Question

I have an intensity/greyscale image, and I have chosen a pixel inside this image. I want to send vectors starting from this pixel in all directions/angles, and I want to sum all the intensities of the pixels touching one vector, for all vectors.

After this step I would like to plot a histogram with the intensities on one axis and the angle on the other axis. I think I can do this last step on my own, but I don’t know how to create these vectors inside my greyscale image and how to get the coordinates of the pixels a vector touches.

I previously did this in C++, which required a lot of code. I am sure this can be done with less effort in MATLAB, but I am quite new to MATLAB, so any help would be appreciated, since I haven’t found anything helpful in the documentation.

Answer 1

It might not be the best way to solve it, but you can do it using a bit of algebra, heres how…
We know the Point-Slope formula of a line passing through point (a,b) with angle theta is:

y = tan(theta) * (x-a) + b

Therefore a simple idea is to compute the intersection of this line with y=const for all const, and read the intensity values at the intersection. You would repeat this for all angles…
A sample code to illustrate the concept:

%% input
point = [128 128];               % pixel location
I = imread('cameraman.tif');     % sample grayscale image

%% calculations
[r c] = size(I);
angles = linspace(0, 2*pi, 4) + rand;
angles(end) = [];
clr = lines( length(angles) );   % get some colors

figure(1), imshow(I), hold on
figure(2), hold on

for i=1:length(angles)
    % line equation
    f = @(x) tan(angles(i))*(x-point(1)) + point(2);

    % get intensities along line
    x = 1:c;
    y = round(f(x));
    idx = ( y<1 | y>r );        % indices of outside intersections
    vals = diag(I(x(~idx), y(~idx)));

    figure(1), plot(x, y, 'Color', clr(i,:))    % plot line
    figure(2), plot(vals, 'Color', clr(i,:))    % plot profile
end
hold off