Image as matrix data in Matlab

Matlab allow us to load digital images and to manipulate the numbers associated with each pixel (color images RGB or gray images).

Usually color data are uint8 variables, therefore it is convenient to convert these numbers to double if products or divisions are involved in the data manipulation.

Load and show an image

The Matlab functions imread and imshow allow us load images to Matlab

imRGB = imread('greens.jpg'); % a color image

imshow(imRGB)

figure() %to show in another window

imGray = imread('coins.png'); % a gray image

imshow(imGray)

%make a copy for later use

imRGBOrig= imRGB;

imGrayOrig= imGray;

Manipulating an image

If you have already load an image you can acces the pixel values. To know its size you have to take into account 3 dimensions: witdh x height x depth. Being depth 1 or 3 according to color or gray images.

[m,n,p]=size(imRGB);

[mm,nn,pp]=size(imGray);

% access to color channel values for pixell 121x121

p121RGB = imRGB(121,121,:);

fprintf('colors RGB = %d %d %d \n', p121RGB); %print in the command window

colors RGB = 23 46 28

p121Gray = imGray(121,121,:);

fprintf('color Gray = %d \n', p121Gray); %print in the command window

color Gray = 68

% modify a pixel value (to red)

imRGB(121,121,1) = 255; %assign max red color channel

imRGB(121,121,2) = 0; %assign min green color channel

imRGB(121,121,3) = 0; %assign min blue color channel

% if you have only a channel (to white)

imGray(121,121,1) = 255; %assign max red color channel

% We can also modify a set of pixels like a matrix

imRGB(140:220,140:220,:)=255; %white square

imshow(imRGB)

Manipulate according color level

We will change the red pixels (until a certain level) to black. All three channels are explored before changing the color.

imRGB = imRGBOrig; % recover original values
for i=1:m
    for j=1:n
        if ( (imRGBOrig(i,j,1)> 100) && (imRGBOrig(i,j,2) < 80) && (imRGBOrig(i,j,3) < 80))
            imRGB(i,j,:)=[0,0,0];
        end
    end
end
figure()
imshow(imRGB);