Lesson 13: Advanced Graphics

Create matrix C. Display an image of the data in C. Add a colorbar to the graph to show the current colormap.

C = [0 2 4 6; 8 10 12 14; 16 18 20 22];
image(C)
colorbar

By default, the CDataMapping property for the image is set to 'direct' so image interprets values in C as indices into the colormap. For example, the bottom right pixel corresponding to the last element in C, 22, uses the 22nd color of the colormap.

Scale the values to the full range of the current colormap by setting the CDataMapping property to 'scaled' when creating the image.

image(C,'CDataMapping','scaled')
colorbar

Alternatively, you can use the imagesc function to scale the values instead of using image(C,'CDataMapping','scaled'). For example, use imagesc(C).

C = [0 2 4 6; 8 10 12 14; 16 18 20 22];
imagesc(C)
colorbar

Place the image so that it lies between 5 and 8 on the x-axis and between 3 and 6 on the y-axis.

Notice that the pixel corresponding to C(1,1) is centered over the point (5,3). The pixel corresponding to C(3,4) is centered over the point (8,6). image positions and orients the rest of the image between those two points.

Colormap Name

The following table lists the predefined colormaps.

Colormap name	Color Scale
parula
jet
hsv
hot
cool
spring
summer
autumn
winter

Colormap name	Color Scale
grey
bone
copper
pink
lines
colorcube
prism
flag
white

Three-Column Matrix

To create a custom colormap, specify map as a three-column matrix of RGB triplets where each row defines one color. An RGB triplet is a three-element row vector whose elements specify the intensities of the red, green, and blue components of the color. The intensities must be in the range [0,1]. For example, this matrix defines a colormap containing five colors.

map = [0.2 0.1 0.5
0.1 0.5 0.8
0.2 0.7 0.6
0.8 0.7 0.3
0.9 1 0];

This table lists the RGB triplet values for common colors.

Specify the number of colors used in a colormap by passing an integer as an input argument to the built-in colormap. Use five colors from the parula colormap.

mesh(peaks)
colormap(parula(5))

Create a custom colormap by defining a three-column matrix of values between 0.0 and 1.0. Each row defines a three-element RGB triplet. The first column specifies the red intensities. The second column specifies the green intensities. The third column specifies the blue intensities.

Use a colormap of blue values by setting the first two columns to zeros.

map = [0 0 0.3
0 0 0.4
0 0 0.5
0 0 0.6
0 0 0.8
0 0 1.0];
surf(peaks)
colormap(map)

Return the colormap values for a specific axes by passing its axes handle to the colormap function.

Create a figure with two subplots and return the axes handles, ax1 and ax2. Add a filled contour plot to each axes and use a different colormap for each axes.

Load the spine data set that returns the image X. Display X using the imagesc function.

Read a sample image.

A = imread('ngc6543a.jpg');

imread returns a 650-by-600-by-3 array, A.

Display the image.

image(A)

When you use image function to display a image, the MATLAB will adjust the image to fill the figure window, so that the image may appear warped. We can force the correct aspect to be displayed by using the axis command.

Display a greyscale, RGB (truecolor), indexed or binary image using imshow. MATLAB includes a TIF file, named corn.tif, that contains three images: a greyscale image, an indexed image, and a truecolor (RGB) image. This example creates a binary image from the greyscale image.

Display an image stored in a file.

imshow('peppers.png');

Read a sample indexed image, corn.tif, into the workspace, and then display it.

[X,map] = imread('corn.tif');
imshow(X,map)

Change the colormap for the image using the colormap function and specifying the target axes as the first input argument. Use the original colormap without the red component.

newmap = map;
newmap(:,1) = 0;
colormap(gca,newmap)

Find information about the example image, ngc6543a.jpg.

info = imfinfo('ngc6543a.jpg');

The info structure contains the following information fields: Filename, FileModDate, FileSize, Format, FormatVersion, Width, Height, BitDepth, ColorType, FormatSignature, NumberOfSamples, CodingMethod, CodingProcess, and Comment.

To display information from the structure, for example CodingMethod, type info.CodingMethod in the command window.

info.CodingMethod

ans =  'Huffman'

To display all the properties in the structure, type info in the command window.

info

info =  struct with fields:
Filename: 'C:\Program Files\MATLAB\R2019a\toolbox\matlab\demos\ngc6543a.jpg'
FileModDate: '01-Oct-1996 13:19:44'
FileSize: 27387
Format: 'jpg'
FormatVersion: ''
Width: 600
Height: 650
BitDepth: 24
ColorType: 'truecolor'
FormatSignature: ''
NumberOfSamples: 3
CodingMethod: 'Huffman'
CodingProcess: 'Sequential'
Comment: {'CREATOR: XV Version 3.00b Rev: 6/15/94 Quality = 75, Smoothing = 0↵'}

Write a 50×50 array of greyscale values to a PNG file in the current folder.

A = rand(50);
imagesc(A)
colormap(grey)
imwrite(A,'mygrey.png')

Write an indexed image array and its associated colormap to a PNG file.

Load sample image data from the file, clown.mat.

load clown

The image array X and its associated colormap, map, are loaded into the MATLAB workspace.

Write the data to a new PNG file.

imwrite(X,map,'myclown.png')

imwrite creates the file, myclown.png, in your current folder.

View the new file by opening it outside of MATLAB.

Write image data to a new PNG file with the built-in MATLAB colormap, copper.

Load sample image data from the file clown.mat.

load clown.mat

The image array X and its associated colormap, map, are loaded into the MATLAB workspace. map is a matrix of 81 RGB vectors.

Define a copper-tone colormap with 81 RGB vectors. Then, write the image data to a PNG file using the new colormap.

newmap = copper(81);
imwrite(X,newmap,'copperclown.png');

imwrite creates the file, copperclown.png, in your current folder.

View the new file by opening it outside of MATLA

Write multiple images to a single multipage TIFF file.

Create two sets of random image data, im1 and im2.

im1 = rand(50,40,3);
im2 = rand(50,50,3);

Write the first image to a new TIFF file. Then, append the second image to the same file.

imwrite(im1,'myMultipageFile.tif')
imwrite(im2,'myMultipageFile.tif','WriteMode','append')

Create a simple plot and assign a handle to it:

clear,close all, clc, clf

x = linspace(-10,10);
y = cos(x);
h = plot(x,y);
title('Trigonometric Function Plot')
xlabel('angle, radians'), ylabel('cos(x)')

The variable h is the handle for the plot. We can change the properties of the graphics object with either dot notation or set function.

set(h,'Color','red') % or
h.Color = 'red';

Set the background color and remove the toolbar for the current figure. Use the gcf command to get the current figure handle.

surf(peaks)
fig = gcf; % current figure handle
fig.Color = [0 0.5 0.5];
fig.ToolBar = 'none';

Plot a sine wave.

x = linspace(0,10);
y = sin(4*x);
plot(x,y)

Set the font size, tick direction, tick length, and y-axis limits for the current axes. Use gca to refer to the current axes.

ax = gca; % current axes
ax.FontSize = 12;
ax.TickDir = 'out';
ax.TickLength = [0.02 0.02];
ax.YLim = [-2 2];

Create an animation for the equation: \(y = k x^2-2\),

In the code, it redraws the graph each time through the loop, instead of creating a new figure window each time. Reassign the y data points to the plot by updating YData object from the plot (The x values stay the same).

close all; clear; clc;

x = -10:0.01:10; % Define the x-values
k = -1; % Set an initial value of k
y = k*x.^2 - 2; % Calculate the first set of y-values
h = plot(x,y); %Create the figure and assign
grid on
axis([-10,10,-100,100]) % Specify the axes
while k - 1 % Start a loop
k = k + 0.01; % Increment k
y = k*x.^2 - 2; % Recalculate y
h.YData = y; % Reassign the y
drawnow % Redraw the graph. now
end

Consider modeling a ball rolling up and over a sine function.

clear, clc

theta = linspace(-pi/2,pi/2,1000); % Define 'hill' parameters
x = linspace(0,20,1000);
y = 20 * cos(theta);
plot(x,y) % Create the plot of the sine wave
axis([-1,21,-1,21]) % Specify the axis
hold on

Next, create a circular object, which will move during the animation.

theta2 = linspace(-pi,pi); % Define parameters used to draw the circle
xc = cos(theta2);
yc = -sin(theta2);
t = patch(xc, yc, 'red'); % Create a circular object
hold off

Finally, create a loop to redraw the ball.

for j = 1:length(theta)
xnew = xc + x(j); % Define new circle position
ynew = yc + y(j);
t.XData = xnew; % update data properties
t.YData = ynew;
drawnow % Display updates
end

Draw a series of plots, capture them as images, and write them into one animated GIF file.

Plot for \(y = {x^n}\) for \(n = 3\).

x = 0:0.01:1;
n = 3;
y = x.^n;
plot(x,y,'LineWidth',3)
title(['y = x^n, n = ' num2str(n) ])

Capture a series of plots for increasing values of \(n\).

n = 1:0.5:5;
nImages = length(n);

fig = figure;
for idx = 1:nImages
y = x.^n(idx);
plot(x,y,'LineWidth',3)
title(['y = x^n, n = ' num2str( n(idx)) ])
drawnow
frame = getframe(fig);
im{idx} = frame2im(frame);
end
close;

Display the series of images in one figure.

figure;
for idx = 1:nImages
subplot(3,3,idx)
imshow(im{idx});
end

Save the nine images into a GIF file. Because three-dimensional data is not supported for GIF files, call rgb2ind to convert the RGB data in the image to an indexed image A with a colormap map. To append multiple images to the first image, call imwrite with the name-value pair argument 'WriteMode','append'.

filename = 'testAnimated.gif'; % Specify the output file name
for idx = 1:nImages
[A,map] = rgb2ind(im{idx},256);
if idx == 1
imwrite(A,map,filename,'gif','LoopCount',Inf,'DelayTime',1);
else
imwrite(A,map,filename,'gif','WriteMode','append','DelayTime',1);
end
end

imwrite writes the GIF file to your current folder. Name-value pair 'LoopCount',Inf causes the animation to continuously loop. 'DelayTime',1 specifies a 1-second delay between the display of each image in the animation.