Question

I’m very new to matlab so sorry if this is a dumb question. I have to following matrices:

im = imread('image.jpg');  %<370x366 double>
[y,x] = find(im); %x & y both <1280x1 double>
theta;  %<370x366 double> computed from gradient of image

I can currently plot points one at a time like this:

plot(x(502) + 120*cos(theta(y(502),x(502))),y(502) + 120*sin(theta(y(502),x(502))));

But what I want to do is some how increment an accumulator array, I want to increment the location of acc by 1 for every time value for that location is found.

So if x(502) + 120*cos(theta(y(502),x(502))),y(502) + 120*sin(theta(y(502),x(502)) = (10,10) then acc(10,10) should be incremented by 1. I’m working with a very large data set so I want to avoid for-loops and use something like this:

acc = zeros(size(im));
%increment value when point lands in that location
acc(y,x) = acc(x + 120*cos(theta(y,x)),y + 120*sin(theta(y,x)),'*b')) + 1;

It would be nice if the 120 could actually be another matrix containing different radius values as well.

Answer 1

Do

i = find(im);

instead of

[y,x] = find(im)

wthis will give you linear indice of non zero values

Also, create a meshgrid

[x,y] = meshgrid(1:366,1:370)

Now you can index both coordinated and values linearly, for example

x(520) is 520-th point x coordinate

im(520) is 520-th point gray value

theta(520) is 520-th gradient value

So, now you can just plot it:

plot(x(i) + 120*cos(theta(i)),y(i) + 120*sin(theta(i)),'b*');

x(i) means a column of i-th values

x(i) + 120*cos(theta(i)) means a column of results

ACCUMULATING

I think in this case it is ok to loop for accumulating:

acc=zeros(size(im));
for ii=1:length(i)
    xx=round(x(ii) + 120*cos(theta(ii)));
    yy=round(y(ii) + 120*sin(theta(ii)));
    acc(xx,yy)=acc(xx,yy)+1;
end;