Merge pull request #3743 from vrabaud:thinning

Misc thinning fixes. #3743

- edges could be modified
- 2 sets of iterations were always done even if the first set did not return any change
- countNonZero is slow compared to hasNonZero

### Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [x] There is a reference to the original bug report and related work
- [x] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.

modules/ximgproc/src/thinning.cpp

@@ -96,6 +96,10 @@ static void thinningIteration(Mat img, int iter, int thinningType){
     Mat marker = Mat::zeros(img.size(), CV_8UC1);
     int rows = img.rows;
     int cols = img.cols;
+    marker.col(0).setTo(1);
+    marker.col(cols - 1).setTo(1);
+    marker.row(0).setTo(1);
+    marker.row(rows - 1).setTo(1);
 
     if(thinningType == THINNING_ZHANGSUEN){
         marker.forEach<uchar>([=](uchar& value, const int postion[]) {
@@ -133,6 +137,7 @@ static void thinningIteration(Mat img, int iter, int thinningType){
             //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) value = 0;
+            // else value = 1;
         });
     }
     if(thinningType == THINNING_GUOHALL){
@@ -170,6 +175,7 @@ static void thinningIteration(Mat img, int iter, int thinningType){
             //int N  = N1 < N2 ? N1 : N2;
             //int m  = iter == 0 ? ((p6 | p7 | (!p9)) & p8) : ((p2 | p3 | (!p5)) & p4);
             //if ((C == 1) && ((N >= 2) && ((N <= 3)) & (m == 0))) value = 0;
+            // else value = 1;
         });
     }
 
@@ -183,16 +189,17 @@ void thinning(InputArray input, OutputArray output, int thinningType){
     // Enforce the range of the input image to be in between 0 - 255
     processed /= 255;
 
-    Mat prev = Mat::zeros(processed.size(), CV_8UC1);
+    Mat prev = processed.clone();
     Mat diff;
 
     do {
         thinningIteration(processed, 0, thinningType);
         thinningIteration(processed, 1, thinningType);
         absdiff(processed, prev, diff);
+        if (!hasNonZero(diff)) break;
         processed.copyTo(prev);
     }
-    while (countNonZero(diff) > 0);
+    while (true);
 
     processed *= 255;
 

modules/ximgproc/test/test_thinning.cpp

@@ -6,9 +6,12 @@
 
 namespace opencv_test { namespace {
 
-static int createTestImage(Mat& src)
+static int createTestImage(Mat1b& src)
 {
-    src = Mat::zeros(Size(256, 256), CV_8UC1);
+    src = Mat1b::zeros(Size(256, 256));
+    // Create a corner point that should not be affected.
+    src(0, 0) = 255;
+
     for (int x = 50; x < src.cols - 50; x += 50)
     {
         cv::circle(src, Point(x, x/2), 30 + x/2, Scalar(255), 5);
@@ -20,13 +23,14 @@ static int createTestImage(Mat& src)
 
 TEST(ximgproc_Thinning, simple_ZHANGSUEN)
 {
-    Mat src;
+    Mat1b src;
     int src_pixels = createTestImage(src);
 
-    Mat dst;
+    Mat1b dst;
     thinning(src, dst, THINNING_ZHANGSUEN);
     int dst_pixels = countNonZero(dst);
     EXPECT_LE(dst_pixels, src_pixels);
+    EXPECT_EQ(dst(0, 0), 255);
 
 #if 0
     imshow("src", src); imshow("dst", dst); waitKey();
@@ -35,13 +39,14 @@ TEST(ximgproc_Thinning, simple_ZHANGSUEN)
 
 TEST(ximgproc_Thinning, simple_GUOHALL)
 {
-    Mat src;
+    Mat1b src;
     int src_pixels = createTestImage(src);
 
-    Mat dst;
+    Mat1b dst;
     thinning(src, dst, THINNING_GUOHALL);
     int dst_pixels = countNonZero(dst);
     EXPECT_LE(dst_pixels, src_pixels);
+    EXPECT_EQ(dst(0, 0), 255);
 
 #if 0
     imshow("src", src); imshow("dst", dst); waitKey();