Displaying images with imshow in Matplotlib - Step-by-Step Execution

Concept Flow - Displaying images with imshow

Load Image Data

↓

Call imshow() with data

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Render Image on Plot

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Show Plot with plt.show()

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Image Displayed on Screen

The flow starts by loading image data, then imshow() is called to prepare the image, followed by rendering and displaying it on screen.

Execution Sample

Matplotlib

import matplotlib.pyplot as plt
import numpy as np
image = np.array([[0, 255], [255, 0]])
plt.imshow(image, cmap='gray')
plt.show()

This code creates a simple 2x2 grayscale image and displays it using imshow.

Execution Table

Step	Action	Input/Parameter	Internal State	Output/Effect
1	Import matplotlib.pyplot as plt	-	plt module loaded	Ready to plot
2	Import numpy as np	-	np module loaded	Ready for array operations
3	Create image array	[[0, 255], [255, 0]]	image variable holds 2x2 array	Image data ready
4	Call plt.imshow()	image array, cmap='gray'	Prepare image for display	Image plot object created
5	Call plt.show()	-	Render plot window	Image displayed on screen
6	End	-	-	Execution complete

💡 All steps executed; image displayed successfully.

Variable Tracker

Variable	Start	After Step 3	After Step 4	After Step 5
plt	Not imported	Imported	Imported	Imported
np	Not imported	Imported	Imported	Imported
image	Undefined	[[0, 255], [255, 0]]	[[0, 255], [255, 0]]	[[0, 255], [255, 0]]
imshow_object	Undefined	Undefined	Undefined	Undefined

Key Moments - 2 Insights

Why do we need to call plt.show() after plt.imshow()?

plt.imshow() prepares the image plot but does not display it. plt.show() opens the window to actually show the image, as seen in execution_table step 5.

What does the cmap='gray' parameter do in imshow()?

cmap='gray' tells imshow to display the image in grayscale colors instead of default colors. This is shown in step 4 where imshow prepares the image with grayscale.

Visual Quiz - 3 Questions

Look at the execution table, what is the state of the variable 'image' after step 3?

A A 2x2 numpy array with values [[0, 255], [255, 0]]

B Undefined

C A plot object

D A grayscale colormap

💡 Hint

Check the 'Internal State' column in row for step 3.

At which step is the image actually displayed on the screen?

A Step 3

B Step 4

C Step 5

D Step 6

💡 Hint

Look at the 'Output/Effect' column for when the image is shown.

If we remove plt.show(), what will happen according to the execution flow?

A Image will still display automatically

B Image will not display on screen

C Code will error out

D Image will display but in color

💡 Hint

Refer to key moment about plt.show() necessity and execution_table step 5.

Concept Snapshot

imshow(image, cmap=None) displays image data as a plot.
Use plt.show() to open the window and see the image.
cmap='gray' shows grayscale images.
Image data is usually a 2D or 3D numpy array.
imshow prepares the image; show renders it.

Full Transcript

This visual execution traces how to display images using matplotlib's imshow function. First, image data is created as a numpy array. Then plt.imshow() is called with this data and optional parameters like cmap='gray' to set grayscale colors. However, imshow only prepares the image plot internally. The actual display happens when plt.show() is called, which opens a window showing the image. Variables like 'image' hold the pixel data, and 'imshow_object' represents the plot. Key points include the need for plt.show() to see the image and how cmap controls colors. The execution table shows each step from imports to display, helping beginners see the process clearly.