little fixes and correction of a conceptual error

Previously, there's no way to the user see the found corners, i've changed that.

In a cout, are write that: "average reprojection err = "
But it isn't a "reprojection error" at all, it is a mean of each EPIPOLAR error, wich occur when the product x' * F * x is not equal to zero.
(x and x' are the same points in the right and left scene)
(the RMS that explain the average absolute reprojection error is given by the return of the stereoCalibrate() function)

At least, i think it's interesting to initialize the camera matrices before.

Thank you all for this amazing code. Apologize my weak english.

samples/cpp/stereo_calib.cpp

@@ -56,7 +56,7 @@ static int print_help()
 
 
 static void
-StereoCalib(const vector<string>& imagelist, Size boardSize, bool useCalibrated=true, bool showRectified=true)
+StereoCalib(const vector<string>& imagelist, Size boardSize,bool displayCorners = false, bool useCalibrated=true, bool showRectified=true)
 {
     if( imagelist.size() % 2 != 0 )
     {
@@ -64,7 +64,6 @@ StereoCalib(const vector<string>& imagelist, Size boardSize, bool useCalibrated=
         return;
     }
 
-    bool displayCorners = false;//true;
     const int maxScale = 2;
     const float squareSize = 1.f;  // Set this to your actual square size
     // ARRAY AND VECTOR STORAGE:
@@ -165,8 +164,8 @@ StereoCalib(const vector<string>& imagelist, Size boardSize, bool useCalibrated=
     cout << "Running stereo calibration ...\n";
 
     Mat cameraMatrix[2], distCoeffs[2];
-    cameraMatrix[0] = Mat::eye(3, 3, CV_64F);
+    cameraMatrix[0] = initCameraMatrix2D(objectPoints,imagePoints[0],imageSize,0);
-    cameraMatrix[1] = Mat::eye(3, 3, CV_64F);
+    cameraMatrix[1] = initCameraMatrix2D(objectPoints,imagePoints[1],imageSize,0);
     Mat R, T, E, F;
 
     double rms = stereoCalibrate(objectPoints, imagePoints[0], imagePoints[1],
@@ -175,6 +174,7 @@ StereoCalib(const vector<string>& imagelist, Size boardSize, bool useCalibrated=
                     imageSize, R, T, E, F,
                     CALIB_FIX_ASPECT_RATIO +
                     CALIB_ZERO_TANGENT_DIST +
+                    CALIB_USE_INTRINSIC_GUESS +
                     CALIB_SAME_FOCAL_LENGTH +
                     CALIB_RATIONAL_MODEL +
                     CALIB_FIX_K3 + CALIB_FIX_K4 + CALIB_FIX_K5,
@@ -209,7 +209,7 @@ StereoCalib(const vector<string>& imagelist, Size boardSize, bool useCalibrated=
         }
         npoints += npt;
     }
-    cout << "average reprojection err = " <<  err/npoints << endl;
+    cout << "average epipolar err = " <<  err/npoints << endl;
 
     // save intrinsic parameters
     FileStorage fs("../data/intrinsics.yml", FileStorage::WRITE);
@@ -399,6 +399,6 @@ int main(int argc, char** argv)
         return print_help();
     }
 
-    StereoCalib(imagelist, boardSize, true, showRectified);
+    StereoCalib(imagelist, boardSize,false, true, showRectified);
     return 0;
 }