Overlaying data on images in Matplotlib - Step-by-Step Execution

Concept Flow - Overlaying data on images

Load Image

↓

Display Image

↓

Prepare Data Points

↓

Overlay Data on Image

↓

Show Final Plot

First, we load and display an image. Then, we prepare data points and overlay them on the image before showing the final combined plot.

Execution Sample

Matplotlib

import matplotlib.pyplot as plt
import numpy as np
img = np.ones((5,5))
plt.imshow(img, cmap='gray')
plt.scatter([1,3], [2,4], color='red')
plt.show()

This code loads a simple image and overlays red points on it.

Execution Table

Step	Action	Data/Variables	Result/Output
1	Import matplotlib and numpy	plt, np	Libraries ready
2	Create image array	img = 5x5 array of ones	White square image created
3	Display image with imshow	img	Image shown in grayscale
4	Prepare data points	x=[1,3], y=[2,4]	Points ready to plot
5	Overlay points with scatter	x, y, color='red'	Red points appear on image
6	Show plot	plt.show()	Final image with points displayed
7	End	-	Execution complete

💡 All steps executed, final plot displayed with image and overlaid data points

Variable Tracker

Variable	Start	After Step 2	After Step 4	Final
img	undefined	5x5 array of ones	5x5 array of ones	5x5 array of ones
x	undefined	undefined	[1, 3]	[1, 3]
y	undefined	undefined	[2, 4]	[2, 4]

Key Moments - 2 Insights

Why do the points appear on top of the image and not replace it?

Because plt.imshow draws the image first, then plt.scatter adds points on the same plot, layering them visually as shown in execution_table steps 3 and 5.

What happens if we call plt.show() before plt.scatter()?

The plot would display only the image without points, since plt.show() ends the current figure display, as seen in execution_table step 6.

Visual Quiz - 3 Questions

Look at the execution table, what is the shape of 'img' after step 2?

A An empty array

B A list of points

C A 5x5 array of ones

D A 3x3 array of zeros

💡 Hint

Check the 'Data/Variables' column in step 2 of the execution_table

At which step do the red points get added on the image?

A Step 5

B Step 3

C Step 4

D Step 6

💡 Hint

Look for the action 'Overlay points with scatter' in the execution_table

If we change the color in plt.scatter to 'blue', how would the output change?

A Points would be red

B Points would be blue

C Image color changes

D No points would show

💡 Hint

Refer to the 'color' parameter in step 5 of the execution_table

Concept Snapshot

Overlaying data on images with matplotlib:
1. Load image data (e.g., numpy array).
2. Display image using plt.imshow().
3. Prepare data points (x, y).
4. Use plt.scatter() to overlay points.
5. Call plt.show() to display combined plot.

Full Transcript

This visual execution shows how to overlay data points on an image using matplotlib. First, we import necessary libraries and create a simple image as a 5x5 array of ones. We display this image with plt.imshow. Next, we prepare data points with x and y coordinates. Using plt.scatter, we overlay these points on the image. Finally, plt.show displays the combined image and points. The execution table traces each step, showing variable states and actions. Key moments clarify why points overlay the image and the importance of calling plt.show at the right time. The quiz tests understanding of image shape, overlay step, and color changes.