how to build an accumulator array in matlab - matlab

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;

Answer 2

The factor (120 in the example) can be matrix of size of im or scalar. The .* will do it.
im = imread('image.jpg');
[y, x] = meshgrid(1 : size(im, 1), 1 : size(im, 2));
factor = 120 * ones(size(im));
theta = gradient(double(image)); % just for example
acc = zeros(size(im));
increment = ((x + factor .* cos(theta)) == 10) & ((y + factor .* sin(theta)) == 10);
acc = acc + increment;
But the comparison to 10 will rarely be true, so you need to allow some range. For example (9,11).
e = 1;
increment = abs((x + factor .* cos(theta)) - 10) < e & abs((y + factor .* sin(theta)) - 10) < e;