Create globalmatting.md

Author: Nerdyvedi

modules/ximgproc/tutorials/globalmatting.md

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+### A Global sampling method for Alpha Matting ###
+
+The Global sampling based method uses all known samples(from a trimap), Unlike other sampling based methods 
+which collect only nearby samples.<br/>
+It first estimates the foreground and background color and then computes the alpha matte.
+
+Input to the model is the Image and the trimap, and the output is the alpha matte.
+
+This [blog post](https://medium.com/vedacv/paper-summary-a-global-sampling-method-for-alpha-matting-490a4217eb2) gives a summary of the paper.
+
+### Results ###
+
+After evaluating this implementation on alphamatting.com, the results are almost as good as the original implementation.
+
+Following were the results:
+
+|     Error type              |      Original implementation    | This implementation  |
+|     -----------             |      ------------------------   | -------------------  |  
+| Sum of absolute differences |       31                        | 31.3                 |
+| Mean square error           |       28.3                      | 29.5                 |
+| Gradient error              |       25                        | 26.3                 |
+| Connectivity error          |       28                        | 36.3                 |
+
+
+Some of the outputs with of this implementation are as follows :
+
+|    Image                    | Trimap                          | Alpha matte(this implementation)  |
+|  --------------             | --------------                  | ------------------------          |
+|![alt text](https://raw.githubusercontent.com/Nerdyvedi/GSOC-Opencv-matting/master/Global-Sampling-Method-for-Alpha-Matting/Input/doll.png "img1" ) |![alt text](https://raw.githubusercontent.com/Nerdyvedi/GSOC-Opencv-matting/master/Global-Sampling-Method-for-Alpha-Matting/Trimap/doll.png "trimap1" ) |![alt text](https://raw.githubusercontent.com/Nerdyvedi/GSOC-Opencv-matting/master/Global-Sampling-Method-for-Alpha-Matting/Results/doll.png "results1" ) |
+|![alt text](https://raw.githubusercontent.com/Nerdyvedi/GSOC-Opencv-matting/master/Global-Sampling-Method-for-Alpha-Matting/Input/troll.png "img2" ) |![alt text](https://raw.githubusercontent.com/Nerdyvedi/GSOC-Opencv-matting/master/Global-Sampling-Method-for-Alpha-Matting/Trimap/troll.png "trimap2" ) |![alt text](https://raw.githubusercontent.com/Nerdyvedi/GSOC-Opencv-matting/master/Global-Sampling-Method-for-Alpha-Matting/Results/troll.png "results2" ) |
+|![alt text](https://raw.githubusercontent.com/Nerdyvedi/GSOC-Opencv-matting/master/Global-Sampling-Method-for-Alpha-Matting/Input/donkey.png "img1" ) |![alt text](https://raw.githubusercontent.com/Nerdyvedi/GSOC-Opencv-matting/master/Global-Sampling-Method-for-Alpha-Matting/Trimap/donkey.png "trimap1" ) |![alt text](https://raw.githubusercontent.com/Nerdyvedi/GSOC-Opencv-matting/master/Global-Sampling-Method-for-Alpha-Matting/Results/donkey.png "results1" ) |