Question

This is a further question based on this answer:

How can I implement a fisheye lens effect (barrel transformation) in MATLAB?

The general solution should work for all background colors and length/width ratios.

Answer 1

As is often the case, there are a number of different ways to do this in MATLAB. I’ll list a few examples for padding RGB images…

Solution #1: Add padding with CAT to make a square image

This solution takes a given color padColor and replicates it using the function REPMAT to create padding of the right size, shape, and color. The padding is then added to the sides of the image using the function CAT:

[r,c,d] = size(rgbImage);  %# Get the image dimensions
nPad = abs(c-r)/2;         %# The padding size
padColor = [1 1 1];        %# RGB triple for pad color (white)
padColor = reshape(padColor,1,1,3);  %# Reshape pad color to 1-by-1-by-3
if c > r                   %# Pad rows
  newImage = cat(1,repmat(padColor,floor(nPad),c),...  %# Top padding
                   rgbImage,...                        %# Image
                   repmat(padColor,ceil(nPad),c));     %# Bottom padding
elseif r > c               %# Pad columns
  newImage = cat(2,repmat(padColor,r,floor(nPad)),...  %# Left padding
                   rgbImage,...                        %# Image
                   repmat(padColor,r,ceil(nPad)));     %# Right padding
end

You can modify the above solution to work for indexed, grayscale, or binary images by replacing the two lines defining the padColor with one of the following:

padColor = uint8(1);    %# For an indexed image (index of color to use)
padColor = uint8(255);  %# For a grayscale image (white)
padColor = true;        %# For a binary image (white)

Solution #2: Make a blank square image and insert the original image

This solution takes a given color padColor and replicates it using the function REPMAT to create a blank square image of that color. The original image is then inserted into this blank image in a centered position:

[r,c,d] = size(rgbImage);  %# Get the image dimensions
padColor = [1 1 1];        %# RGB triple for pad color (white)
padColor = reshape(padColor,1,1,3);  %# Reshape pad color to 1-by-1-by-3
if c > r                   %# Pad rows
  newImage = repmat(padColor,c);  %# Make c-by-c-by-3 matrix of given color
  rowIndex = floor((c-r)/2);      %# Row index for inserting image
  newImage(rowIndex+(1:r),:,:) = rgbImage;     %# Insert the image
elseif r > c               %# Pad columns
  newImage = repmat(padColor,r);  %# Make r-by-r-by-3 matrix of given color
  columnIndex = floor((r-c)/2);   %# Column index for inserting image
  newImage(:,columnIndex+(1:c),:) = rgbImage;  %# Insert the image
end

You can modify the above solution to work for indexed, grayscale, or binary images by replacing the two lines defining the padColor with one of the following:

padColor = uint8(1);    %# For an indexed image (index of color to use)
padColor = uint8(255);  %# For a grayscale image (white)
padColor = true;        %# For a binary image (white)

Solution #3: Use PADARRAY

This solution uses the function PADARRAY to create the padding to make the image square. Unfortunately, there’s no easy way to specify the padding color you want for RGB images when using this solution (see below). However, you can use the 'replicate' argument to have PADARRAY simply replicate the color at the edges of the image where it is adding the padding:

[r,c,d] = size(rgbImage);  %# Get the image dimensions
nPad = abs(c-r)/2;         %# The padding size
if c > r                   %# Pad rows
  newImage = padarray(rgbImage,[floor(nPad) 0],...  %# Pad top
                      'replicate','pre');
  newImage = padarray(newImage,[ceil(nPad) 0],...   %# Pad bottom
                      'replicate','post');
elseif r > c               %# Pad columns
  newImage = padarray(rgbImage,[0 floor(nPad)],...  %# Pad left
                      'replicate','pre');
  newImage = padarray(newImage,[0 ceil(nPad)],...   %# Pad right
                      'replicate','post');
end

This solution will work for indexed, grayscale, or binary images. For these three image types, you have the option to replace the 'replicate' argument with a scalar value that you want to use for the padding (i.e. uint8(255) for white padding in a grayscale image). For RGB images, replacing the 'replicate' argument with a single value will only allow you to create padding colors that are gray shades ranging from white to black (i.e. 1 creates white padding).