finding number of pixels inside each circle

i am drawing circles of different radius on a jpg image. how to find the number of pixels in each circle. i have to compare the pixel distribution ratio in each circle

[Mohammad Abouali]

What's it with counting points in circle? Past few weeks this question has been asked several times by different people! I think we can well say that this question is trending on MATHWORKS :D

[Image Analyst]

What is a "pixel distribution ratio"?

[Dhandapani.S]

say, for inner most circle, there may be 20% of the total pixels of the jpeg image. the next immediate outer circle may contain 40 % of the total jpeg image pixels. this is a kind of segmentation i think. please give me a solution. thank you in advance.

Answer 1

[Image Analyst]

I'm pretty sure I gave you the answer in your other question: http://www.mathworks.com/matlabcentral/answers/169093#comment_261946 How is this any different than that?

[Dhandapani.S]

No the same.thank you. i asked this on 21st jan. the problem was solved with circles. thank you. now i am trying to solve the same with triangles and wanted to find which will be the best circle or traingle?

[Image Analyst]

Just try it and see. Create a mask for triangles with poly2mask().

xTriangle = [x1,x2,x3,x1]; % Make a list of the coordinates of the vertices.
yTriangle = [y1,y2,y3,y1];
[rows, columns, numberOfColorChannels] = size(yourImage);
binaryImage = poly2mask(xTriangle, yTriangle, rows, columns);

[Dhandapani.S]

dear image analyst, you have made the work with concentric circles and i got good results. i am trying with triangles, rectangles and spheres.but cannot accomplish. iget the error with bwboundaries. since bwboundaries is 2d i cannot make it up to sphere. please help me in rectangle and triangle.

Answer 2

[Thorsten]

A well-known "approximation" is :-)

 N = round(radius^2*pi)

You could also create a circle and count the pixels:

 radius = 100;
 x = [-radius: radius];
 [X, Y] = meshgrid(x, x);
 R = sqrt(X.^2 + Y.^2);
 N = numel(find(R<=radius));

Both values are almost the same.

[Dhandapani.S]

thank you. i get 31417 for N. i have a jpeg image. i have to draw circles with different radius keeping the center point of the jpeg image as the circle center. i need to count the pixels in each circle and find the pixel ratio of the jpeg image in each circle. please help me.

[Thorsten]

You can just use any other value for the radius, like

 radius = 50;

and then compute the N to count the pixels in the circle of radius 50. Or what else do you need?