added description of LATCH and fixed indentation

Author: GilLevi

modules/xfeatures2d/include/opencv2/xfeatures2d.hpp

@@ -152,6 +152,19 @@ class CV_EXPORTS LUCID : public DescriptorExtractor
 /** latch Class for computing the LATCH descriptor.
 If you find this code useful, please add a reference to the following paper in your work:
 Gil Levi and Tal Hassner, "LATCH: Learned Arrangements of Three Patch Codes", arXiv preprint arXiv:1501.03719, 15 Jan. 2015
+
+LATCH is a binary descriptor based on learned comparisons of triplets of image patches.
+
+* bytes is the size of the descriptor - can be 64, 32, 16, 8, 4, 2 or 1
+* rotationInvariance - whether or not the descriptor should compansate for orientation changes.
+* half_ssd_size - the size of half of the mini-patches size. For example, if we would like to compare triplets of patches of size 7x7x
+    then the half_ssd_size should be (7-1)/2 = 3.
+
+Note: the descriptor can be coupled with any keypoint extractor. The only demand is that if you use set rotationInvariance = True then 
+    you will have to use an extractor which estimates the patch orientation (in degrees). Examples for such extractors are ORB and SIFT.
+
+Note: a complete example can be found under /samples/cpp/tutorial_code/xfeatures2D/latch_match.cpp
+
 */
 class CV_EXPORTS LATCHDescriptorExtractor : public DescriptorExtractor
 {