How to fill an area within a polygon in Python using matplotlib?

Matplotlib provides several ways to fill areas within polygons. The most common approaches are using Polygon patches, fill() method, or PatchCollection for multiple polygons.

Method 1: Using fill() Method

The simplest way to fill a polygon is using matplotlib's fill() method ?

import matplotlib.pyplot as plt
import numpy as np

# Define polygon vertices (triangle)
x = [1, 4, 2]
y = [1, 2, 4]

plt.figure(figsize=(8, 6))
plt.fill(x, y, color='lightblue', alpha=0.7, edgecolor='blue')
plt.title('Filled Triangle Polygon')
plt.grid(True, alpha=0.3)
plt.show()

Method 2: Using Polygon Patch

For more control over polygon properties, use Polygon patches ?

import matplotlib.pyplot as plt
from matplotlib.patches import Polygon
import numpy as np

# Create random pentagon vertices
vertices = np.array([[1, 1], [3, 1], [4, 3], [2, 4], [0.5, 2.5]])

fig, ax = plt.subplots(figsize=(8, 6))

# Create polygon patch
polygon = Polygon(vertices, closed=True, facecolor='lightgreen', 
                 edgecolor='darkgreen', alpha=0.8, linewidth=2)

ax.add_patch(polygon)
ax.set_xlim(0, 5)
ax.set_ylim(0, 5)
ax.set_title('Filled Pentagon Using Polygon Patch')
ax.grid(True, alpha=0.3)
plt.show()

Method 3: Using PatchCollection for Multiple Polygons

When working with multiple polygons, PatchCollection provides better performance ?

import matplotlib.pyplot as plt
from matplotlib.collections import PatchCollection
from matplotlib.patches import Polygon
import numpy as np

fig, ax = plt.subplots(figsize=(10, 8))

# Create multiple polygons
polygons = []
colors = []

for i in range(5):
    # Generate random polygon vertices
    vertices = np.random.rand(6, 2) * 4 + i
    polygon = Polygon(vertices, closed=True)
    polygons.append(polygon)
    colors.append(i)

# Create patch collection
collection = PatchCollection(polygons, alpha=0.7, cmap='viridis')
collection.set_array(np.array(colors))

ax.add_collection(collection)
ax.set_xlim(0, 8)
ax.set_ylim(0, 6)
ax.set_title('Multiple Filled Polygons')
plt.colorbar(collection, ax=ax)
plt.show()

Customizing Polygon Appearance

You can customize colors, transparency, and edge properties ?

import matplotlib.pyplot as plt
import numpy as np

# Create star-shaped polygon
angles = np.linspace(0, 2*np.pi, 10, endpoint=False)
radii = [1 if i % 2 == 0 else 0.5 for i in range(10)]
x = radii * np.cos(angles)
y = radii * np.sin(angles)

plt.figure(figsize=(8, 8))
plt.fill(x, y, color='gold', alpha=0.8, edgecolor='orange', 
         linewidth=3, linestyle='--')
plt.title('Customized Star Polygon')
plt.axis('equal')
plt.grid(True, alpha=0.3)
plt.show()

Comparison of Methods

Conclusion

Use fill() for simple polygons, Polygon patches for detailed customization, and PatchCollection when working with multiple polygons. Each method offers different levels of control and performance optimization.