matplotlib: how to draw a rectangle on image

How to draw a rectangle on an image, like this:

import matplotlib.pyplot as plt from PIL import Image import numpy as np im = np.array(Image.open('dog.png'), dtype=np.uint8) plt.imshow(im) 

I don't know how to proceed.

4 Answers

You can add a Rectangle patch to the matplotlib Axes.

For example (using the image from the tutorial here):

import matplotlib.pyplot as plt import matplotlib.patches as patches from PIL import Image im = Image.open('stinkbug.png') # Create figure and axes fig, ax = plt.subplots() # Display the image ax.imshow(im) # Create a Rectangle patch rect = patches.Rectangle((50, 100), 40, 30, linewidth=1, edgecolor='r', facecolor='none') # Add the patch to the Axes ax.add_patch(rect) plt.show() 

There is no need for subplots, and pyplot can display PIL images, so this can be simplified further:

import matplotlib.pyplot as plt from matplotlib.patches import Rectangle from PIL import Image im = Image.open('stinkbug.png') # Display the image plt.imshow(im) # Get the current reference ax = plt.gca() # Create a Rectangle patch rect = Rectangle((50,100),40,30,linewidth=1,edgecolor='r',facecolor='none') # Add the patch to the Axes ax.add_patch(rect) 

Or, the short version:

import matplotlib.pyplot as plt from matplotlib.patches import Rectangle from PIL import Image # Display the image plt.imshow(Image.open('stinkbug.png')) # Add the patch to the Axes plt.gca().add_patch(Rectangle((50,100),40,30,linewidth=1,edgecolor='r',facecolor='none')) 

You need use patches.

import matplotlib.pyplot as plt import matplotlib.patches as patches fig2 = plt.figure() ax2 = fig2.add_subplot(111, aspect='equal') ax2.add_patch( patches.Rectangle( (0.1, 0.1), 0.5, 0.5, fill=False # remove background ) ) fig2.savefig('rect2.png', dpi=90, bbox_inches='tight') 

From my understanding matplotlib is a plotting library.

If you want to change the image data (e.g. draw a rectangle on an image), you could use PIL's ImageDraw, OpenCV, or something similar.

Here is PIL's ImageDraw method to draw a rectangle.

Here is one of OpenCV's methods for drawing a rectangle.

Your question asked about Matplotlib, but probably should have just asked about drawing a rectangle on an image.

Here is another question which addresses what I think you wanted to know: Draw a rectangle and a text in it using PIL