Update boid_flockers example include flight direction (#2696)

I updated boid_flockers so that the direction of movement of agents is shown in the simulation.

I did this by changing the marker to caret-down (a triangle with a larger base so that the direction of motion is not ambiguous) and then rotating the marker according to the direction of motion of the agent.

Author: sanika-n

mesa/examples/basic/boid_flockers/agents.py

@@ -58,6 +58,7 @@ def __init__(
         self.separate_factor = separate
         self.match_factor = match
         self.neighbors = []
+        self.angle = 0.0  # represents the angle at which the boid is moving
 
     def step(self):
         """Get the Boid's neighbors, compute the new vector, and move accordingly."""

mesa/examples/basic/boid_flockers/app.py

@@ -1,19 +1,34 @@
 import os
 import sys
 
+from matplotlib.markers import MarkerStyle
+
 sys.path.insert(0, os.path.abspath("../../../.."))
 
 from mesa.examples.basic.boid_flockers.model import BoidFlockers
 from mesa.visualization import Slider, SolaraViz, make_space_component
 
+# Pre-compute markers for different angles (e.g., every 10 degrees)
+MARKER_CACHE = {}
+for angle in range(0, 360, 10):
+    marker = MarkerStyle(10)
+    marker._transform = marker.get_transform().rotate_deg(angle)
+    MARKER_CACHE[angle] = marker
+
 
 def boid_draw(agent):
     neighbors = len(agent.neighbors)
 
+    # Calculate the angle
+    deg = agent.angle
+    # Round to nearest 10 degrees
+    rounded_deg = round(deg / 10) * 10 % 360
+
+    # using cached markers to speed things up
     if neighbors <= 1:
-        return {"color": "red", "size": 20, "marker": "o"}
+        return {"color": "red", "size": 20, "marker": MARKER_CACHE[rounded_deg]}
     elif neighbors >= 2:
-        return {"color": "green", "size": 20, "marker": (3, 0, 60)}
+        return {"color": "green", "size": 20, "marker": MARKER_CACHE[rounded_deg]}
 
 
 model_params = {

mesa/examples/basic/boid_flockers/model.py

@@ -49,6 +49,9 @@ def __init__(
             seed: Random seed for reproducibility (default: None)
         """
         super().__init__(seed=seed)
+        self.agent_angles = np.zeros(
+            population_size
+        )  # holds the angle representing the direction of all agents at a given step
 
         # Set up the space
         self.space = ContinuousSpace(
@@ -79,6 +82,14 @@ def __init__(
         self.average_heading = None
         self.update_average_heading()
 
+    # vectorizing the calculation of angles for all agents
+    def calculate_angles(self):
+        d1 = np.array([agent.direction[0] for agent in self.agents])
+        d2 = np.array([agent.direction[1] for agent in self.agents])
+        self.agent_angles = np.degrees(np.arctan2(d1, d2))
+        for agent, angle in zip(self.agents, self.agent_angles):
+            agent.angle = angle
+
     def update_average_heading(self):
         """Calculate the average heading (direction) of all Boids."""
         if not self.agents:
@@ -96,3 +107,4 @@ def step(self):
         """
         self.agents.shuffle_do("step")
         self.update_average_heading()
+        self.calculate_angles()