Changed RANSAC Homography estimation for speed. Each minimal set is checked for consistency before actually fit a candidate homography.

modules/calib3d/src/_modelest.h

@@ -69,6 +69,7 @@ protected:
     virtual bool getSubset( const CvMat* m1, const CvMat* m2,
                             CvMat* ms1, CvMat* ms2, int maxAttempts=1000 );
     virtual bool checkSubset( const CvMat* ms1, int count );
+    virtual bool isMinimalSetConsistent( const CvMat* m1, const CvMat* m2 ) { return true; };
 
     CvRNG rng;
     int modelPoints;

modules/calib3d/src/fundam.cpp

@@ -68,6 +68,7 @@ public:
 protected:
     virtual void computeReprojError( const CvMat* m1, const CvMat* m2,
                                      const CvMat* model, CvMat* error );
+    virtual bool isMinimalSetConsistent( const CvMat* m1, const CvMat* m2 );
 };
 
 
@@ -287,6 +288,85 @@ cvFindHomography( const CvMat* objectPoints, const CvMat* imagePoints,
     return (int)result;
 }
 
+// We check whether three correspondences for the homography estimation
+// are geometrically consistent (the points in the source image should 
+// maintain the same circular order than in the destination image).
+//
+// The usefullness of this constraint is explained in the paper:
+//
+// "Speeding-up homography estimation in mobile devices"
+// Journal of Real-Time Image Processing. 2013. DOI: 10.1007/s11554-012-0314-1
+// Pablo Marquez-Neila, Javier Lopez-Alberca, Jose M. Buenaposada, Luis Baumela
+CV_IMPL bool
+weakConstraint
+  (
+  const CvMat* srcPoints,
+  const CvMat* dstPoints,
+  int t1,
+  int t2,
+  int t3
+  )
+{
+  const CvPoint2D64f* src = (const CvPoint2D64f*)srcPoints->data.ptr;
+  const CvPoint2D64f* dst = (const CvPoint2D64f*)dstPoints->data.ptr;
+
+  CvMat* A = cvCreateMat( 3, 3, CV_64F );
+  CvMat* B = cvCreateMat( 3, 3, CV_64F );
+
+  double detA;
+  double detB;
+
+  cvmSet(A, 0, 0, src[t1].x);
+  cvmSet(A, 0, 1, src[t1].y);
+  cvmSet(A, 0, 2, 1);
+  cvmSet(A, 1, 0, src[t2].x);
+  cvmSet(A, 1, 1, src[t2].y);
+  cvmSet(A, 1, 2, 1);
+  cvmSet(A, 2, 0, src[t3].x);
+  cvmSet(A, 2, 1, src[t3].y);
+  cvmSet(A, 2, 2, 1);
+
+  cvmSet(B, 0, 0, dst[t1].x);
+  cvmSet(B, 0, 1, dst[t1].y);
+  cvmSet(B, 0, 2, 1);
+  cvmSet(B, 1, 0, dst[t2].x);
+  cvmSet(B, 1, 1, dst[t2].y);
+  cvmSet(B, 1, 2, 1);
+  cvmSet(B, 2, 0, dst[t3].x);
+  cvmSet(B, 2, 1, dst[t3].y);
+  cvmSet(B, 2, 2, 1);
+
+  detA = cvDet(A);
+  detB = cvDet(B);
+
+  cvReleaseMat(&A);
+  cvReleaseMat(&B);
+
+  return (detA*detB >= 0);
+};
+
+// We check whether the minimal set of points for the homography estimation
+// are geometrically consistent. We check if every 3 correspondences sets
+// fulfills the constraint.
+//
+// The usefullness of this constraint is explained in the paper:
+//
+// "Speeding-up homography estimation in mobile devices"
+// Journal of Real-Time Image Processing. 2013. DOI: 10.1007/s11554-012-0314-1
+// Pablo Marquez-Neila, Javier Lopez-Alberca, Jose M. Buenaposada, Luis Baumela
+bool
+CvHomographyEstimator::isMinimalSetConsistent
+  (
+  const CvMat* srcPoints,
+  const CvMat* dstPoints
+  )
+{
+  return weakConstraint(srcPoints, dstPoints, 0, 1, 2) &&
+         weakConstraint(srcPoints, dstPoints, 1, 2, 3) &&
+         weakConstraint(srcPoints, dstPoints, 0, 2, 3) &&
+         weakConstraint(srcPoints, dstPoints, 0, 1, 3);
+}
+
 
 /* Evaluation of Fundamental Matrix from point correspondences.
    The original code has been written by Valery Mosyagin */

modules/calib3d/src/modelest.cpp

@@ -152,6 +152,18 @@ bool CvModelEstimator2::runRANSAC( const CvMat* m1, const CvMat* m2, CvMat* mode
                     return false;
                 break;
             }
+
+            // Here we check for model specific geometrical 
+            // constraints that allow to avoid "runKernel" 
+            // and not checking for inliers if not fulfilled. 
+            //
+            // The usefullness of this constraint for homographies is explained in the paper: 
+            //
+            // "Speeding-up homography estimation in mobile devices"
+            // Journal of Real-Time Image Processing. 2013. DOI: 10.1007/s11554-012-0314-1
+            // Pablo Márquez-Neila, Javier López-Alberca, José M. Buenaposada, Luis Baumela
+            if ( !isMinimalSetConsistent( ms1, ms2 ) )
+                continue;
         }
 
         nmodels = runKernel( ms1, ms2, models );