Blitting for performance in Matplotlib - Step-by-Step Execution

Concept Flow - Blitting for performance

Create Figure & Axes

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Draw Static Background

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Save Background Image

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Update Dynamic Elements

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Restore Background

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Draw Updated Elements

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Blit to Screen

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Repeat Updates Efficiently

Blitting redraws only changing parts of a plot to speed up updates by saving and restoring the static background.

Execution Sample

Matplotlib

import matplotlib.pyplot as plt
fig, ax = plt.subplots()
line, = ax.plot([0], [0], 'ro')
fig.canvas.draw()  # draw the canvas once to initialize
background = fig.canvas.copy_from_bbox(ax.bbox)
line.set_data([1], [1])
fig.canvas.restore_region(background)
ax.draw_artist(line)
fig.canvas.blit(ax.bbox)

This code sets up a plot, saves the background, updates a point, restores background, redraws the point, and blits to update only the changed part.

Execution Table

Step	Action	Variable/Method	Effect	Output/State
1	Create figure and axes	plt.subplots()	Figure and axes objects created	fig, ax ready
2	Plot initial point	ax.plot([0], [0], 'ro')	Red dot at (0,0) drawn	line object created
3	Save background	fig.canvas.copy_from_bbox(ax.bbox)	Static background saved	background image stored
4	Update point data	line.set_data([1], [1])	Point data changed to (1,1)	line updated
5	Restore background	fig.canvas.restore_region(background)	Background restored, erasing old point	canvas reset to background
6	Draw updated point	ax.draw_artist(line)	New point drawn on restored background	line drawn at (1,1)
7	Blit to screen	fig.canvas.blit(ax.bbox)	Only updated area refreshed on screen	fast visual update
8	Repeat updates	Loop or event triggers steps 4-7	Efficient redraw of dynamic elements	smooth animation
9	Exit	No more updates	Stop blitting	final frame displayed

💡 Updates stop when no more changes are needed or animation ends

Variable Tracker

Variable	Start	After Step 3	After Step 4	After Step 5	After Step 6	After Step 7
fig	None	Figure created	Figure created	Figure created	Figure created	Figure created
ax	None	Axes created	Axes created	Axes created	Axes created	Axes created
line	None	Line at (0,0)	Line at (1,1)	Line at (1,1)	Line at (1,1)	Line at (1,1)
background	None	Saved background image	Saved background image	Saved background image	Saved background image	Saved background image

Key Moments - 3 Insights

Why do we save the background before updating the point?

Saving the background (step 3) lets us restore it later (step 5) to erase old drawings without redrawing the whole plot, making updates faster.

What happens if we skip restoring the background before drawing the updated point?

If we skip restoring (step 5), the old point remains visible, causing visual artifacts because the new point draws on top without clearing the old.

Why does blitting improve performance compared to redrawing the entire figure?

Blitting (step 7) updates only the changed area on screen, avoiding full redraws which are slower, so animations or updates run smoothly.

Visual Quiz - 3 Questions

Look at the execution table, what is the state of 'line' after step 4?

A Line at (0,0)

B Line removed

C Line at (1,1)

D Line not created yet

💡 Hint

Check the 'line' variable in variable_tracker after step 4

At which step is the background restored to erase old drawings?

A Step 4

B Step 5

C Step 3

D Step 7

💡 Hint

Look at the 'Action' column in execution_table for restoring background

If we do not call fig.canvas.blit(ax.bbox), what happens?

A The updated point is drawn but screen does not refresh efficiently

B The background is not saved

C The figure is closed

D The point data is not updated

💡 Hint

Refer to step 7 in execution_table about blitting effect

Concept Snapshot

Blitting redraws only changing parts of a plot.
Save static background once.
Restore background before drawing updates.
Draw updated elements.
Use canvas.blit() to refresh efficiently.
Speeds up animations and interactive plots.

Full Transcript

Blitting is a technique in matplotlib to speed up plot updates. First, we create a figure and axes, then draw static parts and save this background. When we want to update dynamic parts like a moving point, we restore the saved background to erase old drawings. Then we draw the updated elements and use blitting to refresh only the changed area on screen. This avoids redrawing the entire figure, making animations smooth and fast. The execution table shows each step from creating the plot, saving background, updating data, restoring background, drawing updated elements, and blitting to screen. Variable tracking shows how the line data changes from (0,0) to (1,1) while the background remains saved. Key moments clarify why saving and restoring background is important and how blitting improves performance. The visual quiz tests understanding of these steps and their effects.