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Merge pull request #8150 from cclauss/patch-1

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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.
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cclauss 2017-02-10 13:12:11 +01:00 committed by Alexander Alekhin
parent 7c3b415d6a
commit a347569350
1 changed files with 8 additions and 9 deletions
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17
doc/py_tutorials/py_imgproc/py_contours/py_contours_begin/py_contours_begin.markdown
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@ -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)