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() in this function so I'm trying to interpolate. Any help would be greatly appreciated. This is what I have so far:

function pic_new=scale_image(pic,scale_zoom)
    [row, col]=size(pic)
    ht_scale=size(pic,1)/scale_zoom*col
    wid_scale=size(pic,2)/scale_zoom*row

    size(ht_scale)
    size(wid_scale)
    x=(0:scale_zoom)*wid_scale
    y=(0:scale_zoom)*ht_scale
    length(x)
    length(y)
    %plotvals=0:0.1:scale_zoom (this is not necessary i think)
    newimg=interp1(pic,x,y,'cubic')
    image(newimg)
end

I think I'm interpolating it very incorrectly :/

I assume you mean that you don't want to use `imresize`, even though you wrote `resize`. Because I'd use `imresize` for this. — Chris A., May 31 '11 at 15:10

score 7 · Answer 1 · edited May 23 '17 at 12:33

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%

Resizing an Image in MATLAB

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