Merge pull request #751 from StevenPuttemans:add_thinning_algorithm

modules/ximgproc/include/opencv2/ximgproc.hpp

@@ -104,6 +104,16 @@ CV_EXPORTS_W void niBlackThreshold( InputArray _src, OutputArray _dst,
                                     double maxValue, int type,
                                     int blockSize, double delta );
 
+/** @brief Applies a binary blob thinning operation, to achieve a skeletization of the input image.
+
+The function transforms a binary blob image into a skeletized form using the technique of Zhang-Suen.
+
+@param src Source 8-bit single-channel image, containing binary blobs, with blobs having 255 pixel values.
+@param dst Destination image of the same size and the same type as src. The function can work in-place.
+ */
+CV_EXPORTS_W void thinning( InputArray src, OutputArray dst);
+
+
 //! @}
 
 }

modules/ximgproc/src/thinning.cpp

+#include "precomp.hpp"
+
+using namespace std;
+
+namespace cv {
+namespace ximgproc {
+
+// Applies a thinning iteration to a binary image
+static void thinningIteration(Mat img, int iter){
+    Mat marker = Mat::zeros(img.size(), CV_8UC1);
+    for (int i = 1; i < img.rows-1; i++)
+    {
+        for (int j = 1; j < img.cols-1; j++)
+        {
+            uchar p2 = img.at<uchar>(i-1, j);
+            uchar p3 = img.at<uchar>(i-1, j+1);
+            uchar p4 = img.at<uchar>(i, j+1);
+            uchar p5 = img.at<uchar>(i+1, j+1);
+            uchar p6 = img.at<uchar>(i+1, j);
+            uchar p7 = img.at<uchar>(i+1, j-1);
+            uchar p8 = img.at<uchar>(i, j-1);
+            uchar p9 = img.at<uchar>(i-1, j-1);
+
+            int A  = (p2 == 0 && p3 == 1) + (p3 == 0 && p4 == 1) +
+                     (p4 == 0 && p5 == 1) + (p5 == 0 && p6 == 1) +
+                     (p6 == 0 && p7 == 1) + (p7 == 0 && p8 == 1) +
+                     (p8 == 0 && p9 == 1) + (p9 == 0 && p2 == 1);
+            int B  = p2 + p3 + p4 + p5 + p6 + p7 + p8 + p9;
+            int m1 = iter == 0 ? (p2 * p4 * p6) : (p2 * p4 * p8);
+            int m2 = iter == 0 ? (p4 * p6 * p8) : (p2 * p6 * p8);
+
+            if (A == 1 && (B >= 2 && B <= 6) && m1 == 0 && m2 == 0)
+                marker.at<uchar>(i,j) = 1;
+        }
+    }
+
+    img &= ~marker;
+}
+
+// Apply the thinning procedure to a given image
+void thinning(InputArray input, OutputArray output){
+    Mat processed = input.getMat().clone();
+    // Enforce the range of the input image to be in between 0 - 255
+    processed /= 255;
+
+    Mat prev = Mat::zeros(processed.size(), CV_8UC1);
+    Mat diff;
+
+    do {
+        thinningIteration(processed, 0);
+        thinningIteration(processed, 1);
+        absdiff(processed, prev, diff);
+        processed.copyTo(prev);
+    }
+    while (countNonZero(diff) > 0);
+
+    processed *= 255;
+
+    output.assign(processed);
+}
+
+} //namespace ximgproc
+} //namespace cv