Colormap and colorbar in MATLAB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this colormap and colorbar code?

Consider the following MATLAB code snippet:

imagesc(peaks(10));
colormap(jet);
colorbar;

What will the colorbar represent?

MATLAB

imagesc(peaks(10));
colormap(jet);
colorbar;

A A colorbar showing a gradient from blue to red representing the data values

B A colorbar showing a gradient from green to yellow representing the data values

C A colorbar with discrete colors only, no gradient

D No colorbar will be displayed

💡 Hint

Think about what the 'jet' colormap looks like and what colorbar does.

The 'jet' colormap in MATLAB creates a smooth gradient from blue to red. The colorbar shows this gradient representing the range of data values in the image.

❓ Predict Output

intermediate

What is the effect of changing colormap to 'hot'?

Given this MATLAB code:

imagesc(magic(5));
colormap(hot);
colorbar;

What color range will the colorbar display?

MATLAB

imagesc(magic(5));
colormap(hot);
colorbar;

A A gradient from purple to pink to white

B A gradient from blue to green to yellow

C A gradient from black through red and yellow to white

D A grayscale gradient from black to white

💡 Hint

Recall the colors in the 'hot' colormap.

The 'hot' colormap starts with black, then red, then yellow, and finally white for the highest values.

❓ Predict Output

advanced

What error occurs with this colormap command?

What error will this MATLAB code produce?

imagesc(rand(5));
colormap('unknownmap');
colorbar;

MATLAB

imagesc(rand(5));
colormap('unknownmap');
colorbar;

A Error: Undefined function or variable 'unknownmap'.

B No error, displays default colormap

C Warning: Colormap ignored, colorbar not shown

D Error: Invalid syntax in colormap function

💡 Hint

Check if 'unknownmap' is a valid colormap name.

MATLAB throws an error because 'unknownmap' is not a recognized colormap name or variable.

❓ Predict Output

advanced

What is the number of colors in this colormap?

Consider this MATLAB code:

imagesc(peaks(20));
colormap(parula(10));
colorbar;

How many distinct colors does the colormap have?

MATLAB

imagesc(peaks(20));
colormap(parula(10));
colorbar;

A 20

B 256

C 64

D 10

💡 Hint

Look at the argument passed to parula.

The number inside parula() specifies how many colors the colormap contains. Here it is 10.

🧠 Conceptual

expert

How does colorbar scale relate to data range?

In MATLAB, if you use:

imagesc(data);
colormap(jet);
colorbar;

and then change the color limits with:

caxis([10 20]);

What effect does this have on the colorbar and image colors?

MATLAB

imagesc(data);
colormap(jet);
colorbar;
caxis([10 20]);

A The colorbar scale remains unchanged; image colors do not change

B The colorbar scale adjusts to show colors only for data values between 10 and 20; values outside are clipped

C The colorbar disappears because caxis limits are set manually

D The image colors invert, showing low values as high colors

💡 Hint

Think about what caxis does to color scaling.

The caxis function sets the data value limits for color mapping. Values below 10 map to the lowest color, above 20 to the highest color, and the colorbar reflects this range.