Finding area center of rectangle

asked Jun 12 '13

Papercut
71 ●3 ●6

updated Jun 12 '13

I am trying to find an area center of various types of rectagles.

(Center of gravity and midpoint of 4 vertices never work so please think in different way)

Please see this image:

image description I have to find the position of position of red dots

Point vertices[4];
Point areaCenter;

I have to find areaCenter so (nearly)

area(areaCenter, vertices[0], vertices[1]) = area(areaCenter, vertices[1], vertices[2]) =

area(areaCenter, vertices[2], vertices[3]) = area(areaCenter, vertices[3], vertices[0])

I tried many different ways to find the mid point but none of them covered every types of rectangles.

Can any1 give me some idea?

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2 answers

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answered Jun 12 '13

unxnut

1644 ●4 ●9 ●28 http://www.cs.umsl.edu...

updated Jun 12 '13

You need to add the positions of non-zero pixels in horizontal (vertical) directions and divide them by the number of nonzero pixels to get the center of gravity.

xsum = 0;
ysum = 0;
num_nonzero_pxl = 0;
for each pixel (p(x,y))
    if pixel p(x,y) is non-zero {
        xsum += x;
        ysum += y;
        num_nonzero_pxl++;
    }
cgix=xsum/num_nonzero_pxl;
cgiy=ysum/num_nonzero_pxl;

Comments

Thanks for the answer. But I don't quite understand what you are trying to say. Can you write some lines of pseudocode please?

Papercut (Jun 12 '13)edit

that's not gonna work. Equally shaped rectangle may have different midpoint depending over its rotation.

Papercut (Jun 12 '13)edit

It should. Remember, you are adding x and y coordinates (the locations) if the pixel is nonzero and not the actual value of pixel.

unxnut (Jun 12 '13)edit

Shouldn't it be cgix=xsum/total_num_nonzero; cgiy=ysum/total_num_nonzero; ?

Papercut (Jun 12 '13)edit

And I am pretty sure the midpoint will be placed outside of rectangle in the rectangle #6 in the image as well as it takes too long.

Papercut (Jun 12 '13)edit

Yes, you are right about the denominator. It should be total_num_nonzero. My bad. Fixed the answer above.

unxnut (Jun 12 '13)edit

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answered Jun 12 '13

Hansg91
251 ●4 ●12

Perhaps this is what you are looking for :

http://en.wikipedia.org/wiki/Centroid#Centroid_of_polygon

Comments

Thanks for the link. I have implemented the Centroid of polygon in your link and even posted my code in the page. However it also has chance to be placed outside of polygon.. :(

Papercut (Jun 13 '13)edit

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