Modelling group dynamics for crowd simulations

This paper investigates a new method to simulate pedestrian crowd movement in a large and complex virtual environment, representing a public space such as a shopping mall. To demonstrate pedestrian dynamics, we consider groups of pedestrians of different size, sharing a crowded environment. A pedestrian has its own characteristics, such as gender, age, position, velocity, and energy. The proposed method uses a multi-group microscopic model to generate real-time trajectories for all people moving in the defined virtual environment. Additionally, a dynamic model is introduced for modelling group behaviour. Based on the proposed method, all pedestrians in each group can continuously adjust their attributes and optimize their path towards the desired visiting targets, while avoiding obstacles and other pedestrians. Simulation results show that the proposed method can describe a realistic simulation of dynamic behaviour.


Introduction
Simulation of crowd dynamics has been studied in different research fields, including computer graphics, virtual reality, robotics, social science, statistical physics, pedestrian, and evacuation dynamics. Broadly speaking, most techniques can be grouped in two main categories: macroscopic and microscopic. The macroscopic techniques focus on the aggregate behaviours of crowds. On the other hand, microscopic techniques are used to create realistic trajectories for individual pedestrians so that they can naturally avoid static and dynamic obstacles, and other pedestrians in the defined virtual environment [1].
Using a similar analogy, a crowd simulation can be either homogeneous or heterogeneous [1,2]. Homogeneous crowds correspond to instances where each agent has a very similar behaviour or goal. Many models have been proposed for homogeneous crowd simulation, e.g. the continuum model [1,3] and the flow − based models [4,5]. In heterogeneous crowds, each agent in the crowd maintains a distinct, observable identity (see [6]). Many techniques have been proposed for simulating heterogeneous crowds [7,8], e.g. agent − based models [9]. In the agent − based model, each agent motion in the crowd is computed separately [1,2]. The computation of an agent motion is split in two distinct tasks: global and local navigation [1,2]. Global navigation aims to compute a long-term collision-free path towards a goal position that only the static obstacles consider. In contrast, local navigation techniques take into account the motion of dynamic obstacles and other agents in the environment and steer each agent towards its goal position [1].
Crowd simulations based on the microscopic model have been widely used in many applications [6]. However, most methods do not implement the process of independently animating pedestrians in real-time. In addition, a crowd model needs to address the dynamic interactions between pedestrians. Generally, people in a crowd move in groups (see Fig. 1), where the behaviour and dynamics of a pedestrian in a group are connected with other pedestrians, and the interaction between pedestrians can significantly influence crowd behaviour. Therefore, crowd simulation methods with a strong computational performance for predicting each pedestrian's movement in each population in real-time are needed.
Broadly speaking, our contribution is the introduction of a mesoscopic model, specifically concerned with people group's dynamics; in particular: • We propose a new method for simulating pedestrian crowd movement in a large and complex virtual environment in a public place such as a shopping mall, • A group of pedestrians can have a variable number of pedestrians with different characteristics, • Pedestrians, either as individuals or in groups, move through the virtual environment in different directions to reach their desired goal points, • A multi-group microscopic model is proposed to generate a real-time trajectory for each pedestrian navigating in the virtual environment.
To describe the group of pedestrians, let us consider a simulated crowd scenario shown in Fig. 1. In this simulated scenario, the crowd is organized in groups. Each group consists of a different Fig. 1 Screenshot of the shopping mall: pedestrians move in different directions to reach their desired goal points on the ground floor of the shopping mall while avoiding the static obstacles and pedestrians number of pedestrian types (male and female) of different ages (old, young, and child) to establish the range of personality variation. Figure 1b and c illustrate an example, the small marked groups of pedestrians consist of "3" pedestrians. The first group (see Fig. 1b) consists of "1" young-man, "1" youngwomen, and "1" old-man. The second group (see Fig. 1c) consists of "1" young-women, "1" old-man, and "1" child. In our model, pedestrians always move with the same group. In any simulation, groups of pedestrians enter the virtual environment through the entrance, and they navigate inside the virtual environment to visit several prefixed target points. Afterward, they move toward the exit to leave the virtual environment.
In the rest of the paper, crowd simulation is the term used to describe the entire dynamics of a scenario, and pedestrians are referred to as agents. Our contribution is organized as follows: Section 2 discusses related research, Section 3 formalizes the problem, Section 4 introduces the proposed method, Section 5 introduces the crowd simulation and summarizes the simulation results. Finally, Section 6 closes the paper with final considerations.

Related work
Simulation of crowd movement and the interaction between pedestrians have been studied in the literature [10]. For example, authors in [3] present an approach for simulating crowds using discrete agents as a single continuous system. Also, a crowd model based on continuum dynamics has been provided by [11] using potential field integrated with global navigation. The crowd flows has been investigated by [12] using the particle methods and continuous pedestrian model. Moreover, the authors in [13] derived the governing equations of motion for simulating different types of pedestrians in the 2D environment. Additionally, there are many other approaches to simulate crowd behaviour based on cognitive modelling and behaviour [14], psychological or sociological factors [15], personality models [8], and stress modelling [1,16].
The effect of groups on crowd movement was investigated in [17] using the least effort cellular automata algorithm. To evaluate the impact of group dynamics on the crowd movement, an agent-based model is proposed by [18,19].
The authors in [20] suggested a multi-group microscopic model based on the interacting particle system coupled with the eikonal equation for describing groups' behaviour. Furthermore, [20] discussed a multi-behaviours microscopic model combined with the social force model and optimal path computation. The authors in [21] provided a literature review from traditional models to recent models to introduce the researchers' crowd simulation models. In a real crowd simulation application, the authors in [2] presented a crowd simulation method for generating the human-like trajectories in the simulated environment based on geometric techniques.
In animation and games, the authors in [22] combined steering behaviours to allow the character to navigate their world in a life-like manner. Moreover, the problem of directing and controlling virtual crowds has been addressed by [23]. To perform multiple virtual agents' path planning, the authors in [24] present a multi-agent navigation graph for each agent in real-time. In literature, several techniques are proposed to animate crowds. Most of these techniques use a simple representation for each agent, i.e. circular shape in 2D plane or cylindrical object in 3D space [1,14].
In real-world applications, crowd movement simulation provides valuable tools for improving efficiency and safety in public places such as airport terminals [18], shopping malls, train stations [14], and theatres [20]. Moreover, crowd movement simulations have been used to develop a level-of-service concept, design elements of pedestrian facilities, planning guidelines [25], safety planning [2], and support transportation planners or managers to design timetables [20]. Furthermore, realistic simulations based on pedestrian behaviour such as obstacle avoidance, stress response, and avoidance of other pedestrians have many applications such as computer game designers, movies, and virtual environments [2]. Considerable effort has been made on locomotion, realistic pedestrian movement behaviours, path planning, and navigation in large virtual environments [15].
Finally, a crowd manifests itself with dynamics that are a combination of individual, group, and global dynamics in a given environment. Although the complexity of a very busy scene might seem chaotic at first, such as a train station or an airport concourse, individual and group dynamics shape the dynamics of the crowd as a whole and an in-depth study at microscopic or mesoscopic level can discern the underlying dynamics. This duality of individual versus crowd dynamics has been studied by social scientists, who employed nonlinear dynamics to model trajectories and interactions. Notable examples are the research contributions of Epstein [26,27] and Helbing [28][29][30].
With respect to the state-of-the-art discussed above, our work proposes a method for simulating the crowd dynamics of pedestrians in a very complex environment. In this method, the crowd consists of a large number of pedestrians in multiple groups with a different number and various types of pedestrians with different intentions. Pedestrians in each group continuously adjust their paths and update their attributes independently in real-time.

Problem formulation
We study a crowd simulation of pedestrians with different group sizes and category, moving through a virtual environment for a limited amounted of time. Each group of pedestrians visits several goal points with varying sequences of the visit. Moreover, the number of each type of pedestrian in each group is different, and each type of pedestrian has its own attributes, such as gender, age, position, velocity, and energy. In the virtual environment, there are several obstacles at different locations that prevent pedestrians from moving through, and pedestrians need to keep a certain distance for safety reasons. For simulating the crowd movement, it is required to formulate pedestrians with a wide variety of characteristics. Based on the different schedule models, pedestrians are appropriately introduced into the virtual environment. After that, they will navigate inside the virtual environment to visit several goal points, and then they will leave the virtual environment. Sometimes, pedestrians may consider undertaking other activities; this can happen between two sequential visits. In this study, pedestrians have to adjust their attributes and optimize their paths continuously in the virtual environment. Moreover, pedestrians in a group must move with the group to their destination, while avoiding stationary obstacles and other pedestrians (the dynamic obstacles) when they come in close proximity.

Proposed method for crowd simulation
The proposed method uses the multi-group microscopic model for generating a real-time trajectory for each pedestrian while navigating in the walking area. Moreover, an agent is introduced in the proposed method for modelling pedestrian's behaviours, where each type of pedestrian has its own attributes. During the simulation, pedestrians move in groups toward their destination points in the simulated working environment, and avoid obstacles and other pedestrians when they move closer. Moreover, the goal point for each group of pedestrians changes dynamically, and the desired goal point is defined at each step from the list of goal points. In the proposed method, pedestrians carry out various crowd activities, such as tuning their parameters, avoiding collisions, optimizing their paths, and changing their behaviour. In this work, pedestrians use local knowledge to avoid local collisions and interact with other pedestrians. Furthermore, pedestrians use global knowledge for longterm planning and provide goal-directing capability.
In this study, pedestrians continuously adjust their position based on the proposed method in real-time. Therefore, visualizing their motions is important to understand the trajectories that pedestrians follow. For that reason, the virtual environment and the virtual pedestrians are imported into the 3D animation software package Autodesk Maya [31] via a custom Python script. Then, the Maya's key-frame system is used to visualize the simulated scenario. In the proposed method, we created Python script models and embedded them into the Autodesk Maya for generating real-time trajectories.
The proposed method consists of three main steps: environment setting, motion computation and steering behaviours, and locomotion, described in the three following sections.

Environment setting
The first step is to introduce the working environment, obstacles, pedestrians, and goal points. Moreover, this step identifies the initial state of the pedestrian's attributes: gender, age, position, velocity, and energy. Also, interactions between pedestrians with the environment have to be defined.

Remark 1
During this simulation, the virtual pedestrian's position is defined along the x, y, and z axes. We assumed that the pedestrian is restricted to move in a 2D space in the proposed method. Therefore, the pedestrian's position is defined by two independent coordinates, x and z, and the value of the y− coordinate is set to 0.

Virtual environment and obstacles
The walking area of the virtual environment is considered a two-dimensional (2D) workspace (C) for all simulations. The workspace centre is located on the origin point, and the continuous workspace is divided into square grid cells (x = 1, z = 1) unit . In the proposed method, all the grid cells of the given workspace meet the following equation: where w and l represent the width and length of the simulated workspace and C (x,z) represents the grid cells in the workspace. Afterwards, the obstacle model is imported into the scene to prevent the pedestrians from walking through the space occupied by obstacles. The region of space occupied by the obstacles is denoted by C obs , and the obstaclefree region is represented by C free . The grid cells have integer coordinates in the form C (x,z) ∈ C , with 1 ≤ x ≤ w , and 1 ≤ z ≤ l . A given cell can correspond either to a navigable area C (x,z) or to a space occupied by the obstacles C (x,z) ∈ C obs . In this study, the obstacles are distributed at different locations in the free space. A safety zone with radius R is created around the obstacles to avoid the possibility of overlapping the paths traced by pedestrians with the obstacle boundaries. The steps for constructing the workspace and obstacles are illustrated in Algorithm 1, and a brief description of each step is provided as follows: Step 1: Define the number of obstacles n3DObstacles.
Algorithm 1-Line(1) Step 2: Import the models of the workspace and obstacles into the scene. Algorithm 1-Lines(2,10) Step 3: Determine the values of width w and length l of the simulated workspace from the model in the scene. Algorithm 1-Line (3) Step 4: Divide the continuous workspace into several square grid cells, and determine the coordinates of each grid cell in the form Step 5: Set the value of each grid cell in the free space C free inside the working environment C to "0". Algorithm 1-Line(7) Step 6: Create a safety zone around each obstacle with a radius of R. There are n3DObstacles obstacles in the scene. Each safety zone consists of a set of square grid cells; the total number of the grid cells occupied by obstacles is equal to nObstacles. The centre of the grid cells occupied by obstacles is denoted by the matrix C obs(h,l) , (h = 1, 2) , (l = 1...nObstacles) . Algorithm 1-Line(13-21).
Step 7: Set the value of each grid cell of the space occupied by obstacles C obs around each obstacle with a radius of R inside the working environment C to "1". Algorithm 1-Line(17).
Step 8: Create 2D array from the grid cell values, where the grid cells C (x,z) can correspond either to the navigable area C (x,z) ∈ C free or to a space occupied by obstacles C (x,z) ∈ C obs . Algorithm 1-Lines(22-28).

Pedestrians
After generating the workspace with obstacles, the virtual pedestrians are introduced into the scene. First, pedestrians are assigned attributes; these include gender, age, position, velocity, and energy. Algorithm 2 describes this in detail. A brief description of each step follows: Step 1: Define the number of groups (nGroups), number of pedestrian's type nTypes, and constant c p1 . Algorithm 2-Line(1).
Step 2: Import the models of the pedestrians into the scene.
Step 3: Generate the crowd with several groups (nGroups), where pedestrians in each group ( Pedestrians group ) consist of a different number of pedestrians ( nPedestrians group ). Algorithm 2-Line(6-24).
Step 4: Create pedestrians with their attributes such as (position, velocity, energy level,... etc.) for each type (nTypes) of pedestrians. This study classified pedestrians into seven different types (nTypes="7") to establish the range of variation; each type of pedestrian has associated characteristics. Algorithm 2-Lines(13-18).
Step 5: Formulate all pedestrians in the crowd ( Pedestrians crowd ) with all their attributes by concatenation of all pedestrians ( nPedestrians group and Pedestrians group ) of all groups. Algorithm 2-Line(20-21).
Step 6: Determine the total number of pedestrians in the crowd ( nPedestrians total ) by summing all pedestrians in all groups. Algorithm 2-Line(22).

Goal points
Each group of pedestrians is assumed to have different goal points to visit in terms of the number and sequence of the goal points. The goal points in the simulated environment correspond to a specific region in the environment. The steps for formulating the list of goal points for each group in the simulated environment are illustrated in Algorithm 3, and a brief description follows: Step 1: Define the number of the goal points (nGoals) and their locations ( Goals group ) in the crowd. Algorithm 3-Line(1).
Step 2: Denote the list of all goal points existing in the scene by GoalPoints list , where we obtained the values of x − coordinates ( Goals x ) and z − coordinate ( Goals z ) of each goal in the scene; the y − coordinate ( Goals y ) is set to zero vector. Algorithm 3-Line(3) Step 3: Create the number of goal points randomly ( nRandom goals ) between [1,nGoals] for each group ( g r ), and also the sequence of the goal points is generated randomly. Algorithm 3-Line(6-14) Step 4: Create the starting and the final destination points for each group ( g r ), which are located outside the working environment. The x − coordinate for both the start and the end points are generated randomly between [(−w∕2), (w∕2)] , the y − coordinate is set to "0", and the value of z − coordinate is equal to l∕2 + c s1 . The value of constant c s1 represents how far the goal point from the outside of the working environment in z direction. Algorithm 3-Lines (15)(16)(17)(18)(19)(20)(21)(22)(23)(24) Step 5: Formulate the overall goal points' array of the crowd ( Goals crowd ) by concatenating of goal points' array of all groups ( Goals group ). Algorithm 3-Line(27)

Initilization of a pedestrian
Each pedestrian is assigned a position and a velocity. Pedestrians and groups are distributed randomly outside the virtual environment. Then, they start to move to reach the goal point in the list of goal points. Steps to compute the initial value of Position p e and Velocity p e for each pedestrian are illustrated in Algorithm 4, and a brief description follows: Step 1: First, this algorithm starts with the previously created input data: number of groups nGroups, num-ber goal points nGoals, all pedestrians in the crowd Pedestrians crowd , number of pedestrians in the crowd nPedestrians crowd , the list of goal points of all groups of pedestrians in the crowd Goals crowd , and define constants c f 1 , c f 2 and c f 3 . Algorithm 4-Line(1).
Step 2: Create random position for each pedestrian Position p e ( p e = 1, .., nPedestrians group [g r ] ) in each group g r , ( g r = 1, ..., nGroups ) around the starting point Goals group [1] . The distance between the randomly created point(x, z) and the starting point depends on the value of constant c f 1 . The starting point represents the first goal point in the goal points list Goals group that belongs to the group g r . Algorithm 4-Lines(6-11). At intitialization, all pedestrians' characteristics are set, including dynamics' attributes position and velocity, and personal attributes, such as gender, age, and energy. The groups' initial configurations in terms of the number of the pedestrian, type of pedestrian, number of goal points and their sequence, pedestrian's attributes, and most of the other variables are generated at random.
In this study, pedestrians ( Pedestrians crowd [g r ] , g r = 1, ..., nGroups ) enter the simulated environment one at the same time. Pedestrians in the crowd need to decide whether and when (key-frame kFrame) they start to move from the starting point. Pedestrian enter the virtual environment following a specific schedule, as illustrated in Algorithm 5. Based on this algorithm, each group g r , ( g r = 1, ..., nGroups ) starts moving at randomly created keyframes ( kFrame activate [g r ] ) between (1, nFrames*c a1 ), where nFrames represents the total number of frames and c a1 is a constant.

Motion computation and steering behaviours
Pedestrians' dynamics include actions such as obstacle avoidance, pedestrian collision, avoidance, flocking with neighbouring pedestrians, goal − directing , and combined steering behaviours. Pedestrian behaviours are discussed in the following subsections. In each key frame, pedestrians move from the current position to the new updated position at a variable walking pace. The new updated position and velocity are calculated based on the steering behaviours with the current pedestrian's position and velocity.
Obstacle avoidance To direct pedestrians to avoid the collision, we employed an extended version of a previous method developed by us, called boundary node method (BNM) which is used to generate a collision-free path for each pedestrian when they move close to the obstacles in real-time. The details of the BNM method are given in [32,33].
Using the global path planning in real-time contexts is difficult for modelling pedestrians' behaviours by using the agent-based model because it becomes computationally expensive. Therefore, the agent-based models used in this study separate local collision avoidance from global path planning. Based on the extended BNM method, each pedestrian in the crowd is simulated by a nine-node quadrilateral element. The nodes are denoted by p(q), (q = 1...9) , the centroid node p(5) represents the pedestrian's location, as illustrated in Fig. 2, and the nodes p(1 → 4) with p(6 → 9) represent the eight-boundary nodes that help pedestrians to move forward and avoid obstacles. As shown in Fig. 2b, pedestrian and boundary nodes p(q), (q = 1...9) are restricted to move in eight possible directions e(u), (u = 1...8) in the workspace. The BNM method uses the length and width of obstacles to generate a collision-free path. However, the height of obstacles (that would allow pedestrians to identify them) is not taken into account by this method. More details on the BNM method for path planning and its applicability to various problems are provided in [32]. While the distance between the pedestrian's position and centre of the obstacle is less than (r+R), the pedestrian interferes with the safety zone around obstacles, where R and r represent the radius of the safety zone around the obstacle and pedestrian, respectively. Based on the obstacles avoidance procedure, the pedestrian and boundary nodes are steered to avoid obstacles and change their motion direction by selecting a new position in the free space C free .
In order to explain the obstacle avoidance and demonstrate how the pedestrians are steered to avoid obstacles and change their moving direction with the help boundary nodes, let us consider a very simple example with only one pedestrian, as shown in Fig. 3. In the first iteration of the simulation, t = 1 , the pedestrian (p(5)) is initially located at the start point and the boundary nodes p(1 → 4) and p(6 → 9) are generated around the pedestrian's position. This is illustrated in Fig. 3a, where the boundary nodes are represented by blue star objects and pedestrian is represented by the red star object. At the iteration number t = 2 , as the pedestrian wants to move forward with the boundary nodes from the current Position p e (see Fig. 3a) to the new Position p e (see Fig. 3b), all the obstacles in the working environment are examined for possible collisions. Let us assume that the distance between the pedestrian position and the center of the obstacle is greater than the sum of the radius of the safety zone around the obstacle and pedestrian (R + r) . In this case, there is no interference with the obstacle, and the pedestrian can safely change its position to a new one. In the third iteration t = 3 , as the pedestrian moves toward the goal position, nodes p(1), p (4), and p (7) interfere with the safety zone around the obstacle. In this case, the pedestrian needs to investigate the workspace to find the next position without colliding against obstacles. This implies that the Fig. 3 Obstacle avoidance using BNM: demonstrates the single pedestrian exploration in a 2D environment with obstacles pedestrian can only move in the x-direction, in this case: therefore, the movement of the pedestrian is restricted to either the left or right direction. Changing the motion direction depends on values E(2) and E(8) at the location p(2) and p (8). The values of E(2) and E (8) are calculated based on the potential function. The procedure of calculating the potential value of E based on the new proposed potential function is illustrated in detail in our previous work [32]. The results of the calculated potential values of E(2) and E(8) based on the potential function show that E(2) < E (8) . This implies that the next position of the pedestrian must be in the right direction. The same procedure is repeated for the iteration number t = 3 − 6 by shifting the pedestrian to the right until it passes the block of obstacles. At t = 8 , as illustrated in Fig. 3c, the pedestrian moves forward without collision with any obstacles. Similarly, for the iterations t = 9 − 11 , the pedestrian moves forward until it reaches the goal position as shown in Fig. 3d Pedestrian collision avoidance During the crowd simulation, pedestrians need to stay together and maintain the group while pedestrians must keep a certain distance (spaces) between themselves. The study of group behaviors in a crowd simulation can help in better understanding and respecting different personal spaces. However, the culture, relationship between pedestrians, and crowd density are essential factors affecting personal space. Besides, in certain circumstances, identifying the minimum personal space requirements is needed to protect health and limit the spread of potential disease.
Each moving pedestrian represents a dynamic obstacle for the remaining pedestrians in the scene. Pedestrians should not reside within the personal space requirement, the socalled private sphere, of the other pedestrians. Usually, each pedestrian has a safety zone with a radius r, and pedestrians should keep a certain distance from the other neighbouring pedestrians. We assume that the distance between pedestrians should not be less than (2r). Suppose that the distance between current and neighbouring pedestrian drops below a specific value (2r). In such a case, the current pedestrian changes its motion direction in surrounding areas to prevent the collision. For this situation, we have proposed an appropriate reactive behaviour, called pedestrian collision avoidance, for avoiding collision between pedestrians walking closer. The current pedestrian steers to either left or right for a newly calculated position, located on the third vertex of an equilateral triangle. The overall process of pedestrian collision avoidance is illustrated in Algorithm 6, and the concept of collision avoidance is explained in the following steps: Step 1: First, this algorithm starts with the previously created input data: position of all pedestrians in the crowd Positions crowd , new calculated pedestrian's position Position p e , current pedestrian's position Position p e , number of all pedestrians in the crowd nPedestrians total s. Algorithm 6-Line(1).
Step 2: Check for interference between current pedestrian Position p e and all pedestrians Position p e ( p e = 1, ..., nPedestrians total ) in the crowd, if the distance between current pedestrian Position p e and the other pedestrians in the crowd is less than 2 × r , then the interference will occur and a new position Position p e needs to be calculated. Algorithm 6-Line(2-6).
Step 3: Suppose a collision happens between pedestrians, the proposed method implements pedestrian collision avoidance to avoid collision between pedestrians. There are two possibilities for the new pedestrian's position ( right − hand side or left − hand side ) located on the third vertex of equilateral triangles. The pedestrian will choose right − hand side or left − hand side to avoid the collision based on the random value. Algorithm 6-lines(7-13). Flocking behaviour We have considered a sort of cooperation among pedestrians moving with the same group. The pedestrians' steering behaviours in the same group demonstrate how pedestrians react, and pedestrians outside of the group are ignored. We considered steering behaviours such as separation, cohesion, and alignment related to the pedestrians. These behaviours influence the pedestrians' motion, more precisely, their position and velocity. The separation steering behaviour gives a pedestrian the ability to maintain a certain separation distance from other pedestrians in the same group, as shown in Fig. 4a. The separation steering behaviour is used to prevent pedestrians from crowding together. Moreover, the cohesion steering behaviour gives a pedestrian the ability to cohere with the other pedestrians, as shown in Fig. 4b. At the same time, the alignment steering behaviour tends to align pedestrians with its neighbouring pedestrians, as demonstrated in Fig. 4c. For details on the steering behaviours, see [22,34].
Goal-directed behaviour Pedestrians in each group walk towards a well-defined goal at a certain point in the virtual environment modelled by a goal-directed behaviour. The distance between the current position of the pedestrian and the current active goal is calculated. If the distance between these two points is less than a certain value, then a new goal point is assigned to the current group. Suppose the current goal point is the last destination point. In that case, the group will terminate the simulation, and this group will not be active anymore. Otherwise, the goal_directed steering behavior calculates the goal vector and normalizes the goal vector by applying a weighting goal weight , as illustrated in Algorithm 7.
Combining behaviours Combining behaviours can happen in two ways: (1) switching, (2) blending [22]. As the walking circumstance changes in the simulated environment, the pedestrian may "switch" between behavioural modes. Alternatively, these behaviours, which are acting in parallel, are commonly "blended" together. For example, in normal situations, as the pedestrians move around through the simulated environment toward their destination, they blend both flocking and goal-directed behaviours to a single steering force vector to allow the group to walk toward their goals. Furthermore, suppose that a group of pedestrians approaches an obstacle or other pedestrians in the same or different groups. This situation leads to a behavioural switch from moving to collision avoidance. All pedestrians try to avoid collisions with obstacles and pedestrians, and pedestrians do not prefer to remain with the group while avoiding obstacles. The proposed method uses a Python script to evaluate pedestrians' behaviours, to compute a new state of pedestrians, and also generates 3D scenes and views in the animation software package Autodesk Maya [31], as illustrated in the following subsection.

Locomotion
The proposed method computes the pedestrians' movement taking into account the steering behaviours, and generates locomotion for each pedestrian in the virtual environment in real-time. In this study, the Maya's key-frame is used for 3D animating motion for a large crowd of multi-groups of pedestrians walking around through the virtual environment toward their destination points. As an implementation of the pedestrian movement, we used different types of pedestrians for this simulation. Then, we imported pedestrians into Maya via Python scripts. Subsequently, Maya allows the imported models to control the pedestrians. Based on the proposed method, the position and the velocity of each virtual pedestrian in the scene are updated in real-time.

Crowd simulation
This section describes the environment and the settings that we have considered for the proposed method.
In this simulation, pedestrians in different group sizes enter the virtual environment through the main entrance. The entrance's width is "15" unit (we assume that all distances in this study are measured in unit of length). The width w and the length l of the simulated environment are set to "105" and "330" unit, respectively. Inside the simulated environment, there are "8" shops (goal points) that the pedestrians want to visit ("4" shops on both the right-and left-hand sides). Each goal point corresponds to a specific region (shop) in the shopping mall environment. Moreover, there are "2" general service points. These points are considered a temporary goal point. We add them to the list of the goal points when the group needs them. The   [22] In this study, pedestrians are divided in groups before entering the shopping mall. Each group consists of multiple pedestrian types (male and female) of different ages (old, young, and child) to establish the range of personality variation. Several examples of the virtual pedestrian [35] are shown in Fig. 6. Each pedestrian has a different energy level, i.e. an old pedestrian has a lower energy level than a young pedestrian. The energy level of a person will decrease as he/she walks through the environment. Pedestrians rest at the sitting place in a service point when their energy level goes below the minimum energy level. During the simulation, each pedestrian is considered a dynamic obstacle for other pedestrians. Each pedestrian has a personal space requirement with a radius, r, where other pedestrians cannot interfere. In this simulation, the radius of the safety space around pedestrians r is set to 0.9 units. The minimum distance between pedestrians is set to 2r units, such that pedestrians do not interfere with the other neighbouring pedestrians. In this work, the proposed methods are implemented in Python, and the crowd simulation was carried out on a PC with Intel(R) core(TM) i5-8300H CPU 2.30GHz, 8RAM. The source code is available per request by sending an email to the authors.

Crowd simulation results
In our simulations, we present pedestrian movement in a crowd made of "200" different small groups, each with a distinct set of goal points at known locations. The total number of pedestrians in the crowd is the sum of all pedestrians in all groups, which is equal to "696" pedestrians ("696"). Pedestrians' group size distributions in the crowd are illustrated in Table 1. As shown in the table, the groups' size is different in terms of the number of pedestrians. For example, "8" groups ( 4% of the whole number groups) are seen in the crowd. The number of each pedestrian's type (types of pedestrian="7") in each group is generated randomly between [0, N c ] , and we assumed that N c = 1 , and the obtained results are presented in Table 2. From the table, it is observed that the total number of each type of pedestrian in the crowd is different. For example, the total number of the first type of pedestrian ( type 1 ) is "108" pedestrians ( 15.5% of the total number of pedestrians in the crowd).
Seven different types of characters (see Fig. 6) are used to generate "200" groups of pedestrians with "696" pedestrians. Initially, all groups of pedestrians are scattered in the front of the virtual environment around the starting point, as shown   Fig. 7a. In this scenario, the x − coordinate for the starting and ending points for pedestrians is generated randomly between [−50, w + 50] . Moreover, the value of z − coordinate is set to l + c s1 , where c s1 is determined randomly between [10,60]. Afterwards, groups were appropriately introduced into the shopping mall environment based on the defined schedule.
Each group starts moving into the mall at a randomly created key-frame between (1, nFrames*c a1 ), where c a1 is set to 0.5. Figure 8 illustrates the activation's' key − frames of all groups; the blue point objects represent the key − frames for activating groups to move. The red circle represents the groups' size in terms of the number of pedestrians; a larger circle represents a group with a higher number of pedestrians.
At each key − frame , pedestrians' attributes and their positions are calculated and updated independently in realtime. The maximum number of frames nFrames is set to "200" key − frames . The simulation results of the pedestrians' routes for all groups at each key − frame are illustrated in Fig. 7b. The figure shows that the proposed method allows pedestrians to navigate the walking area from the starting point to the final destination point. The results show that the proposed method prevents pedestrians from colliding with the obstacles in the scene by using the obstacle avoidance method, as illustrated in Fig. 7b "Obstacles' locations". As the pedestrians move closer to an obstacle or other pedestrians, they need to change their trajectory to avoid potential collisions. Our method exploits the high performance of BNM for collision avoidance. Moreover, our BNM method makes sure pedestrians never get stuck behind obstacles or against walls, always guaranteeing a collision-free path.
Different screenshots of the simulation results from different viewpoints are presented in Figs. 1 and 9. As shown in Fig. 1b&c , pedestrians attempt to walk in different directions to reach their desired goal points while avoiding obstacles and other pedestrians in the scene. All pedestrians remain with the group during the simulation. Different types of walking states are demonstrated in Fig. 9b, a closeup of a particular part of the scene. For example, a pedestrian (A) who has walked alone and changing his motion direction to avoid stairs on his right. Moreover, a pedestrian (B) has a limited space to move after leaving the shop − 6 . At the same time pedestrian (C) has a free moving space to move. At the end of the simulation, all pedestrians are terminated   Figure 10 demonstrates the situation when a pedestrian tries to pass an obstacle (the stairs). At this point, the pedestrian checks the path for collision with obstacles. If the pedestrian were to move along the current direction, it would collide with the obstacle, as shown in the figure. Therefore, the pedestrian changes his direction of motion accordingly. The BNM method is employed to find a collision-free path and guide the pedestrian to turn left and then right to overcome the obstacle. Afterwards, the pedestrian moves undisturbed to reach its destination point. Similar actions are taken when two or more pedestrians are in danger of colliding. An illustrative example of combined obstacle and pedestrian avoidance is shown in Figs. 11 and 12. In this example, we consider a group of two pedestrians moving forward to their goal point. Firstly, at key − frame 94 , as shown in Fig. 11a, if the pedestrians moved along the current direction, they would collide with the obstacle (a vase) because the obstacle blocks the path. Therefore, the proposed method calculates their new direction of motion as shown in Fig. 11b&c at key − frame 95 and 96.
Secondly, after the pedestrians pass the obstacle, a collision may happen between pedestrians at the key − frame 97. When the distance between pedestrians goes below 2 × r , then pedestrian (B) employs the avoidance method to resolve this situation (see Fig. 12a). Based on this method, pedestrian (B) has to change his motion direction to the right to avoid collision with its neighbouring pedestrian (A). Then pedestrians (A) and (B) keep moving at key − frame 98 to reach their destination point, as shown in Fig. 12b. The red circle objects represent the pedestrians' movement, which can be seen clearly near the obstacle in the walking area. The results show that the pedestrians can avoid collisions and reach their goal points by using the proposed method.
In order to illustrate the pedestrian's movement in the simulated environment, different screenshot views of a simple illustrative example is presented in Fig. 14 at different stages of the simulation, as shown in Fig. 13. A pedestrian (we consider a group that consists of one pedestrian) Fig. 10 Screenshot of the simulation: shows a single pedestrian trying to move through the walking area toward the goal point, and he starts to change his motion direction near the safety area around the stairs at key − frames (a) 118, (b) 122, and (c) 124 Fig. 11 Screenshot of the simulation: shows a group of pedestrians, consists of two pedestrians (pedestrian A and pedestrian B), using the obstacle avoidance method to pass the obstacle. Afterwards, pedestrians keep moving toward their desired goal points: (a) collision avoidance between pedestrians and obstacle at keyfram = 94 , (b) collision avoidance between pedestrian A and obstacle at keyfram = 95 , and (c) collision avoidance between pedestrian B and obstacle at keyfram = 96 Fig. 12 Screenshot of the simulation: shows a group of pedestrians, consists of two pedestrians (pedestrian A and B), passing each other using the pedestrian avoidance method. (a) Collision avoidance takes place between pedestrians A and B at key − frame 97 . (b) Pedestrian B changes the motion direction to avoid pedestrian A without any collision at key − frame 98 , and then they keep moving to reach their goal point activates to move to enter the walking area at a randomly created key-frame, as shown in Figs. 14a, 13 left (a0) and 13 right (b0). Afterwards, the pedestrian enters the walking area through the entrance at key-frame "65", as illustrated in Figs. 14a, 13 left (a1) and 13 right (b1), then pass through the walking area to reach the goal points while avoiding obstacles. The simulation results in Figs. 14b, 13 left (a2) and 13 right (b2), present the time that the pedestrian comes closer to an obstacle at key-frame "100", where the pedestrian turns to the left to pass the obstacle without collision. As the pedestrian reaches the first goal point, the current goal point updates to the second goal point in the goal point list. Then the pedestrian moves toward the new goal point, as shown in Fig. 14c, 13 left (a3) and 13 right (b3) at key-frame "145". The pedestrian keeps changing the motion direction in the walking area to determine a collision-free path and track the goal points. The simulation results at several locations at key-frame "165", "200", "230", "240", and "270" are presented in Figs. 14d → h , 13 left ( a4 → a8 ) and 13 right ( b4 → b8 ). In this simulation, the pedestrian visits the service point in the middle of the simulated environment, as shown in Fig. 14i, 13 left (a9) and 13 right (b9) at key-frame "287". Subsequently, the pedestrian moves toward the entrance/exit to leave the working environment, as illustrated in Figs. 14j, 13 left (a10) and 13 right (b10) at key-frame "310". At the last stage of the simulation, the pedestrian continues moving until it reaches the final destination point located at the End − point as shown in Figs. 13 left (a11) and 13 right (b11).
The performance of our implementation was evaluated by simulating pedestrian movements in the virtual environment with a different number of groups (from 1 → 10 groups ) and different group sizes. For each pedestrian in each group, the proposed method is used to generate pedestrians' routes as shown in Fig. 1(a). We performed "200" independent runs for each simulated scenario for "200" frames. At each independent run, the average values for the number of pedestrians and the total computational time required to generate the pedestrians' route are calculated. The obtained results are presented in Fig. 15. The results demonstrate that the average value of the computational time is roughly linear, not increasing significantly with increasing the number of pedestrians in the scene. Moreover, the obtained results show that the proposed method can generate the routes for pedestrians navigating in the virtual environment to visit several destination points within a reasonable computational time.

Conclusions
This study develops a new method for simulating pedestrian crowd movement in a virtual environment. We have considered a public space such as a virtual working environment. A large number of virtual pedestrians of Fig. 13 Pedestrian's trajectory: the graph shows pedestrians' movement in different directions in the simulated walking area at different keyframes: 65, 100, 145, 165, 200, 230, 240, 270, 287, and 310. The red circle objects represent the pedestrians' trace in the virtual environment at each key-frame. The left graph shows the generated planned pedestrian's trajectory, and the right graph shows the simulation result of the pedestrian's trajectory in the virtual environment after the simulation runs for 500 keyframes different sizes and types move as groups inside the virtual environment. Each group follows different directions to reach their goal points. The proposed method generates a real-time route for each pedestrian walking in the virtual environment. The developed method demonstrates how the pedestrians adapt their dynamics to avoid static obstacles and other pedestrians. Based on our method, each pedestrian in each group constantly adjust their paths and parameters in real-time. The obtained results show that the developed method performs well to generate the pedestrian crowd movement with "200" different small groups of pedestrians in a given virtual environment. After 200 keyframes, we obtained "4235" values for pedestrian velocity, where the maximum value of the pedestrian's velocity reaches 1.14 units/timestep. The mean and the standard deviation of the calculated velocity are equal to 0.905 units/timestep and 0.216, respectively. We concluded that the increasing number of groups of pedestrians has the reverse effect on the pedestrian mean velocity in the crowd.  list of random number for key-frame to activate groups r, R radius of the safety zone around pedestrian and obstacles,unit Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.

Fig. 15
The average value of the number of pedestrians and the computational time required to find the routes for pedestrians in each group