entire 3minutes 20 second of the game. We decided to tell the user a target score to maintain the gamehomeostasis - a process of achieving optimal levels of affective well-being by engaging inactivities that raise arousal if it is currently too low, or activities that lower arousal if it is toohigh .3.2.3 Beta Testing and ModificationWe conducted beta tests and surveys with 8 people. We told the testers that the maximumpossible score is 118, and we asked users to press the spacebar whenever they feel the latency.Their scores ranged from is 20 to 62, with an average of 43.5. In the survey, testers respond thatthe game mechanic is simplistic enough for them to get familiar with 20 seconds. Testers thoughtthe possible motivational target score for the experiment should be ranged from 60 to 100. Someof the testers did not feel lag until the 400 to 500 ms ping. Testers experienced different levels ofanxiety varying from person to person in the last minute of the game, but most testers forgot topress the spacebar when they felt the latency. To obtain the subjective measure from thespacebar, we have to remind the participants to press the spacebar when they feel the lag beforeeach game.
16The target score for the beta test is 118 - the maximum possible score; the target score forthe experiment is adjusted according to the top score from beta testing. The top score from betatesting is 80.3.3 Player Performance OutputOne objective measure of Quality of Experience is player performance. We predict thatthe player will play less optimally with higher latency. We are particularly interested in clickcounts, hit rate (miss rate), and spacebar counts (for subjective Quality of Experience) versuslatency, so the user events and the corresponding time need to be recorded. Sushi Shooter andSquare Dodger record each player’s action, such as mouse clicks and spacebar presses, alongwith the result of the action, such as missing the target in Sushi Shooter or being hit by a squarein Square Dodger. 3.4 Stress Measurement DesignStress is defined as a state of mental or emotional strain caused by adverse circumstances.We measure the heart rate and emotions of the player to evaluate the change of stress level.3.4.1 Emotional DetectionThe hardware that was used for detecting participants’ face is a 1080p webcam fromLogitech as the figure 3.4.1a.
17Figure 3.4.1aThe software that was used for analyzing participants’ emotion，namedEmotionDetectionAsset, is developed by the RAGE project of the Open University of theNetherlands(OUNL) onAsset]. This asset isa client-side software component that can detect emotions from players’ faces in real-time. Itreturns a string representing six basic emotions: happiness, sadness, surprise, fear, disgust, anger,and can also detect the neutral face. It is recommended by the authors to use this software ingames to collect emotion data during playtesting, which is what we did for our project.The EmotionDetectionAsset was run in the background to monitor the player’s emotionwhile playing the game with different delay ranges. The data for the six basic emotion wasextracted to analyze the relationship between network delay and the player’s emotion.
183.4.2 Heart Rate MeasurementFigure 3.4.2aHeart rate, or pulse, is the number of times the heart beats per minute. We chose to usethe eMotion Faros sensor [http://ecg.biomation.com/faros.htm] to obtain users’ ECG,accelerometer x, accelerometer y, accelerometer z, marker, and HRV(heart rate variability).These data are in EDF format. We used EDFbrowser to convert the EDF file to a plain text file.Each line of the text file contains the time and the above six fields. We parse the file with thestart time from the game output files to generate a CSV file with attributes of time andcorresponding HRV.Figure 3.4.2b
19Figure 3.4.2cFigure 3.4.2dHeart rate variability (HRV) measures the specific changes in time (or variability)between successive heartbeats. The time between beats is measured in milliseconds and is calledan “R-R interval” or “inter-beat interval”(IBI) . Researchers have found that an increase inHRV is related to increased self-control abilities, greater social skills, and better abilities to copewith stress, among other findings . HR can be found by60 1000average HRV.
203.5 Testing EnvironmentFigure 3.5The experiment was conducted at Zoo lab in Fuller laboratories of WPI. The room was isolatedfrom outside to eliminate potential distraction factors.3.6 ProcedureThe procedure followed for our user study was:1. Confirm the email address with the participants and ask them to sign the Consent Form.2. Ask participants to fill out the online Demographic Question survey.3. Instruct participants on how to wear the heart sensor.4. Open the emotion detection software in the background, adjust the angle of the camera tomake sure it can detect the participants’ face.
215. Remind participants to press spacebar whenever they feel lag.6. Provide basic instruction for Sushi Shooter controls.7. Remind participants of the target score (80).8. Ask participants to play Sushi Shooter.9. Save the emotion output file and reopen the emotion webcam.10. Provide instructions on for Square Dodger controls11. Remind participants of the game goal.12. Ask participants to play Square Dodger.13. Save the emotion output file.14. Ask participants to fill out the online opinion summary questionnaire.15. Debrief the participants and collect the heart rate sensor.
22Section 4 AnalysisThis chapter contains an overview of the test demographics, the tools used for data parsing, andthe analysis of user performance, heart rate variability, and emotion data.4.1 DemographicsFigure 4.a: DemographicsWe recruited participants from Worcester Polytechnic Institute students. There were 36participants with 21 males and 15 females (Figure 4.a: top left). Participants have different levelsof computer/console gaming ability (Figure 4.a: bottom left and bottom right). The average
23computer gaming ability of the participants is 2.83. The average console gaming ability of theparticipants is 220.127.116.11 Data ParsingThe performance and emotion raw data are in text format. The heart rate variability dataare in EDF format. We used EDFbrowser ( https://www.teuniz.net/edfbrowser/ ) to export theEDF files to text files. We used the Python CSV module( https://docs.python.org/2/library/csv.html ) to create CSV files from the text files. We usedPython Pandas module ( http://pandas.pydata.org/pandas-docs/stable/ ) and Google Spreadsheet tocalculate the average, counts, and aggregation of data attributes.4.3 User Performance Analysis4.3.1 Sushi ShooterFigure 4.b.1 Sushi Shooter: Click vs Added LagBoxplot: distribution of all participantsFigure 4.b.2 Sushi Shooter: Click vs Added LagBar chart: average click counts with standard errorFigure 4.b.1 shows a boxplot of the distribution of users clicks counts at six latency levels. Thex-axis represents added lag in milliseconds; the y-axis represents the number of mouse clicks.
24Figure 4.b.2 shows a bar chart of the average number of clicks for all participants at six latencylevels. The x-axis represents the added lag in milliseconds; the y-axis represents the averagenumber of clicks for all participants. The black bar on top of each colored bar represents thestandard error. These graphs show a decreasing trend which means users tend to click less asadded lag increases.Figure 4.b.3 Sushi Shooter:Hit vs Added LagBoxplot: distribution of all participantsFigure 4.b.4 Sushi Shooter: Hit vs Added LagBar chart: average hit counts with standard errorFigure 4.b.3 shows a boxplot of the distribution of users hit counts at six latency levels. Thex-axis represents added lag in milliseconds; the y-axis represents the number of bullets that hitthe sushi. Show with standard error bars, Figure 4.b.4 is a bar chart of the average number of hitsfor all participants in six latency levels. The x-axis represents the added lag in milliseconds; they-axis represents the average number of misses for all participants. These graphs show an overalldecreasing trend which means users tend to hit the sushi fewer time as the added lag increases.
25Figure 4.b.5 Sushi Shooter: Miss vs Added LagBoxplot: distribution of all participantsFigure 4.b.6 Sushi Shooter: Miss vs Added LagBar chart: average miss counts with standard errorFigure 4.b.5 shows a boxplot of the distribution of users misses counts at six latency levels. Thex-axis represents added lag in milliseconds; the y-axis represents the number of bullets missesthe sushi. This graph shows that users tend to miss the same number of shooting from 200 to 500milliseconds of added lag. There is no visual trend in this graph which suggests there is no spacecorrelation of added lag and miss rate. Figure 4.b.6 is a bar chart of the average number ofmisses for all participants at six latency levels shown with standard error bars. The x-axisrepresents added lag in milliseconds; the y-axis represents the average number of hits for allparticipants. This graph shows that, on average, users tend to miss the same number of timeswhen shooting from 200 to 500 millisecond of added lag.
26Figure 4.b.7 Sushi Shooter: Lag (spacebar) vs Added LagBoxplot: distribution of all participantsFigure 4.b.8 Sushi Shooter: Lag (spacebar) vs Added LagBar chart: average spacebar counts with standard errorFigure 4.b.7 shows a boxplot of the distribution of users spacebar presses at six latency levels.The x-axis represents added lag in milliseconds; the y-axis represents the number of space barpresses. This graph shows an increasing trend from 0 to 300 ms added lag and a decreasing trendfrom 300 to 500 ms added lag. There is no apparent visual correlation between added lag anduser self-report of lag.Figure 4.b.8 is a bar chart of the average number of users spacebar presses for all participants atsix latency levels shown with standard error bars. The x-axis represents added lag inmilliseconds; the y-axis represents the average number of spacebar presses. This graph showsthat, on average, users tend to press the spacebar same number of times from 0 to 200millisecond of added lag. The standard error bars of 0, 400,
of games. For example, for a MOBA (Multiplayer Online Battle Arena) game like League of Legends (Riot Games, 2009), players sometimes are required to aim in a certain direction or area to hit other champions instead of targeting other champions directly like some actions in First Person Shooters (FPS) games .
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