Merge pull request #22164 from lamm45:hough-angles

Fix angle discretization in Hough transforms

modules/imgproc/include/opencv2/imgproc.hpp

@@ -2028,13 +2028,13 @@ transform.
 
 @param image 8-bit, single-channel binary source image. The image may be modified by the function.
 @param lines Output vector of lines. Each line is represented by a 2 or 3 element vector
-\f$(\rho, \theta)\f$ or \f$(\rho, \theta, \textrm{votes})\f$ . \f$\rho\f$ is the distance from the coordinate origin \f$(0,0)\f$ (top-left corner of
-the image). \f$\theta\f$ is the line rotation angle in radians (
-\f$0 \sim \textrm{vertical line}, \pi/2 \sim \textrm{horizontal line}\f$ ).
+\f$(\rho, \theta)\f$ or \f$(\rho, \theta, \textrm{votes})\f$, where \f$\rho\f$ is the distance from
+the coordinate origin \f$(0,0)\f$ (top-left corner of the image), \f$\theta\f$ is the line rotation
+angle in radians ( \f$0 \sim \textrm{vertical line}, \pi/2 \sim \textrm{horizontal line}\f$ ), and
 \f$\textrm{votes}\f$ is the value of accumulator.
 @param rho Distance resolution of the accumulator in pixels.
 @param theta Angle resolution of the accumulator in radians.
-@param threshold Accumulator threshold parameter. Only those lines are returned that get enough
+@param threshold %Accumulator threshold parameter. Only those lines are returned that get enough
 votes ( \f$>\texttt{threshold}\f$ ).
 @param srn For the multi-scale Hough transform, it is a divisor for the distance resolution rho.
 The coarse accumulator distance resolution is rho and the accurate accumulator resolution is
@@ -2043,8 +2043,9 @@ parameters should be positive.
 @param stn For the multi-scale Hough transform, it is a divisor for the distance resolution theta.
 @param min_theta For standard and multi-scale Hough transform, minimum angle to check for lines.
 Must fall between 0 and max_theta.
-@param max_theta For standard and multi-scale Hough transform, maximum angle to check for lines.
-Must fall between min_theta and CV_PI.
+@param max_theta For standard and multi-scale Hough transform, an upper bound for the angle.
+Must fall between min_theta and CV_PI. The actual maximum angle in the accumulator may be slightly
+less than max_theta, depending on the parameters min_theta and theta.
  */
 CV_EXPORTS_W void HoughLines( InputArray image, OutputArray lines,
                               double rho, double theta, int threshold,
@@ -2072,7 +2073,7 @@ And this is the output of the above program in case of the probabilistic Hough t
 line segment.
 @param rho Distance resolution of the accumulator in pixels.
 @param theta Angle resolution of the accumulator in radians.
-@param threshold Accumulator threshold parameter. Only those lines are returned that get enough
+@param threshold %Accumulator threshold parameter. Only those lines are returned that get enough
 votes ( \f$>\texttt{threshold}\f$ ).
 @param minLineLength Minimum line length. Line segments shorter than that are rejected.
 @param maxLineGap Maximum allowed gap between points on the same line to link them.
@@ -2091,13 +2092,14 @@ The function finds lines in a set of points using a modification of the Hough tr
 @param lines Output vector of found lines. Each vector is encoded as a vector<Vec3d> \f$(votes, rho, theta)\f$.
 The larger the value of 'votes', the higher the reliability of the Hough line.
 @param lines_max Max count of Hough lines.
-@param threshold Accumulator threshold parameter. Only those lines are returned that get enough
+@param threshold %Accumulator threshold parameter. Only those lines are returned that get enough
 votes ( \f$>\texttt{threshold}\f$ ).
 @param min_rho Minimum value for \f$\rho\f$ for the accumulator (Note: \f$\rho\f$ can be negative. The absolute value \f$|\rho|\f$ is the distance of a line to the origin.).
 @param max_rho Maximum value for \f$\rho\f$ for the accumulator.
 @param rho_step Distance resolution of the accumulator.
 @param min_theta Minimum angle value of the accumulator in radians.
-@param max_theta Maximum angle value of the accumulator in radians.
+@param max_theta Upper bound for the angle value of the accumulator in radians. The actual maximum
+angle may be slightly less than max_theta, depending on the parameters min_theta and theta_step.
 @param theta_step Angle resolution of the accumulator in radians.
  */
 CV_EXPORTS_W void HoughLinesPointSet( InputArray point, OutputArray lines, int lines_max, int threshold,

modules/imgproc/src/hough.cpp

@@ -68,6 +68,18 @@ struct hough_cmp_gt
     const int* aux;
 };
 
+static inline int
+computeNumangle( double min_theta, double max_theta, double theta_step )
+{
+    int numangle = cvFloor((max_theta - min_theta) / theta_step) + 1;
+    // If the distance between the first angle and the last angle is
+    // approximately equal to pi, then the last angle will be removed
+    // in order to prevent a line to be detected twice.
+    if ( numangle > 1 && fabs(CV_PI - (numangle-1)*theta_step) < theta_step/2 )
+        --numangle;
+    return numangle;
+}
+
 static void
 createTrigTable( int numangle, double min_theta, double theta_step,
                  float irho, float *tabSin, float *tabCos )
@@ -130,7 +142,7 @@ HoughLinesStandard( InputArray src, OutputArray lines, int type,
 
     CV_CheckGE(max_theta, min_theta, "max_theta must be greater than min_theta");
 
-    int numangle = cvRound((max_theta - min_theta) / theta);
+    int numangle = computeNumangle(min_theta, max_theta, theta);
     int numrho = cvRound(((max_rho - min_rho) + 1) / rho);
 
 #if defined HAVE_IPP && IPP_VERSION_X100 >= 810 && !IPP_DISABLE_HOUGH
@@ -475,7 +487,7 @@ HoughLinesProbabilistic( Mat& image,
     int width = image.cols;
     int height = image.rows;
 
-    int numangle = cvRound(CV_PI / theta);
+    int numangle = computeNumangle(0.0, CV_PI, theta);
     int numrho = cvRound(((width + height) * 2 + 1) / rho);
 
 #if defined HAVE_IPP && IPP_VERSION_X100 >= 810 && !IPP_DISABLE_HOUGH
@@ -792,7 +804,7 @@ static bool ocl_HoughLines(InputArray _src, OutputArray _lines, double rho, doub
     }
 
     UMat src = _src.getUMat();
-    int numangle = cvRound((max_theta - min_theta) / theta);
+    int numangle = computeNumangle(min_theta, max_theta, theta);
     int numrho = cvRound(((src.cols + src.rows) * 2 + 1) / rho);
 
     UMat pointsList;
@@ -846,7 +858,7 @@ static bool ocl_HoughLinesP(InputArray _src, OutputArray _lines, double rho, dou
     }
 
     UMat src = _src.getUMat();
-    int numangle = cvRound(CV_PI / theta);
+    int numangle = computeNumangle(0.0, CV_PI, theta);
     int numrho = cvRound(((src.cols + src.rows) * 2 + 1) / rho);
 
     UMat pointsList;
@@ -956,7 +968,7 @@ void HoughLinesPointSet( InputArray _point, OutputArray _lines, int lines_max, i
     int i;
     float irho = 1 / (float)rho_step;
     float irho_min = ((float)min_rho * irho);
-    int numangle = cvRound((max_theta - min_theta) / theta_step);
+    int numangle = computeNumangle(min_theta, max_theta, theta_step);
     int numrho = cvRound((max_rho - min_rho + 1) / rho_step);
 
     Mat _accum = Mat::zeros( (numangle+2), (numrho+2), CV_32SC1 );

modules/imgproc/test/test_houghlines.cpp

@@ -329,6 +329,17 @@ TEST(HoughLinesPointSet, regression_21029)
     EXPECT_TRUE(lines.empty());
 }
 
+TEST(HoughLines, regression_21983)
+{
+    Mat img(200, 200, CV_8UC1, Scalar(0));
+    line(img, Point(0, 100), Point(100, 100), Scalar(255));
+    std::vector<Vec2f> lines;
+    HoughLines(img, lines, 1, CV_PI/180, 90, 0, 0, 0.001, 1.58);
+    ASSERT_EQ(lines.size(), 1U);
+    EXPECT_EQ(lines[0][0], 100);
+    EXPECT_NEAR(lines[0][1], 1.57179642, 1e-4);
+}
+
 INSTANTIATE_TEST_CASE_P( ImgProc, StandartHoughLinesTest, testing::Combine(testing::Values( "shared/pic5.png", "../stitching/a1.png" ),
                                                                            testing::Values( 1, 10 ),
                                                                            testing::Values( 0.05, 0.1 ),