finding number of pixels inside each circle

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Dhandapani.S
Dhandapani.S 2015-1-21
评论： Dhandapani.S 2015-2-1
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
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Dhandapani.S
Dhandapani.S 2015-1-21
编辑：Image Analyst 2015-1-26
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.

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Image Analyst
Image Analyst 2015-1-26
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?
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Image Analyst
Image Analyst 2015-1-26
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
Dhandapani.S 2015-2-1
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.

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Thorsten
Thorsten 2015-1-21
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.
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Dhandapani.S
Dhandapani.S 2015-1-21
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
Thorsten 2015-1-26
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?

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