diff --git a/doc/tutorials/imgproc/imgtrans/hough_circle/hough_circle.rst b/doc/tutorials/imgproc/imgtrans/hough_circle/hough_circle.rst index feb3bf1cf4..43f7cd0d74 100644 --- a/doc/tutorials/imgproc/imgtrans/hough_circle/hough_circle.rst +++ b/doc/tutorials/imgproc/imgtrans/hough_circle/hough_circle.rst @@ -7,7 +7,29 @@ Goal ===== In this tutorial you will learn how to: -* Use the OpenCV functions :hough_circles:`HoughCircles <>` to detect circles in an image. +* Use the OpenCV function :hough_circles:`HoughCircles <>` to detect circles in an image. + +Theory +======= + +Hough Circle Transform +------------------------ + +* The Hough Circle Transform works in a *roughly* analogous way to the Hough Line Transform explained in the previous tutorial. +* In the line detection case, a line was defined by two parameters :math:`(r, \theta)`. In the circle case, we need three parameters to define a circle: + + .. math:: + + C : ( x_{center}, y_{center}, r ) + + where :math:`(x_{center}, y_{center})` define the center position (gree point) and :math:`r` is the radius, which allows us to completely define a circle, as it can be seen below: + + .. image:: images/Hough_Circle_Tutorial_Theory_0.jpg + :alt: Result of detecting circles with Hough Transform + :height: 200pt + :align: center + +* For sake of efficiency, OpenCV implements a detection method slightly trickier than the standard Hough Transform: *The Hough gradient method*. For more details, please check the book *Learning OpenCV* or your favorite Computer Vision bibliography Code ====== @@ -70,9 +92,87 @@ Code return 0; } + +Explanation +============ + + +#. Load an image + + .. code-block:: cpp + + src = imread( argv[1], 1 ); + + if( !src.data ) + { return -1; } + +#. Convert it to grayscale: + + .. code-block:: cpp + + cvtColor( src, src_gray, CV_BGR2GRAY ); + +#. Apply a Gaussian blur to reduce noise and avoid false circle detection: + + .. code-block:: cpp + + GaussianBlur( src_gray, src_gray, Size(9, 9), 2, 2 ); + +#. Proceed to apply Hough Circle Transform: + + .. code-block:: cpp + + vector circles; + + HoughCircles( src_gray, circles, CV_HOUGH_GRADIENT, 1, src_gray.rows/8, 200, 100, 0, 0 ); + + with the arguments: + + * *src_gray*: Input image (grayscale) + * *circles*: A vector that stores sets of 3 values: :math:`x_{c}, y_{c}, r` for each detected circle. + * *CV_HOUGH_GRADIENT*: Define the detection method. Currently this is the only one available in OpenCV + * *dp = 1*: The inverse ratio of resolution + * *min_dist = src_gray.rows/8*: Minimum distance between detected centers + * *param_1 = 200*: Upper threshold for the internal Canny edge detector + * *param_2* = 100*: Threshold for center detection. + * *min_radius = 0*: Minimum radio to be detected. If unknown, put zero as default. + * *max_radius = 0*: Maximum radius to be detected. If unknown, put zero as default + +#. Draw the detected circles: + + .. code-block:: cpp + + for( size_t i = 0; i < circles.size(); i++ ) + { + Point center(cvRound(circles[i][0]), cvRound(circles[i][1])); + int radius = cvRound(circles[i][2]); + // circle center + circle( src, center, 3, Scalar(0,255,0), -1, 8, 0 ); + // circle outline + circle( src, center, radius, Scalar(0,0,255), 3, 8, 0 ); + } + + You can see that we will draw the circle(s) on red and the center(s) with a small green dot + +#. Display the detected circle(s): + + .. code-block:: cpp + + namedWindow( "Hough Circle Transform Demo", CV_WINDOW_AUTOSIZE ); + imshow( "Hough Circle Transform Demo", src ); + +#. Wait for the user to exit the program + + .. code-block:: cpp + + waitKey(0); + + Result ======= - + +The result of running the code above with a test image is shown below: + .. image:: images/Hough_Circle_Tutorial_Result.jpg :alt: Result of detecting circles with Hough Transform :align: center diff --git a/doc/tutorials/imgproc/imgtrans/hough_circle/images/Hough_Circle_Tutorial_Theory_0.jpg b/doc/tutorials/imgproc/imgtrans/hough_circle/images/Hough_Circle_Tutorial_Theory_0.jpg new file mode 100644 index 0000000000..8b729ca2fd Binary files /dev/null and b/doc/tutorials/imgproc/imgtrans/hough_circle/images/Hough_Circle_Tutorial_Theory_0.jpg differ