Canny lower threshold

modules/core/include/visp3/core/vpImageFilter.h

@@ -72,6 +72,9 @@ class VISP_EXPORT vpImageFilter
   static void canny(const vpImage<unsigned char> &I, vpImage<unsigned char> &Ic, unsigned int gaussianFilterSize,
                     float thresholdCanny, unsigned int apertureSobel);
 
+  static void canny(const vpImage<unsigned char> &I, vpImage<unsigned char> &Ic, unsigned int gaussianFilterSize,
+                    float lowerThresholdCanny, float higherThresholdCanny, unsigned int apertureSobel);
+
   /*!
     Apply a 1x3 derivative filter to an image pixel.
 

modules/core/src/image/vpImageFilter.cpp

@@ -297,24 +297,83 @@ int main()
 */
 void vpImageFilter::canny(const vpImage<unsigned char> &Isrc, vpImage<unsigned char> &Ires,
                           unsigned int gaussianFilterSize, float thresholdCanny, unsigned int apertureSobel)
+{
+  vpImageFilter::canny(Isrc, Ires, gaussianFilterSize, thresholdCanny / 3.f, thresholdCanny, apertureSobel);
+}
+
+/*!
+  Apply the Canny edge operator on the image \e Isrc and return the resulting
+  image \e Ires.
+
+  The following example shows how to use the method:
+
+  \code
+#include <visp3/core/vpImage.h>
+#include <visp3/core/vpImageFilter.h>
+
+int main()
+{
+  // Constants for the Canny operator.
+  const unsigned int gaussianFilterSize = 5;
+  const float upperThresholdCanny = 15;
+  const float lowerThresholdCanny = 5;
+  const unsigned int apertureSobel = 3;
+
+  // Image for the Canny edge operator
+  vpImage<unsigned char> Isrc;
+  vpImage<unsigned char> Icanny;
+
+  // First grab the source image Isrc.
+
+  // Apply the Canny edge operator and set the Icanny image.
+  vpImageFilter::canny(Isrc, Icanny, gaussianFilterSize, lowerThresholdCanny, upperThresholdCanny, apertureSobel);
+  return (0);
+}
+  \endcode
+
+  \param Isrc : Image to apply the Canny edge detector to.
+  \param Ires : Filtered image (255 means an edge, 0 otherwise).
+  \param gaussianFilterSize : The size of the mask of the Gaussian filter to
+  apply (an odd number).
+  \param lowerThreshold : The lower threshold for the Canny operator. Values lower
+  than this value are rejected. If negative, it will be set to one third
+  of the thresholdCanny .
+  \param upperThreshold : The upper threshold for the Canny operator. Only value
+  greater than this value are marked as an edge. If negative, it will be automatically
+  computed, along with the lower threshold. Otherwise, the lower threshold will be set to one third
+  of the thresholdCanny .
+  \param apertureSobel : Size of the mask for the Sobel operator (odd number).
+*/
+void vpImageFilter::canny(const vpImage<unsigned char> &Isrc, vpImage<unsigned char> &Ires,
+                          unsigned int gaussianFilterSize, float lowerThreshold, float upperThreshold, unsigned int apertureSobel)
 {
 #if defined(HAVE_OPENCV_IMGPROC)
-  cv::Mat img_cvmat, cv_I_blur, edges_cvmat;
+  cv::Mat img_cvmat, cv_I_blur, cv_dx, cv_dy, edges_cvmat;
   vpImageConvert::convert(Isrc, img_cvmat);
   cv::GaussianBlur(img_cvmat, cv_I_blur, cv::Size((int)gaussianFilterSize, (int)gaussianFilterSize), 0, 0);
-  float upperCannyThresh = thresholdCanny;
-  float lowerCannyThresh = thresholdCanny / 3.f;
+  cv::Sobel(cv_I_blur, cv_dx, CV_16S, 1, 0, apertureSobel);
+  cv::Sobel(cv_I_blur, cv_dy, CV_16S, 0, 1, apertureSobel);
+  float upperCannyThresh = upperThreshold;
+  float lowerCannyThresh = lowerThreshold;
   if (upperCannyThresh < 0) {
     upperCannyThresh = computeCannyThreshold(img_cvmat, &cv_I_blur, lowerCannyThresh);
   }
-  cv::Canny(cv_I_blur, edges_cvmat, lowerCannyThresh, upperCannyThresh, (int)apertureSobel);
+  else if (lowerCannyThresh < 0) {
+    lowerCannyThresh = upperCannyThresh / 3.f;
+  }
+  cv::Canny(cv_dx, cv_dy, edges_cvmat, lowerCannyThresh, upperCannyThresh, false);
   vpImageConvert::convert(edges_cvmat, Ires);
 #else
   (void)apertureSobel;
-  if (thresholdCanny < 0) {
+  float upperCannyThresh = upperThreshold;
+  float lowerCannyThresh = lowerThreshold;
+  if (upperCannyThresh < 0) {
     throw(vpException(vpException::badValue, "OpenCV imgproc module missing to be able to compute automatically the Canny thresholds"));
   }
-  vpCannyEdgeDetection edgeDetector(gaussianFilterSize, 0.1, thresholdCanny * 0.5, thresholdCanny);
+  else if (lowerCannyThresh < 0) {
+    lowerCannyThresh = upperCannyThresh / 3.;
+  }
+  vpCannyEdgeDetection edgeDetector(gaussianFilterSize, 0.1, lowerCannyThresh, upperCannyThresh);
   Ires = edgeDetector.detect(Isrc);
 #endif
 }

modules/imgproc/include/visp3/imgproc/vpCircleHoughTransform.h

@@ -81,8 +81,10 @@ class VISP_EXPORT vpCircleHoughTransform
     int m_sobelKernelSize; /*!< Size of the Sobel kernels used to compute the gradients. Must be an odd number.*/
 
     // // Edge detection attributes
-    float m_cannyThresh; /*!< The threshold for the Canny operator. Only value greater than this value are marked as an edge.
-                               A negative value makes the algorithm compute this threshold automatically.*/
+    float m_lowerCannyThresh; /*!< The lower threshold for the Canny operator. Values lower than this value are rejected.
+                               A negative value makes the algorithm compute the lower threshold automatically.*/
+    float m_upperCannyThresh; /*!< The upper threshold for the Canny operator. Only values greater than this value are marked as an edge.
+                               A negative value makes the algorithm compute the upper and lower thresholds automatically.*/
     int m_edgeMapFilteringNbIter; /*!< Number of iterations of 8-neighbor connectivity filtering to apply to the edge map*/
 
     // // Center candidates computation attributes
@@ -107,7 +109,8 @@ class VISP_EXPORT vpCircleHoughTransform
       : m_gaussianKernelSize(5)
       , m_gaussianStdev(1.f)
       , m_sobelKernelSize(3)
-      , m_cannyThresh(-1.f)
+      , m_lowerCannyThresh(-1.f)
+      , m_upperCannyThresh(-1.f)
       , m_edgeMapFilteringNbIter(1)
       , m_centerXlimits(std::pair<int, int>(std::numeric_limits<int>::min(), std::numeric_limits<int>::max()))
       , m_centerYlimits(std::pair<int, int>(std::numeric_limits<int>::min(), std::numeric_limits<int>::max()))
@@ -129,8 +132,10 @@ class VISP_EXPORT vpCircleHoughTransform
      * \param[in] gaussianKernelSize Size of the Gaussian filter kernel used to smooth the input image. Must be an odd number.
      * \param[in] gaussianStdev Standard deviation of the Gaussian filter.
      * \param[in] sobelKernelSize Size of the Sobel kernels used to compute the gradients. Must be an odd number.
-     * \param[in] cannyThresh The threshold for the Canny operator. Only value greater than this value are marked as an edge.
-                          A negative value makes the algorithm compute this threshold automatically.
+     * \param[in] lowerCannyThresh The lower threshold for the Canny operator. Values lower than this value are rejected.
+                          A negative value makes the algorithm compute this threshold and the lower one automatically.
+     * \param[in] upperCannyThresh The upper threshold for the Canny operator. Only values greater than this value are marked as an edge.
+                          A negative value makes the algorithm compute this threshold and the lower one automatically.
      * \param[in] edgeMapFilterNbIter Number of 8-neighbor connectivity filtering iterations to apply to the edge map.
      * \param[in] centerXlimits Minimum and maximum position on the horizontal axis of the center of the circle we want to detect.
      * \param[in] centerYlimits Minimum and maximum position on the vertical axis of the center of the circle we want to detect.
@@ -147,7 +152,8 @@ class VISP_EXPORT vpCircleHoughTransform
         const int &gaussianKernelSize
       , const float &gaussianStdev
       , const int &sobelKernelSize
-      , const float &cannyThresh
+      , const float &lowerCannyThresh
+      , const float &upperCannyThresh
       , const int &edgeMapFilterNbIter
       , const std::pair<int, int> &centerXlimits
       , const std::pair<int, int> &centerYlimits
@@ -163,7 +169,8 @@ class VISP_EXPORT vpCircleHoughTransform
       : m_gaussianKernelSize(gaussianKernelSize)
       , m_gaussianStdev(gaussianStdev)
       , m_sobelKernelSize(sobelKernelSize)
-      , m_cannyThresh(cannyThresh)
+      , m_lowerCannyThresh(lowerCannyThresh)
+      , m_upperCannyThresh(upperCannyThresh)
       , m_edgeMapFilteringNbIter(edgeMapFilterNbIter)
       , m_centerXlimits(centerXlimits)
       , m_centerYlimits(centerYlimits)
@@ -185,7 +192,7 @@ class VISP_EXPORT vpCircleHoughTransform
       txt += "\tGaussian filter kernel size = " + std::to_string(m_gaussianKernelSize) + "\n";
       txt += "\tGaussian filter standard deviation = " + std::to_string(m_gaussianStdev) + "\n";
       txt += "\tSobel filter kernel size = " + std::to_string(m_sobelKernelSize) + "\n";
-      txt += "\tCanny edge filter threshold = " + std::to_string(m_cannyThresh) + "\n";
+      txt += "\tCanny edge filter thresholds = [" + std::to_string(m_lowerCannyThresh) + " ; " + std::to_string(m_upperCannyThresh) + "]\n";
       txt += "\tEdge map 8-neighbor connectivity filtering number of iterations = " + std::to_string(m_edgeMapFilteringNbIter) + "\n";
       txt += "\tCenter horizontal position limits: min = " + std::to_string(m_centerXlimits.first) + "\tmax = " + std::to_string(m_centerXlimits.second) +"\n";
       txt += "\tCenter vertical position limits: min = " + std::to_string(m_centerYlimits.first) + "\tmax = " + std::to_string(m_centerYlimits.second) +"\n";
@@ -271,7 +278,8 @@ class VISP_EXPORT vpCircleHoughTransform
         throw vpException(vpException::badValue, "Sobel Kernel size should be odd.");
       }
 
-      params.m_cannyThresh = j.value("cannyThresh", params.m_cannyThresh);
+      params.m_lowerCannyThresh = j.value("lowerCannyThresh", params.m_lowerCannyThresh);
+      params.m_upperCannyThresh = j.value("upperCannyThresh", params.m_upperCannyThresh);
       params.m_edgeMapFilteringNbIter = j.value("edgeMapFilteringNbIter", params.m_edgeMapFilteringNbIter);
 
       params.m_centerXlimits = j.value("centerXlimits", params.m_centerXlimits);
@@ -320,7 +328,8 @@ class VISP_EXPORT vpCircleHoughTransform
           {"gaussianKernelSize", params.m_gaussianKernelSize},
           {"gaussianStdev", params.m_gaussianStdev},
           {"sobelKernelSize", params.m_sobelKernelSize},
-          {"cannyThresh", params.m_cannyThresh},
+          {"lowerCannyThresh", params.m_lowerCannyThresh},
+          {"upperCannyThresh", params.m_upperCannyThresh},
           {"edgeMapFilteringNbIter", params.m_edgeMapFilteringNbIter},
           {"centerXlimits", params.m_centerXlimits},
           {"centerYlimits", params.m_centerYlimits},
@@ -482,12 +491,15 @@ class VISP_EXPORT vpCircleHoughTransform
    * Set the threshold for the Canny operator.
    * Only value greater than this value are marked as an edge.
    * If negative, the threshold is automatically computed.
-   * \param[in] canny_threshold : Canny filter upper threshold. When set to -1 (default), compute
+   * \param[in] lowerCannyThreshold : Canny filter lower threshold. When set to -1 (default), compute
+   * automatically this threshold.
+   * \param[in] upperCannyThreshold : Canny filter upper threshold. When set to -1 (default), compute
    * automatically this threshold.
    */
-  inline void setCannyThreshold(const float &canny_threshold)
+  inline void setCannyThreshold(const float &lowerCannyThreshold, const float &upperCannyThreshold)
   {
-    m_algoParams.m_cannyThresh = canny_threshold;
+    m_algoParams.m_lowerCannyThresh = lowerCannyThreshold;
+    m_algoParams.m_upperCannyThresh = upperCannyThreshold;
   }
 
   /*!
@@ -680,7 +692,7 @@ class VISP_EXPORT vpCircleHoughTransform
    */
   inline float getCannyThreshold() const
   {
-    return m_algoParams.m_cannyThresh;
+    return m_algoParams.m_upperCannyThresh;
   }
 
   /*!

modules/imgproc/src/vpCircleHoughTransform.cpp

@@ -220,16 +220,19 @@ void
 vpCircleHoughTransform::edgeDetection(const vpImage<unsigned char> &I)
 {
 #if defined(HAVE_OPENCV_IMGPROC)
-  float cannyThresh = m_algoParams.m_cannyThresh;
-  float lowerThresh;
+  float upperCannyThresh = m_algoParams.m_upperCannyThresh;
+  float lowerCannyThresh = m_algoParams.m_lowerCannyThresh;
   // Apply the Canny edge operator to compute the edge map
   // The canny method performs Gaussian blur and gradient computation
-  if (m_algoParams.m_cannyThresh < 0.) {
-    cannyThresh = vpImageFilter::computeCannyThreshold(I, lowerThresh);
+  if (m_algoParams.m_upperCannyThresh < 0.) {
+    upperCannyThresh = vpImageFilter::computeCannyThreshold(I, lowerCannyThresh);
   }
-  vpImageFilter::canny(I, m_edgeMap, m_algoParams.m_gaussianKernelSize, cannyThresh, m_algoParams.m_sobelKernelSize);
+  else if (m_algoParams.m_lowerCannyThresh < 0) {
+    lowerCannyThresh = upperCannyThresh / 3.;
+  }
+  vpImageFilter::canny(I, m_edgeMap, m_algoParams.m_gaussianKernelSize, lowerCannyThresh, upperCannyThresh, m_algoParams.m_sobelKernelSize);
 #else
-  m_cannyVisp.setCannyThresholds(-1, m_algoParams.m_cannyThresh);
+  m_cannyVisp.setCannyThresholds(m_algoParams.m_lowerCannyThresh, m_algoParams.m_upperCannyThresh);
   m_cannyVisp.setGradients(m_dIx, m_dIy);
   m_edgeMap = m_cannyVisp.detect(I);
 #endif

tutorial/imgproc/hough-transform/config/detector_full.json

@@ -1,5 +1,6 @@
 {
-  "cannyThresh": 150.0,
+  "lowerCannyThresh": 50.0,
+  "upperCannyThresh": 150.0,
   "centerMinDistance": 15.0,
   "centerThresh": 100.0,
   "centerXlimits": [

tutorial/imgproc/hough-transform/config/detector_half.json

@@ -1,5 +1,6 @@
 {
-  "cannyThresh": 150.0,
+  "lowerCannyThresh": 50.0,
+  "upperCannyThresh": 150.0,
   "centerMinDistance": 15.0,
   "centerThresh": 50.0,
   "centerXlimits": [

tutorial/imgproc/hough-transform/config/detector_img.json

@@ -1,5 +1,6 @@
 {
-  "cannyThresh": -1.0,
+  "lowerCannyThresh": -1.0,
+  "upperCannyThresh": -1.0,
   "centerMinDistance": 5.0,
   "centerThresh": 100.0,
   "centerXlimits": [

tutorial/imgproc/hough-transform/config/detector_quarter.json

@@ -1,5 +1,6 @@
 {
-  "cannyThresh": 150.0,
+  "lowerCannyThresh": 50.0,
+  "upperCannyThresh": 150.0,
   "centerMinDistance": 15.0,
   "centerThresh": 25.0,
   "centerXlimits": [

tutorial/imgproc/hough-transform/tutorial-circle-hough.cpp

@@ -199,9 +199,11 @@ int main(int argc, char **argv)
   const float def_gaussianSigma = 1.f;
   const int def_sobelKernelSize = 3;
 #ifdef HAVE_OPENCV_IMGPROC
-  const float def_cannyThresh = 150.f;
+  const float def_lowerCannyThresh = 50.f;
+  const float def_upperCannyThresh = 150.f;
 #else
-  const float def_cannyThresh = 25.f;
+  const float def_lowerCannyThresh = 8.f;
+  const float def_upperCannyThresh = 25.f;
 #endif
   const int def_nbEdgeFilteringIter = 2;
   const std::pair<int, int> def_centerXlimits = std::pair<int, int>(0, 640);
@@ -221,7 +223,8 @@ int main(int argc, char **argv)
   int opt_gaussianKernelSize = def_gaussianKernelSize;
   float opt_gaussianSigma = def_gaussianSigma;
   int opt_sobelKernelSize = def_sobelKernelSize;
-  float opt_cannyThresh = def_cannyThresh;
+  float opt_lowerCannyThresh = def_lowerCannyThresh;
+  float opt_upperCannyThresh = def_upperCannyThresh;
   int opt_nbEdgeFilteringIter = def_nbEdgeFilteringIter;
   std::pair<int, int> opt_centerXlimits = def_centerXlimits;
   std::pair<int, int> opt_centerYlimits = def_centerYlimits;
@@ -263,9 +266,10 @@ int main(int argc, char **argv)
       opt_sobelKernelSize = atoi(argv[i + 1]);
       i++;
     }
-    else if (argName == "--canny-thresh" && i + 1 < argc) {
-      opt_cannyThresh = static_cast<float>(atof(argv[i + 1]));
-      i++;
+    else if (argName == "--canny-thresh" && i + 2 < argc) {
+      opt_lowerCannyThresh = static_cast<float>(atof(argv[i + 1]));
+      opt_upperCannyThresh = static_cast<float>(atof(argv[i + 2]));
+      i += 2;
     }
     else if (argName == "--edge-filter" && i + 1 < argc) {
       opt_nbEdgeFilteringIter = atoi(argv[i + 1]);
@@ -322,7 +326,7 @@ int main(int argc, char **argv)
         << "\t [--gaussian-kernel <kernel-size>] (default: " << def_gaussianKernelSize << ")" << std::endl
         << "\t [--gaussian-sigma <stddev>] (default: " << def_gaussianSigma << ")" << std::endl
         << "\t [--sobel-kernel <kernel-size>] (default: " << def_sobelKernelSize << ")" << std::endl
-        << "\t [--canny-thresh <canny-thresh>] (default: " << def_cannyThresh << ")" << std::endl
+        << "\t [--canny-thresh <lower-canny-thresh upper-canny-thresh>] (default: " << def_lowerCannyThresh << " ; " << def_upperCannyThresh << ")" << std::endl
         << "\t [--edge-filter <nb-iter>] (default: " << def_nbEdgeFilteringIter << ")" << std::endl
         << "\t [--radius-limits <radius-min> <radius-max>] (default: min = " << def_minRadius << ", max = " << def_maxRadius << ")" << std::endl
         << "\t [--dilatation-repet <nb-repetitions>] (default: " << def_dilatationRepet << ")" << std::endl
@@ -363,9 +367,9 @@ int main(int argc, char **argv)
         << "\t\tDefault: " << def_gaussianSigma << std::endl
         << std::endl
         << "\t--canny-thresh" << std::endl
-        << "\t\tPermit to set the upper threshold of the Canny edge detector." << std::endl
-        << "\t\tMust be a positive value." << std::endl
-        << "\t\tDefault: " << def_cannyThresh << std::endl
+        << "\t\tPermit to set the lower and upper thresholds of the Canny edge detector." << std::endl
+        << "\t\tIf a value is negative, it will be automatically computed." << std::endl
+        << "\t\tDefault: " << def_upperCannyThresh << std::endl
         << std::endl
         << "\t--edge-filter" << std::endl
         << "\t\tPermit to set the number of iteration of 8-neighbor filter iterations of the result of the Canny edge detector." << std::endl
@@ -478,7 +482,8 @@ int main(int argc, char **argv)
     algoParams(opt_gaussianKernelSize
       , opt_gaussianSigma
       , opt_sobelKernelSize
-      , opt_cannyThresh
+      , opt_lowerCannyThresh
+      , opt_upperCannyThresh
       , opt_nbEdgeFilteringIter
       , opt_centerXlimits
       , opt_centerYlimits