Merge pull request #8150 from cclauss/patch-1

cv2.findContours() no longer modifies source image (#8150)

* cv2.findContours() no longer modifies source image

Since OpenCV 3.2, cv2.findContours() no longer modifies the source image but returns a modified image as the first of three return parameters.  ??? Do I have that correct ???

Also fixed up the code blocks to be valid markdown.

* ```python --> @code{.py}

Enables syntax highlighting in docs.

doc/py_tutorials/py_imgproc/py_contours/py_contours_begin/py_contours_begin.markdown

@@ -17,8 +17,7 @@ detection and recognition.
 
 -   For better accuracy, use binary images. So before finding contours, apply threshold or canny
     edge detection.
--   findContours function modifies the source image. So if you want source image even after
-    finding contours, already store it to some other variables.
+-   Since OpenCV 3.2, findContours() no longer modifies the source image but returns a modified image as the first of three return parameters.
 -   In OpenCV, finding contours is like finding white object from black background. So remember,
     object to be found should be white and background should be black.
 
@@ -28,12 +27,12 @@ import numpy as np
 import cv2
 
 im = cv2.imread('test.jpg')
-imgray = cv2.cvtColor(im,cv2.COLOR_BGR2GRAY)
-ret,thresh = cv2.threshold(imgray,127,255,0)
-im2, contours, hierarchy = cv2.findContours(thresh,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)
+imgray = cv2.cvtColor(im, cv2.COLOR_BGR2GRAY)
+ret, thresh = cv2.threshold(imgray, 127, 255, 0)
+im2, contours, hierarchy = cv2.findContours(thresh, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)
 @endcode
 See, there are three arguments in **cv2.findContours()** function, first one is source image, second
-is contour retrieval mode, third is contour approximation method. And it outputs the contours and
+is contour retrieval mode, third is contour approximation method. And it outputs a modified image, the contours and
 hierarchy. contours is a Python list of all the contours in the image. Each individual contour is a
 Numpy array of (x,y) coordinates of boundary points of the object.
 
@@ -49,15 +48,15 @@ contours which should be passed as a Python list, third argument is index of con
 drawing individual contour. To draw all contours, pass -1) and remaining arguments are color,
 thickness etc.
 
-To draw all the contours in an image:
+* To draw all the contours in an image:
 @code{.py}
 cv2.drawContours(img, contours, -1, (0,255,0), 3)
 @endcode
-To draw an individual contour, say 4th contour:
+* To draw an individual contour, say 4th contour:
 @code{.py}
 cv2.drawContours(img, contours, 3, (0,255,0), 3)
 @endcode
-But most of the time, below method will be useful:
+* But most of the time, below method will be useful:
 @code{.py}
 cnt = contours[4]
 cv2.drawContours(img, [cnt], 0, (0,255,0), 3)