diff --git a/modules/imgproc/include/visp3/imgproc/vpCircleHoughTransform.h b/modules/imgproc/include/visp3/imgproc/vpCircleHoughTransform.h
index e4f1426b5a..a434a7c813 100644
--- a/modules/imgproc/include/visp3/imgproc/vpCircleHoughTransform.h
+++ b/modules/imgproc/include/visp3/imgproc/vpCircleHoughTransform.h
@@ -108,7 +108,9 @@ class VISP_EXPORT vpCircleHoughTransform
 
     // // Circle candidates computation attributes
     float m_circleProbaThresh;  /*!< Probability threshold in order to keep a circle candidate.*/
-    float m_circlePerfectness; /*!< The scalar product radius RC_ij . gradient(Ep_j) >=  m_circlePerfectness * || RC_ij || * || gradient(Ep_j) || to add a vote for the radius RC_ij. */
+    float m_circlePerfectness; /*!< The threshold for the colinearity between the gradient of a point
+                                    and the radius it would form with a center candidate to be able to vote.
+                                    The formula to get the equivalent angle is: `angle = acos(circle_perfectness)`. */
 
     // // Circle candidates merging attributes
     float m_centerMinDist; /*!< Maximum distance between two circle candidates centers to consider merging them.*/
@@ -164,7 +166,9 @@ class VISP_EXPORT vpCircleHoughTransform
      * \param[in] dilatationKernelSize Kernel size of the dilatation that is performed to detect the maximum number of votes for the center candidates.
      * \param[in] centerThresh Minimum number of votes a point must exceed to be considered as center candidate.
      * \param[in] circleProbabilityThresh Probability threshold in order to keep a circle candidate.
-     * \param[in] circlePerfectness The scalar product radius RC_ij . gradient(Ep_j) >=  m_circlePerfectness * || RC_ij || * || gradient(Ep_j) || to add a vote for the radius RC_ij.
+     * \param[in] circlePerfectness The threshold for the colinearity between the gradient of a point
+                                    and the radius it would form with a center candidate to be able to vote.
+                                    The formula to get the equivalent angle is: `angle = acos(circle_perfectness)`.
      * \param[in] centerMinDistThresh  Two circle candidates whose centers are closer than this threshold are considered for merging.
      * \param[in] mergingRadiusDiffThresh Maximum radius difference between two circle candidates to consider merging them.
      * \param[in] averagingWindowSize Size of the averaging window around the maximum number of votes to compute the
@@ -384,7 +388,9 @@ class VISP_EXPORT vpCircleHoughTransform
     }
 
     /**
-     * \brief Get the threshold for the scalar product between the radius and the gradient to count a vote.
+     * \brief Get the threshold for the colinearity between the gradient of a point
+     * and the radius it would form with a center candidate to be able to vote.
+     * The formula to get the equivalent angle is: `angle = acos(circle_perfectness)`.
      *
      * \return float The threshold.
      */
@@ -870,7 +876,10 @@ class VISP_EXPORT vpCircleHoughTransform
   }
 
   /*!
-   * Set circles perfectness. The scalar product radius RC_ij . gradient(Ep_j) >=  m_circlePerfectness * || RC_ij || * || gradient(Ep_j) || to add a vote for the radius RC_ij.
+   * \brief Set circles perfectness, which corresponds to the threshold of the colinearity
+   * between the gradient of a point and the radius it would form with a center candidate
+   * to be able to vote.
+   * The formula to get the equivalent angle is: `angle = acos(circle_perfectness)`.
    * \param[in] circle_perfectness : Circle perfectness. Value between 0 and 1. A perfect circle has value 1.
    */
   void setCirclePerfectness(const float &circle_perfectness)
diff --git a/modules/imgproc/src/vpCircleHoughTransform.cpp b/modules/imgproc/src/vpCircleHoughTransform.cpp
index 9a457f74a5..7a15b56999 100644
--- a/modules/imgproc/src/vpCircleHoughTransform.cpp
+++ b/modules/imgproc/src/vpCircleHoughTransform.cpp
@@ -705,8 +705,8 @@ vpCircleHoughTransform::computeCircleCandidates()
 
     for (auto edgePoint : m_edgePointsList) {
       // For each center candidate CeC_i, compute the distance with each edge point EP_j d_ij = dist(CeC_i; EP_j)
-      float rx = edgePoint.first  - centerCandidate.first;
-      float ry = edgePoint.second - centerCandidate.second;
+      float rx = edgePoint.second  - centerCandidate.second;
+      float ry = edgePoint.first - centerCandidate.first;
       float r2 = rx * rx + ry * ry;
       if ((r2 > rmin2) && (r2 < rmax2)) {
         float gx = m_dIx[edgePoint.first][edgePoint.second];
diff --git a/tutorial/imgproc/hough-transform/tutorial-circle-hough.cpp b/tutorial/imgproc/hough-transform/tutorial-circle-hough.cpp
index d89f7bcb51..e6f3b84e7e 100644
--- a/tutorial/imgproc/hough-transform/tutorial-circle-hough.cpp
+++ b/tutorial/imgproc/hough-transform/tutorial-circle-hough.cpp
@@ -70,7 +70,7 @@ int main(int argc, char **argv)
   const int def_dilatationKernelSize = 5;
   const float def_centerThresh = 70.f;
   const float def_circleProbaThresh = 0.725f;
-  const float def_circlePerfectness = 0.65f;
+  const float def_circlePerfectness = 0.85f;
   const float def_centerDistanceThresh = 5.f;
   const float def_radiusDifferenceThresh = 5.f;
   const int def_averagingWindowSize = 5;