Add default value for LUCID feature. Add note about color image requirement.

Add CV_Assert for color image.

modules/xfeatures2d/include/opencv2/xfeatures2d.hpp

@@ -132,6 +132,8 @@ public:
 
 An image descriptor that can be computed very fast, while being
 about as robust as, for example, SURF or BRIEF.
+
+@note It requires a color image as input.
  */
 class CV_EXPORTS_W LUCID : public Feature2D
 {
@@ -140,7 +142,7 @@ public:
      * @param lucid_kernel kernel for descriptor construction, where 1=3x3, 2=5x5, 3=7x7 and so forth
      * @param blur_kernel kernel for blurring image prior to descriptor construction, where 1=3x3, 2=5x5, 3=7x7 and so forth
      */
-    CV_WRAP static Ptr<LUCID> create(const int lucid_kernel, const int blur_kernel);
+    CV_WRAP static Ptr<LUCID> create(const int lucid_kernel = 1, const int blur_kernel = 2);
 };
 
 

modules/xfeatures2d/src/lucid.cpp

@@ -100,12 +100,14 @@ namespace cv {
         // gliese581h suggested filling a cv::Mat with descriptors to enable BFmatcher compatibility
         // speed-ups and enhancements by gliese581h
         void LUCIDImpl::compute(InputArray _src, std::vector<KeyPoint> &keypoints, OutputArray _desc) {
-            if (_src.getMat().empty())
+            cv::Mat src_input = _src.getMat();
+            if (src_input.empty())
                 return;
+            CV_Assert(src_input.depth() == CV_8U && src_input.channels() == 3);
 
             Mat_<Vec3b> src;
 
-            blur(_src.getMat(), src, cv::Size(b_kernel, b_kernel));
+            blur(src_input, src, cv::Size(b_kernel, b_kernel));
 
             int x, y, j, d, p, m = (l_kernel*2+1)*(l_kernel*2+1)*3, width = src.cols, height = src.rows, r, c;