title: Fill circle with gradient
body: <p>I want fill circle with gradient color, like I show on bottom. I can't figure out easy way, how to do that.
  I can make more circles, but transitions are visible.</p>
  
  <pre><code>cv::circle(img, center, circle_radius * 1.5, cv::Scalar(1.0, 1.0, 0.3), CV_FILLED);
  cv::circle(img, center, circle_radius * 1.2, cv::Scalar(1.0, 1.0, 0.6), CV_FILLED);
  cv::circle(img, center, circle_radius, cv::Scalar(1.0, 1.0, 1.0), CV_FILLED);
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/g7neN.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/g7neN.png" alt="enter image description here"></a></p>