Resizing an Image in MATLAB

Question

I\'m trying to create a function the scales an image based on a value (scale_zoom) for a homework assignment. I don\'t want to use the MATLAB built in function resize()

Answer 1

I had previously answered a question about scaling images using nearest-neighbor interpolation, and much of the code and explanations I used there apply here. The main difference is the final interpolation step. Here's how you can write your function, using INTERP2 and generalizing for a 2-D grayscale or 3-D RGB image of any type:

function pic_new = scale_image(pic,scale_zoom)

  oldSize = size(pic);                               %# Old image size
  newSize = max(floor(scale_zoom.*oldSize(1:2)),1);  %# New image size
  newX = ((1:newSize(2))-0.5)./scale_zoom+0.5;  %# New image pixel X coordinates
  newY = ((1:newSize(1))-0.5)./scale_zoom+0.5;  %# New image pixel Y coordinates
  oldClass = class(pic);  %# Original image type
  pic = double(pic);      %# Convert image to double precision for interpolation

  if numel(oldSize) == 2  %# Interpolate grayscale image

    pic_new = interp2(pic,newX,newY(:),'cubic');

  else                    %# Interpolate RGB image

    pic_new = zeros([newSize 3]);  %# Initialize new image
    pic_new(:,:,1) = interp2(pic(:,:,1),newX,newY(:),'cubic');  %# Red plane
    pic_new(:,:,2) = interp2(pic(:,:,2),newX,newY(:),'cubic');  %# Green plane
    pic_new(:,:,3) = interp2(pic(:,:,3),newX,newY(:),'cubic');  %# Blue plane

  end

  pic_new = cast(pic_new,oldClass);  %# Convert back to original image type

end

And you can test it as follows:

img = imread('peppers.png');      %# Load the sample peppers image
newImage = scale_image(img,0.3);  %# Scale it to 30%