最近在做關於圖像識別的東西，用到了尋找輪廓函數cv2.findContours和繪製輪廓函數cv2.drawContours
先看看cv2.findContours的定義：
如果嫌囉嗦（不想看英語），直接下拉，下面我有寫用法與總結
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<std::vector<cv::Point> >).
. @param hierarchy Optional output vector (e.g. std::vector<cv::Vec4i>), 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的定義：
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，看的頭蒙。我來總結一下吧
cv2.findContours一共有：
def findContours(image, mode, method, contours=None, hierarchy=None, offset=None):
這麼幾個參數：
image : 傳入的圖像，
注意一定要是二值圖像，（cv2讀圖片時，用 cv2.imread(imgpath , 0) 即可，注意第二個參數是0）
mode : 輪廓檢索方式
cv2.RETR_TREE:建立一個等級樹結構的輪廓
cv2.RETR_EXTERNAL:只檢測外輪廓
cv2.RETR_LIST:檢測的輪廓不建立等級關係
cv2.RETR_CCOMP:建立兩個等級的輪廓，上面一層爲外邊界，裏面一層爲內孔的邊界信息
method : 輪廓近似方法
cv2.CHAIN_APPROX_NONE: 存儲所有邊界點
cv2.CHAIN_APPROX_SIMPLE: 壓縮垂直、水平、對角方向，只保留端點
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後返回值只有兩個：contours, hierarchy，之前的返回值有三個image, contours, hierarchy
一般這樣搭配使用：
contours, hierarchy = cv2.findContours(img, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_NONE)
當使用cv2.CHAIN_APPROX_NONE存儲所有輪廓點時，返回的contours是一個列表，每一項是一個輪廓信息，每一個輪廓信息是一個(n , 1 , 2)維的numpy.ndarray，n是座標的個數。
利用cv2.drawContours可以將之前找到的輪廓畫出來
def drawContours(image, contours, contourIdx, color, thickness=None, lineType=None, hierarchy=None, maxLevel=None, offset=None):
別看有這麼多參數，實際上用的最多的是前幾個：
image：
傳入的圖像，在這張圖上畫，注意，drawContours本身會直接修改image，不需要返回值，所以在傳入圖片前先備份一下原圖，一般用：img = image * 1，這裏image是np.array，乘一相當於複製了一份，注注意：如果img = image這樣直接賦值，是沒有用的，修改img照樣會修改image，這是因爲深拷貝與淺拷貝的不同，這裏image * 1是一個小技巧，但正規的深拷貝應該用copy這個函數，（還得導包，所以偷個懶，直接乘 1）
contours:
這個是一個列表，注意：是一個列表，是一個列表，是一個列表，可以直接將剛纔用findContours找出來的contours給他，也可以是自己給出的一組座標信息。
contourIdx：顧名思義，就是要畫的輪廓下標, -1表示畫出全部的輪廓。比如剛纔找出來的contours，在將-1給到contourIdx，就會把所有輪廓畫出來。
color：畫線的顏色：例如（0,0,255）
thickness: 畫線的粗細，-1則爲填充
一般用法：
cv2.drawContours(img, contours, -1, (255, 255, 255), -1)
注意：如果想畫contours中的某個輪廓，一定要取出來放到列表裏，像這樣：
c = []
c.append(contours[3])
cv2.drawContours(img, c, -1, (255, 255, 255), -1)