Image manipulation in matlab

Question

I am a very new programmer (this is my second exercise), and I have been asked to shift an image (of a Galaxy) by a sinusoidal shift. I have tried to write a programme to do this as follows:

My programme:

GalaxyImage = imread('hs-2013-06-a-web.jpg');
image(GalaxyImage)
GalaxyImage=double(GalaxyImage)/255;
image(Sess2GalaxyIm(GalaxyImage));

My function:

function [GalaxySlanted] = Sess2GalaxyIm(GalaxyImage)
    A = 20;
    k = 3;
    Y = size(GalaxyImage, 1);
    X = size(GalaxyImage, 2);
    max_shift = ceil(A*2);
    GalaxySlanted = zeros(Y, X+max_shift, 3);
    for y=1:Y
        local_shift = ceil(A*sind(k*y)); 
        local_x = 1:X;
        local_x = local_x + local_shift;
        GalaxySlanted(y, local_x, :)=GalaxyImage(y, :, :);
    end
end

At the moment, when I run it, it seems to just be shifting my image by a constant amount and I don't see why.

Answer 1

There is a possible issue with how you define local_x. You start by adding some padding onto the image to compensate for the fact that the output x could be up to "A" pixels shifted on either side:

max_shift = ceil(A*2);
GalaxySlanted = zeros(Y, X+max_shift, 3);

However, in the loop, you ignore this and only start off with local_x as 1:X - so if the output from sind is negative you will be trying to index negative numbers into GalaxySlanted and get a resulting error. Changing that line to local_x = (1:X)+A; should work (with oMiD's modification of the shift).

Another quick note - there is actually a function im2double which will handle the image conversion, and scaling, in one go.

Answer 2

Your image's shift is constant because in local_shift = ceil(A*sind(k*Y)); you used constant values for A,K and Y. may be this modification makes your code correct:(y instead of Y)

local_shift = ceil(A*sind(k*y));