pyhton opencv中cv2.findContours与cv2.drawContours

最近在做关于图像识别的东西，用到了寻找轮廓函数cv2.findContours和绘制轮廓函数cv2.drawContours
先看看cv2.findContours的定义：
如果嫌啰嗦（不想看英语），直接下拉，下面我有写用法与总结

def findContours(image, mode, method, contours=None, hierarchy=None, offset=None): # real signature unknown; restored from __doc__
    """
    findContours(image, mode, method[, contours[, hierarchy[, offset]]]) -> contours, hierarchy
    .   @brief Finds contours in a binary image.
    .   
    .   The function retrieves contours from the binary image using the algorithm @cite Suzuki85 . The contours
    .   are a useful tool for shape analysis and object detection and recognition. See squares.cpp in the
    .   OpenCV sample directory.
    .   @note Since opencv 3.2 source image is not modified by this function.
    .   
    .   @param image Source, an 8-bit single-channel image. Non-zero pixels are treated as 1's. Zero
    .   pixels remain 0's, so the image is treated as binary . You can use #compare, #inRange, #threshold ,
    .   #adaptiveThreshold, #Canny, and others to create a binary image out of a grayscale or color one.
    .   If mode equals to #RETR_CCOMP or #RETR_FLOODFILL, the input can also be a 32-bit integer image of labels (CV_32SC1).
    .   @param contours Detected contours. Each contour is stored as a vector of points (e.g.
    .   std::vector >).
    .   @param hierarchy Optional output vector (e.g. std::vector), containing information about the image topology. It has
    .   as many elements as the number of contours. For each i-th contour contours[i], the elements
    .   hierarchy[i][0] , hierarchy[i][1] , hierarchy[i][2] , and hierarchy[i][3] are set to 0-based indices
    .   in contours of the next and previous contours at the same hierarchical level, the first child
    .   contour and the parent contour, respectively. If for the contour i there are no next, previous,
    .   parent, or nested contours, the corresponding elements of hierarchy[i] will be negative.
    .   @param mode Contour retrieval mode, see #RetrievalModes
    .   @param method Contour approximation method, see #ContourApproximationModes
    .   @param offset Optional offset by which every contour point is shifted. This is useful if the
    .   contours are extracted from the image ROI and then they should be analyzed in the whole image
    .   context.
    """
    pass

cv2.drawContours的定义：

def drawContours(image, contours, contourIdx, color, thickness=None, lineType=None, hierarchy=None, maxLevel=None, offset=None): # real signature unknown; restored from __doc__
    """
    drawContours(image, contours, contourIdx, color[, thickness[, lineType[, hierarchy[, maxLevel[, offset]]]]]) -> image
    .   @brief Draws contours outlines or filled contours.
    .   
    .   The function draws contour outlines in the image if \f$\texttt{thickness} \ge 0\f$ or fills the area
    .   bounded by the contours if \f$\texttt{thickness}<0\f$ . The example below shows how to retrieve
    .   connected components from the binary image and label them: :
    .   @include snippets/imgproc_drawContours.cpp
    .   
    .   @param image Destination image.
    .   @param contours All the input contours. Each contour is stored as a point vector.
    .   @param contourIdx Parameter indicating a contour to draw. If it is negative, all the contours are drawn.
    .   @param color Color of the contours.
    .   @param thickness Thickness of lines the contours are drawn with. If it is negative (for example,
    .   thickness=#FILLED ), the contour interiors are drawn.
    .   @param lineType Line connectivity. See #LineTypes
    .   @param hierarchy Optional information about hierarchy. It is only needed if you want to draw only
    .   some of the contours (see maxLevel ).
    .   @param maxLevel Maximal level for drawn contours. If it is 0, only the specified contour is drawn.
    .   If it is 1, the function draws the contour(s) and all the nested contours. If it is 2, the function
    .   draws the contours, all the nested contours, all the nested-to-nested contours, and so on. This
    .   parameter is only taken into account when there is hierarchy available.
    .   @param offset Optional contour shift parameter. Shift all the drawn contours by the specified
    .   \f$\texttt{offset}=(dx,dy)\f$ .
    .   @note When thickness=#FILLED, the function is designed to handle connected components with holes correctly
    .   even when no hierarchy date is provided. This is done by analyzing all the outlines together
    .   using even-odd rule. This may give incorrect results if you have a joint collection of separately retrieved
    .   contours. In order to solve this problem, you need to call #drawContours separately for each sub-group
    .   of contours, or iterate over the collection using contourIdx parameter.
    """
    pass

好吧，一堆英文，看的头蒙。我来总结一下吧
cv2.findContours一共有：

def findContours(image, mode, method, contours=None, hierarchy=None, offset=None):

这么几个参数：
image : 传入的图像，
注意一定要是二值图像，（cv2读图片时，用 cv2.imread(imgpath , 0) 即可，注意第二个参数是0）

mode : 轮廓检索方式
cv2.RETR_TREE:建立一个等级树结构的轮廓
cv2.RETR_EXTERNAL:只检测外轮廓
cv2.RETR_LIST:检测的轮廓不建立等级关系
cv2.RETR_CCOMP:建立两个等级的轮廓，上面一层为外边界，里面一层为内孔的边界信息

method : 轮廓近似方法

cv2.CHAIN_APPROX_NONE: 存储所有边界点
cv2.CHAIN_APPROX_SIMPLE: 压缩垂直、水平、对角方向，只保留端点
cv2.CHAIN_APPROX_TX89_L1: 使用teh-Chini近似算法
cv2.CHAIN_APPROX_TC89_KCOS: 使用teh-Chini近似算法

返回值：注意，在opencv3.2后返回值只有两个：contours, hierarchy，之前的返回值有三个image, contours, hierarchy

一般这样搭配使用：

contours, hierarchy = cv2.findContours(img, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_NONE)

当使用cv2.CHAIN_APPROX_NONE存储所有轮廓点时，返回的contours是一个列表，每一项是一个轮廓信息，每一个轮廓信息是一个(n , 1 , 2)维的numpy.ndarray，n是坐标的个数。

利用cv2.drawContours可以将之前找到的轮廓画出来

def drawContours(image, contours, contourIdx, color, thickness=None, lineType=None, hierarchy=None, maxLevel=None, offset=None):

别看有这么多参数，实际上用的最多的是前几个：
image：
传入的图像，在这张图上画，注意，drawContours本身会直接修改image，不需要返回值，所以在传入图片前先备份一下原图，一般用：img = image * 1，这里image是np.array，乘一相当于复制了一份，注注意：如果img = image这样直接赋值，是没有用的，修改img照样会修改image，这是因为深拷贝与浅拷贝的不同，这里image * 1是一个小技巧，但正规的深拷贝应该用copy这个函数，（还得导包，所以偷个懒，直接乘 1）

contours:
这个是一个列表，注意：是一个列表，是一个列表，是一个列表，可以直接将刚才用findContours找出来的contours给他，也可以是自己给出的一组坐标信息。

contourIdx：顾名思义，就是要画的轮廓下标, -1表示画出全部的轮廓。比如刚才找出来的contours，在将-1给到contourIdx，就会把所有轮廓画出来。

color：画线的颜色：例如（0,0,255）

thickness: 画线的粗细，-1则为填充

一般用法：

cv2.drawContours(img, contours, -1, (255, 255, 255), -1)

注意：如果想画contours中的某个轮廓，一定要取出来放到列表里，像这样：

c = []
c.append(contours[3])
cv2.drawContours(img, c, -1, (255, 255, 255), -1)