count of points in particular region from graph
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graph is plot of scattered points , graph is also divided into grids of size 64*64. i need the count of number of points in each grid .
if a 32*32 (-1024 to 1024)matrix is taken such that each grid is represented by a value(count ) in matrix . any help is highly appretiated thanks
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Accepted Answer
DGM
on 6 May 2021
Edited: DGM
on 6 May 2021
Look at histcounts2():
% make some scattered points
npoints = 1000;
x = randn(npoints,1)*500;
y = randn(npoints,1)*300;
% plot them (just for sake of demonstration)
scatter(x,y); grid on;
xlim([-1024 1024])
ylim([-1024 1024])
% define bin edges and find bin counts
edx = -1024:64:1024;
edy = -1024:64:1024;
counts = histcounts2(x,y,edx,edy).';
Play around with it with some asymmetric data so you understand the orientation of the output. You may find it more intuitive to transpose the counts array like I did here.
If you want to visualize the results:
imshow(counts,[])
(scaled up, obviously)
2 Comments
DGM
on 6 May 2021
Edited: DGM
on 6 May 2021
Something like this.
% find sums of annular rings of counts
s = size(counts);
maxr = min(s(1:2))/2;
annularcounts = zeros(maxr,1);
corners = [s(1) s(2)]+1;
for a = 1:floor(maxr)
corners = corners-1;
l = corners-a+1;
A = counts(a:corners(1),a);
B = counts(corners(1),a:corners(2)).';
C = counts(corners(1):-1:a,corners(2));
D = counts(a,corners(2):-1:a).';
annularcounts(a) = sum(cat(1,A,B,C,D));
end
plot(maxr:-1:1,annularcounts); grid on
xlabel('radius from center of histogram')
ylabel('total count in annulus')
Which kind of makes sense. The peak density is at the center, but the area is also the smallest.
More Answers (1)
KSSV
on 6 May 2021
You can proceed like below demo example:
clc; clear all ;
a = 1; b = 10 ;
x = (b-a).*rand(1000,1) + a;
y = (b-a).*rand(1000,1) + a;
[X,Y] = meshgrid(a:b,a:b) ;
plot(x,y,'.r')
hold on
plot(X,Y,'k',X',Y','k')
% number of points lying inside box (1,1) and (2,2)
idx = (x > 1) & (x <= 2) ;
idy = (y > 1) & (y <= 2) ;
id = idx & idy ;
plot(x(id),y(id),'ob')
fprintf('The number of points lying inside box (1,1) and (2,2) are %d\n',nnz(id))
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