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Fix angle discretization in Hough transforms

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In some situations the last value was missing from the discrete theta
values. Now, the last value is chosen such that it is close to the
user-provided maximum theta, while the distance to pi remains always
at least theta_step/2. This should avoid duplicate detections.

A better way would probably be to use max_theta as is and adjust the
resolution (theta_step) instead, such that the discretization would
always be uniform (in a circular sense) when full angle range is used.
This commit is contained in:
lamm45
2022-06-26 19:13:12 -04:00
parent 5e1c9099e8
commit 1f0bfc8d83
3 changed files with 41 additions and 16 deletions
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modules/imgproc/src/hough.cpp
+17 -5
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@@ -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 );