1) Modified cv::ximgproc::niBlackThreshold to support binarization using popular…

1) Modified cv::ximgproc::niBlackThreshold to support binarization using popular Niblack inspired techniques. Sauvola, Wolf, and NICK support added.
2) Changed niblack_thresholding.cpp sample to allow experimentation with these new methods
3) Added/updated docs where appropriate

modules/ximgproc/doc/ximgproc.bib

@@ -222,3 +222,40 @@
   pages={191--196},
   year={1997}
 }
+
+@book{Niblack1985,
+  title={An introduction to digital image processing},
+  author={Niblack, Wayne},
+  year={1985},
+  publisher={Strandberg Publishing Company}
+}
+
+@inproceedings{Sauvola1997,
+  title={Adaptive document binarization},
+  author={Sauvola, Jaakko and Seppanen, Tapio and Haapakoski, Sami and Pietikainen, Matti},
+  booktitle={Document Analysis and Recognition, 1997., Proceedings of the Fourth International Conference on},
+  volume={1},
+  pages={147--152},
+  year={1997},
+  organization={IEEE}
+}
+
+@article{Wolf2004,
+  title={Extraction and recognition of artificial text in multimedia documents},
+  author={Wolf, Christian and Jolion, J-M},
+  journal={Pattern Analysis \& Applications},
+  volume={6},
+  number={4},
+  pages={309--326},
+  year={2004},
+  publisher={Springer}
+}
+
+@inproceedings{Khurshid2009,
+  title={Comparison of Niblack inspired Binarization methods for ancient documents},
+  author={Khurshid, Khurram and Siddiqi, Imran and Faure, Claudie and Vincent, Nicole},
+  booktitle={IS\&T/SPIE Electronic Imaging},
+  pages={72470U--72470U},
+  year={2009},
+  organization={International Society for Optics and Photonics}
+}

modules/ximgproc/include/opencv2/ximgproc.hpp

@@ -79,10 +79,21 @@ enum ThinningTypes{
     THINNING_GUOHALL      = 1  // Thinning technique of Guo-Hall
 };
 
+/**
+* @brief Specifies the binarization method to use in cv::ximgproc::niBlackThreshold
+*/
+enum LocalBinarizationMethods{
+	BINARIZATION_NIBLACK = 0, //!< Classic Niblack binarization. See @cite Niblack1985 .
+	BINARIZATION_SAUVOLA = 1, //!< Sauvola's technique. See @cite Sauvola1997 .
+	BINARIZATION_WOLF = 2,    //!< Wolf's technique. See @cite Wolf2004 .
+	BINARIZATION_NICK = 3     //!< NICK technique. See @cite Khurshid2009 .
+};
+
 //! @addtogroup ximgproc
 //! @{
 
-/** @brief Applies Niblack thresholding to input image.
+/** @brief Performs thresholding on input images using Niblack's technique or some of the
+popular variations it inspired.
 
 The function transforms a grayscale image to a binary image according to the formulae:
 -   **THRESH_BINARY**
@@ -91,8 +102,9 @@ The function transforms a grayscale image to a binary image according to the for
     \f[dst(x,y) =  \fork{0}{if \(src(x,y) > T(x,y)\)}{\texttt{maxValue}}{otherwise}\f]
 where \f$T(x,y)\f$ is a threshold calculated individually for each pixel.
 
-The threshold value \f$T(x, y)\f$ is the mean minus \f$ delta \f$ times standard deviation
-of \f$\texttt{blockSize} \times\texttt{blockSize}\f$ neighborhood of \f$(x, y)\f$.
+The threshold value \f$T(x, y)\f$ is determined based on the binarization method chosen. For
+classic Niblack, it is the mean minus \f$ k \f$ times standard deviation of
+\f$\texttt{blockSize} \times\texttt{blockSize}\f$ neighborhood of \f$(x, y)\f$.
 
 The function can't process the image in-place.
 
@@ -103,14 +115,17 @@ used with the THRESH_BINARY and THRESH_BINARY_INV thresholding types.
 @param type Thresholding type, see cv::ThresholdTypes.
 @param blockSize Size of a pixel neighborhood that is used to calculate a threshold value
 for the pixel: 3, 5, 7, and so on.
-@param delta Constant multiplied with the standard deviation and subtracted from the mean.
-Normally, it is taken to be a real number between 0 and 1.
+@param k The user-adjustable parameter used by Niblack and inspired techniques. For Niblack, this is
+normally a value between 0 and 1 that is multiplied with the standard deviation and subtracted from
+the mean.
+@param binarizationMethod Binarization method to use. By default, Niblack's technique is used.
+Other techniques can be specified, see cv::ximgproc::LocalBinarizationMethods.
 
 @sa  threshold, adaptiveThreshold
  */
 CV_EXPORTS_W void niBlackThreshold( InputArray _src, OutputArray _dst,
                                     double maxValue, int type,
-                                    int blockSize, double delta );
+                                    int blockSize, double k, int binarizationMethod = BINARIZATION_NIBLACK );
 
 /** @brief Applies a binary blob thinning operation, to achieve a skeletization of the input image.
 

modules/ximgproc/samples/niblack_thresholding.cpp

@@ -16,6 +16,7 @@ Mat_<uchar> src;
 int k_ = 8;
 int blockSize_ = 11;
 int type_ = THRESH_BINARY;
+int method_ = BINARIZATION_NIBLACK;
 
 void on_trackbar(int, void*);
 
@@ -34,6 +35,7 @@ int main(int argc, char** argv)
     namedWindow("Niblack", WINDOW_AUTOSIZE);
     createTrackbar("k", "Niblack", &k_, 20, on_trackbar);
     createTrackbar("blockSize", "Niblack", &blockSize_, 30, on_trackbar);
+    createTrackbar("method", "Niblack", &method_, 3, on_trackbar);
     createTrackbar("threshType", "Niblack", &type_, 4, on_trackbar);
     on_trackbar(0, 0);
     waitKey(0);
@@ -47,7 +49,8 @@ void on_trackbar(int, void*)
     int blockSize = 2*(blockSize_ >= 1 ? blockSize_ : 1) + 1; // 3,5,7,...,61
     int type = type_;  // THRESH_BINARY, THRESH_BINARY_INV,
                        // THRESH_TRUNC, THRESH_TOZERO, THRESH_TOZERO_INV
+    int method = method_; //BINARIZATION_NIBLACK, BINARIZATION_SAUVOLA, BINARIZATION_WOLF, BINARIZATION_NICK
     Mat dst;
-    niBlackThreshold(src, dst, 255, type, blockSize, k);
+    niBlackThreshold(src, dst, 255, type, blockSize, k, method);
     imshow("Niblack", dst);
 }

modules/ximgproc/src/niblack_thresholding.cpp

@@ -47,12 +47,15 @@ namespace cv {
 namespace ximgproc {
 
 void niBlackThreshold( InputArray _src, OutputArray _dst, double maxValue,
-        int type, int blockSize, double delta )
+        int type, int blockSize, double k, int binarizationMethod )
 {
     // Input grayscale image
     Mat src = _src.getMat();
     CV_Assert(src.channels() == 1);
     CV_Assert(blockSize % 2 == 1 && blockSize > 1);
+    if (binarizationMethod == BINARIZATION_SAUVOLA) {
+	CV_Assert(src.depth() == CV_8U);
+    }
     type &= THRESH_MASK;
 
     // Compute local threshold (T = mean + k * stddev)
@@ -61,13 +64,35 @@ void niBlackThreshold( InputArray _src, OutputArray _dst, double maxValue,
     Mat thresh;
     {
         // note that: Var[X] = E[X^2] - E[X]^2
-        Mat mean, sqmean, stddev;
+        Mat mean, sqmean, variance, stddev, sqrtVarianceMeanSum;
+        double srcMin, stddevMax;
         boxFilter(src, mean, CV_32F, Size(blockSize, blockSize),
                 Point(-1,-1), true, BORDER_REPLICATE);
         sqrBoxFilter(src, sqmean, CV_32F, Size(blockSize, blockSize),
                 Point(-1,-1), true, BORDER_REPLICATE);
-        sqrt(sqmean - mean.mul(mean), stddev);
-        thresh = mean + stddev * static_cast<float>(delta);
+        variance = sqmean - mean.mul(mean);
+        sqrt(variance, stddev);
+        switch (binarizationMethod)
+        {
+        case BINARIZATION_NIBLACK:
+		thresh = mean + stddev * static_cast<float>(k);
+		break;
+        case BINARIZATION_SAUVOLA:
+		thresh = mean.mul(1. + static_cast<float>(k) * (stddev / 128.0 - 1.));
+		break;
+        case BINARIZATION_WOLF:
+		minMaxIdx(src, &srcMin);
+		minMaxIdx(stddev, NULL, &stddevMax);
+		thresh = mean - static_cast<float>(k) * (mean - srcMin - stddev.mul(mean - srcMin) / stddevMax);
+		break;
+        case BINARIZATION_NICK:
+		sqrt(variance + sqmean, sqrtVarianceMeanSum);
+		thresh = mean + static_cast<float>(k) * sqrtVarianceMeanSum;
+		break;
+        default:
+		CV_Error( CV_StsBadArg, "Unknown binarization method" );
+		break;
+        }
         thresh.convertTo(thresh, src.depth());
     }