Image Remapping

In this demo, we show how to use the OpenCV function cv.remap to implement simple remapping routines.

Sources:

Contents

Theory

Remapping is the process of taking pixels from one place in the image and locating them in another position in a new image.

To accomplish the mapping process, it might be necessary to do some interpolation for non-integer pixel locations, since there will not always be a one-to-one-pixel correspondence between source and destination images.

We can express the remap for every pixel location as:

where is the remapped image, the source image and is the mapping function that operates on .

Let's think in a quick example. Imagine that we have an image and, say we want to do a remap such that:

What would happen? It is easily seen that the image would flip in the direction. For instance, consider the input image:

observe how the red circle changes positions with respect to x (considering the horizontal direction):

In OpenCV, the function cv.remap offers a simple remapping implementation.

Code

This program:

load input image

src = imread(fullfile(mexopencv.root(), 'test', 'tsukuba.png'));
imshow(src), title('Source')

prepare for building mapping matrices (for x and y coordinates), we are going to perform 4 different mappings

[rows,cols,~] = size(src);
[X,Y] = meshgrid((1:cols)-1, (1:rows)-1);  % 0-based indices
map = struct('x',[], 'y',[]);

1) reduce the picture to half its size and center it in the middle

map(1).x = zeros(rows,cols);
map(1).y = zeros(rows,cols);
idx = (X > cols*0.25 & X < cols*0.75 & Y > rows*0.25 & Y < rows*0.75);
map(1).x(idx) = 2*(X(idx) - cols*0.25) + 0.5;
map(1).y(idx) = 2*(Y(idx) - rows*0.25) + 0.5;

2) turn the image upside down (flip-y)

map(2).x = X;
map(2).y = rows - Y;

3) reflect the image from left to right (flip-x)

map(3).x = cols - X;
map(3).y = Y;

4) combination of 3 and 4 (flip-xy)

map(4).x = cols - X;
map(4).y = rows - Y;

apply mapping matrices to source image, and show results

pos = get(0, 'DefaultFigurePosition');
figure('Position',pos.*[0.5 0.5 1.3 1.3])
for k=1:4
    dst = cv.remap(src, map(k).x, map(k).y);
    subplot(2,2,k), imshow(dst), title(sprintf('map(%d)',k))
end

show mapping matrices

figure('Position',pos.*[0.5 1 2 1])
for k=1:4
    subplot(2,4,k), imshow(map(k).x, []), title(sprintf('map(%d).X',k))
    subplot(2,4,k+4), imshow(map(k).y, []), title(sprintf('map(%d).Y',k))
end