Animation basics in MATLAB - Step-by-Step Execution

Concept Flow - Animation basics

Initialize figure and plot

↓

Start loop over frames

↓

Update plot data

↓

Refresh display with drawnow

↓

Pause for animation speed

↓

Check if more frames

↓

Repeat

The animation starts by setting up a plot, then repeatedly updates the plot data inside a loop, refreshing the display each time until all frames are shown.

Execution Sample

MATLAB

x = linspace(0,2*pi,100);
fig = figure;
h = plot(x,sin(x));
for k = 1:50
    y = sin(x + k*0.1);
    set(h, 'YData', y);
    drawnow;
    pause(0.05);
end

This code animates a sine wave moving to the right by updating the plot's Y data in a loop.

Execution Table

Step	k (frame)	y (sin values)	Action	Display Update
1	1	sin(x + 0.1)	Set YData to y	Plot shows sine wave shifted by 0.1
2	2	sin(x + 0.2)	Set YData to y	Plot updates to sine wave shifted by 0.2
3	3	sin(x + 0.3)	Set YData to y	Plot updates to sine wave shifted by 0.3
...	...	...	...	...
50	50	sin(x + 5.0)	Set YData to y	Plot updates to sine wave shifted by 5.0
51	End	N/A	Loop ends	Animation stops

💡 Loop ends after 50 frames, animation complete

Variable Tracker

Variable	Start	After 1	After 2	After 3	...	After 50	Final
k	N/A	1	2	3	...	50	End
y	sin(x + 0)	sin(x + 0.1)	sin(x + 0.2)	sin(x + 0.3)	...	sin(x + 5.0)	N/A

Key Moments - 3 Insights

Why do we use 'set(h, 'YData', y)' instead of re-plotting inside the loop?

Using 'set' updates the existing plot efficiently without creating a new plot each time, as shown in the execution_table rows where only YData changes but the plot handle 'h' stays the same.

What is the role of 'drawnow' in the animation loop?

'drawnow' forces MATLAB to refresh the figure window immediately after updating YData, so the animation appears smooth, as seen in each step's Display Update.

Why do we include 'pause(0.05)' in the loop?

'pause' slows down the loop so the animation is visible to our eyes, otherwise it would run too fast to see changes, matching the timing between steps in the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what is the value of k and what action is performed?

A k=3, re-plot entire graph

B k=2, set YData to sin(x + 0.2)

C k=3, set YData to sin(x + 0.3)

D k=1, set YData to sin(x + 0.1)

💡 Hint

Check row 3 in execution_table for k and Action columns

At which step does the animation loop end according to the execution_table?

A Step 50

B Step 51

C Step 1

D Step 100

💡 Hint

Look at the exit_note and last row in execution_table

If we remove 'pause(0.05)', what would happen to the animation speed?

A Animation would run faster and might appear jumpy

B Animation would run slower

C Animation would stop

D No change in animation speed

💡 Hint

Refer to key_moments explanation about 'pause' slowing the loop

Concept Snapshot

Animation basics in MATLAB:
- Initialize figure and plot once
- Use a loop to update plot data (e.g., YData)
- Use 'set' to update plot efficiently
- Call 'drawnow' to refresh display
- Use 'pause' to control animation speed
- Loop ends after desired frames

Full Transcript

This animation example in MATLAB starts by creating a figure and plotting a sine wave. Then, inside a loop from 1 to 50, it updates the sine wave by shifting it horizontally. The plot's YData is updated using 'set', and 'drawnow' refreshes the figure so the change is visible. A short pause slows the loop so the animation can be seen clearly. The loop ends after 50 frames, completing the animation.

Practice

1. What is the main purpose of using drawnow in a MATLAB animation loop?

easy

A. To update the figure window and show the latest plot changes immediately

B. To pause the animation for a fixed time

C. To clear the figure before plotting new data

D. To set the axis limits for the plot

Solution

1. Step 1: Understand the role of drawnow

   drawnow forces MATLAB to update the figure window immediately with any changes made to the plot.

2. Step 2: Differentiate from other commands

   Unlike pause which delays execution, drawnow just refreshes the display without stopping the code.

3. Final Answer:

   To update the figure window and show the latest plot changes immediately -> Option A

4. Quick Check:

   drawnow updates display immediately [OK]

Hint: Remember: drawnow refreshes the plot instantly [OK]

Common Mistakes:

• Confusing drawnow with pause
• Thinking drawnow clears the plot
• Using drawnow to set axis limits

2. Which of the following is the correct syntax to create a simple animation loop in MATLAB?

easy

A. loop i in 1:10 { plot(i, i^2); pause(0.5); drawnow; }

B. for i = 1 to 10; plot(i, i^2); pause(0.5); drawnow; end

C. for i = 1:10; plot(i, i^2); pause(0.5); drawnow; end

D. while i <= 10; plot(i, i^2); pause(0.5); drawnow; i++; end

Solution

1. Step 1: Check MATLAB loop syntax

   MATLAB uses for variable = range syntax, so for i = 1:10 is correct.

2. Step 2: Verify loop body and commands

   The commands plot, pause, and drawnow are correctly placed and separated by semicolons.

3. Final Answer:

   for i = 1:10; plot(i, i^2); pause(0.5); drawnow; end -> Option C

4. Quick Check:

   MATLAB for-loop uses '=' and ':' [OK]

Hint: MATLAB loops use 'for var = range' syntax [OK]

Common Mistakes:

• Using 'for i = 1 to 10' which is invalid
• Using curly braces {} for loops
• Using i++ which is not MATLAB syntax

3. What will be the visible effect of running this MATLAB code?

figure;
axis([0 10 0 100]);
for x = 1:10
    y = x^2;
    plot(x, y, 'ro');
    drawnow;
    pause(0.3);
end

medium

A. Only the last red dot at (10,100) is visible

B. All red dots appear at once after the loop finishes

C. The plot window stays blank with no dots

D. Red dots appear one by one at points (1,1), (2,4), ..., (10,100)

Solution

1. Step 1: Understand the loop plotting behavior

   Each iteration plots a red circle at (x, x^2) and calls drawnow to update the figure immediately.

2. Step 2: Effect of axis limits and pause

   Axis limits prevent resizing, and pause(0.3) delays so dots appear one by one.

3. Final Answer:

   Red dots appear one by one at points (1,1), (2,4), ..., (10,100) -> Option D

4. Quick Check:

   drawnow + pause shows points sequentially [OK]

Hint: drawnow + pause shows animation step-by-step [OK]

Common Mistakes:

• Thinking all points appear at once
• Assuming plot clears previous points
• Ignoring axis limits effect

4. Identify the error in this MATLAB animation code:

for t = 1:5
    plot(t, t^2, 'bo');
    pause(0.5);
end

medium

A. Missing drawnow to update the plot window

B. Incorrect plot marker symbol 'bo'

C. Loop variable t should start from 0

D. Pause duration is too short for animation

Solution

1. Step 1: Check for plot update command

   The code plots points but does not call drawnow, so the figure may not update during the loop.

2. Step 2: Understand effect of missing drawnow

   Without drawnow, MATLAB delays updating the figure until the loop ends, so animation won't show step-by-step.

3. Final Answer:

   Missing drawnow to update the plot window -> Option A

4. Quick Check:

   Always use drawnow to refresh animation frames [OK]

Hint: Use drawnow inside loops to refresh plots [OK]

Common Mistakes:

• Thinking pause alone updates the plot
• Changing loop start without reason
• Confusing marker symbols with errors

5. You want to animate a moving sine wave in MATLAB without flickering or resizing. Which approach is best?

hard

A. Clear figure inside loop, plot new sine wave, use pause only

B. Set fixed axis limits before the loop, update plot data inside loop, use drawnow and pause

C. Plot sine wave once outside loop, then update axis limits inside loop

D. Use pause inside loop without drawnow or axis limits

Solution

1. Step 1: Prevent flickering by fixing axis limits

   Setting axis limits before the loop stops MATLAB from resizing the plot each frame, avoiding flicker.

2. Step 2: Update plot data and refresh display properly

   Update the sine wave data inside the loop, then call drawnow to refresh and pause to control speed.

3. Final Answer:

   Set fixed axis limits before the loop, update plot data inside loop, use drawnow and pause -> Option B

4. Quick Check:

   Fixed axis + drawnow + pause = smooth animation [OK]

Hint: Fix axis limits before loop to avoid flicker [OK]

Common Mistakes:

• Clearing figure each loop causing flicker
• Not fixing axis limits causing resizing
• Using pause without drawnow