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how to find the radius using the center point only...
the only given point is the center point...
how to get or calculate for the radius...

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Short answer: a center point alone does not give a unique radius — infinitely many circles share the same center. As pointed out, you need at least one additional constraint (a point on the circumference, a bounding box, a known diameter/scale, tangent lines, an image, etc.) to compute a radius.

If the circle is a rendered object in an image and you do have the center pixel, there are practical options. The fastest is the outward scan method (what described): walk from the center along several directions and stop at the first background/edge pixel; the distance gives an integer-radius estimate. That method is simple and fast but brittle with anti-aliasing, soft edges, or noise. More robust approaches are:

  • Preprocess (grayscale, blur, threshold or Canny edge detection).
  • Use Hough Circle Transform to detect circle parameters (center and radius) reliably.
  • For filled disks, compute a distance transform of the binary mask and read the distance value at the center for a subpixel radius estimate.

When you do have at least one boundary point, the radius is simply the Euclidean distance between the center and that point. Example (Python-style):

dx = x_edge - x_center
dy = y_edge - y_center
r = (dx*dx + dy*dy) ** 0.5

Other geometry cases: if the circle is inscribed in a known rectangle, r = min(width, height) / 2. If it must circumscribe the rectangle, r = 0.5 * sqrt(width^2 + height^2). For noisy or subpixel accuracy use many edge points and a least-squares circle fit (algebraic or geometric fit).

Cautions: watch for integer coordinates, partial/occluded circles, anti-aliasing, and image scale. For automated, robust results prefer edge detection + Hough or distance-transform + subpixel refinement rather than single-direction scans.

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Member Avatar for gusano79

Do you mean the radius of a circle? You can't get a radius from just its center point; that's not enough information. Or are you talking about something else?

Member Avatar for Reverend Jim

If you have the centre point of a rendered circle you can find the radius by scanning pixels either horizontally or vertically until you find an edge, providing the circle is filled with a uniform colour and you can detect the edge by examining the pixels. I had to write something similar for someone to auto crop a few thousand New Yorker cartoons. The cartoons were all black and white and bounded by a rectangle surrounded by whitespace.

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