Improved 3D OpenPose doc

Author: gineshidalgo99

doc/openpose_3d_reconstruction_demo.md

@@ -39,7 +39,11 @@ This demo assumes n arbitrary stereo cameras, FLIR company (former Point Grey).
 
 
 ### Calibrate Cameras
-You must manually get the intrinsic and extrinsic parameters of your cameras and introduce them on: `openpose3d\cameraParameters.hpp`. We used the 8-distortion-parameter version of OpenCV. By default, the program uses 3 cameras, but you can add or remove cameras from `cameraParameters.hpp` by adding or removing elements to `INTRINSICS`, `DISTORTIONS` and `M_EACH_CAMERA`. `INTRINSICS` corresponds to the intrinsic parameters, `DISTORTIONS` to the distortion coefficients, and `M_EACH_CAMERA` corresponds to the extrinsic parameters of the cameras with respect to camera 1 as origin.
+The user must manually get the intrinsic and extrinsic parameters of your cameras and introduce them on: `include\openpose3d\cameraParameters.hpp`.
+
+The program uses 3 cameras by default, but cameras can be added or removed from `cameraParameters.hpp` by adding or removing elements to `INTRINSICS`, `DISTORTIONS` and `M_EACH_CAMERA`. `INTRINSICS` corresponds to the intrinsic parameters, `DISTORTIONS` to the distortion coefficients, and `M_EACH_CAMERA` corresponds to the extrinsic parameters of the cameras with respect to camera 1, i.e. camera 1 is considered the coordinates origin.
+
+3D OpenPose uses the 8-distortion-parameter version of OpenCV by default. Internally, the distortion parameters are used by the OpenCV function [undistort()](http://docs.opencv.org/3.2.0/da/d54/group__imgproc__transform.html#ga69f2545a8b62a6b0fc2ee060dc30559d) to rectify the images. This function can take either 4-, 5- or 8-parameter distortion coefficients (OpenCV 3.X also adds a 12- and 14-parameter alternatives). Therefore, either version (4, 5, 8, 12 or 14) will work in 3D OpenPose, the user just needs to modify the `DISTORTION_i` variables in `cameraParameters.hpp` with the desired number of distortion parameters for each camera.