How to save an array as a grayscale image with Matplotlib/Numpy?

To save an array as a grayscale image with Matplotlib/NumPy, we can use imshow() with the gray colormap and savefig() to save the image to disk.

Basic Example

Here's how to create and save a grayscale image from a NumPy array ?

import numpy as np
import matplotlib.pyplot as plt

# Set figure size
plt.rcParams["figure.figsize"] = [7.50, 3.50]
plt.rcParams["figure.autolayout"] = True

# Create random data with 5x5 dimension
arr = np.random.rand(5, 5)

# Display as grayscale image
plt.imshow(arr, cmap='gray')
plt.colorbar()  # Add colorbar to show intensity scale
plt.title('Grayscale Image from Array')

# Save the image
plt.savefig('/images/grayscale_output.png', dpi=300, bbox_inches='tight')
plt.show()

Saving Without Axes and Borders

To save just the image data without matplotlib's axes and borders ?

import numpy as np
import matplotlib.pyplot as plt

# Create sample data
data = np.random.rand(100, 100)

# Create figure without axes
fig, ax = plt.subplots()
ax.imshow(data, cmap='gray')
ax.axis('off')  # Remove axes

# Save with tight bounding box
plt.savefig('/images/clean_grayscale.png', bbox_inches='tight', 
            pad_inches=0, dpi=150)
plt.show()

Using Different Grayscale Ranges

Control the intensity range with vmin and vmax parameters ?

import numpy as np
import matplotlib.pyplot as plt

# Create data with specific range
data = np.array([[0, 0.3, 0.6],
                 [0.2, 0.5, 0.8],
                 [0.4, 0.7, 1.0]])

fig, (ax1, ax2) = plt.subplots(1, 2, figsize=(10, 4))

# Default range (0-1)
ax1.imshow(data, cmap='gray')
ax1.set_title('Default Range')
ax1.axis('off')

# Custom range (0.2-0.8)
ax2.imshow(data, cmap='gray', vmin=0.2, vmax=0.8)
ax2.set_title('Custom Range (0.2-0.8)')
ax2.axis('off')

plt.tight_layout()
plt.savefig('/images/grayscale_ranges.png', dpi=200)
plt.show()

Key Parameters

Conclusion

Use plt.imshow() with cmap='gray' to display arrays as grayscale images. Use plt.savefig() with appropriate parameters to save high-quality images to disk.