OpenCV之findContours函数解读

OpenCV之findContours函数解读

http://blog.csdn.net/u012062327

参考：opencv documentation

findContours（）函数原型：

void findContours(InputOutputArray image, OutputArrayOfArrays contours, OutputArray hierarchy, int mode, int method, Point offset=Point()) 

Parameters: 
image – Source, an 8-bit single-channel image.

contours – Detected contours. Each contour is stored as a vector of points. 

hierarchy – Optional output 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] , hiearchy[i][1] , hiearchy[i][2] , and hiearchy[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. 

mode – Contour retrieval mode 
CV_RETR_EXTERNAL retrieves only the extreme outer contours. It sets hierarchy[i][2]=hierarchy[i][3]=-1 for all the contours. 
CV_RETR_LIST retrieves all of the contours without establishing any hierarchical relationships. 
CV_RETR_CCOMP retrieves all of the contours and organizes them into a two-level hierarchy. At the top level, there are external boundaries of the components. At the second level, there are boundaries of the holes. If there is another contour inside a hole of a connected component, it is still put at the top level. 
CV_RETR_TREE retrieves all of the contours and reconstructs a full hierarchy of nested contours. This full hierarchy is built and shown in the OpenCV contours.c demo. 

method – Contour approximation method 
CV_CHAIN_APPROX_NONE stores absolutely all the contour points. That is, any 2 subsequent points (x1,y1) and (x2,y2) of the contour will be either horizontal, vertical or diagonal neighbors, that is, max(abs(x1-x2),abs(y2-y1))==1. 
CV_CHAIN_APPROX_SIMPLE compresses horizontal, vertical, and diagonal segments and leaves only their end points. For example, an up-right rectangular contour is encoded with 4 points. 
CV_CHAIN_APPROX_TC89_L1,CV_CHAIN_APPROX_TC89_KCOS applies one of the flavors of the Teh-Chin chain approximation algorithm. 
 
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.  



参数说明
image输入图像必须为一个2值单通道图像
contours参数为检测的轮廓数组，每一个轮廓用一个point类型的vector表示
hiararchy参数和轮廓个数相同，每个轮廓contours[ i ]对应4个hierarchy元素hierarchy[ i ][ 0 ] ~hierarchy[ i ][ 3 ]，分别表示后一个轮廓、前一个轮廓、父轮廓、内嵌轮廓的索引编号，如果没有对应项，该值设置为负数。
mode表示轮廓的检索模式
CV_RETR_EXTERNAL表示只检测外轮廓
CV_RETR_LIST检测的轮廓不建立等级关系
CV_RETR_CCOMP建立两个等级的轮廓，上面的一层为外边界，里面的一层为内孔的边界信息。如果内孔内还有一个连通物体，这个物体的边界也在顶层。
CV_RETR_TREE建立一个等级树结构的轮廓。具体参考contours.c这个demo
method为轮廓的近似办法
CV_CHAIN_APPROX_NONE存储所有的轮廓点，相邻的两个点的像素位置差不超过1，即max（abs（x1-x2），abs（y2-y1））==1
CV_CHAIN_APPROX_SIMPLE压缩水平方向，垂直方向，对角线方向的元素，只保留该方向的终点坐标，例如一个矩形轮廓只需4个点来保存轮廓信息
CV_CHAIN_APPROX_TC89_L1，CV_CHAIN_APPROX_TC89_KCOS使用teh-Chinl chain 近似算法
offset表示代表轮廓点的偏移量，可以设置为任意值。对ROI图像中找出的轮廓，并要在整个图像中进行分析时，这个参数还是很有用的。

findContours后会对输入的2值图像改变，所以如果不想改变该2值图像，需创建新mat来存放，findContours后的轮廓信息contours可能过于复杂不平滑，可以用approxPolyDP函数对该多边形曲线做适当近似
contourArea函数可以得到当前轮廓包含区域的大小，方便轮廓的筛选
findContours经常与drawContours配合使用，用来将轮廓绘制出来。
drawContours函数原型：
drawContours(InputOutputArray image, InputArrayOfArrays contours, int contourIdx, const Scalar& color, int thickness=1, int lineType=8, InputArray hierarchy=noArray(), int maxLevel=INT_MAX, Point offset=Point() ) 

image表示目标图像；
contours表示输入的轮廓组，每一组轮廓由点vector构成；
contourIdx指明画第几个轮廓，如果该参数为负值，则画全部轮廓；
color为轮廓的颜色；
thickness为轮廓的线宽，如果为负值或CV_FILLED表示填充轮廓内部；
lineType为线型；
hierarchy轮廓结构信息；
maxLevel 绘画轮廓的最大值，只有hierarchy存在是才有用

    drawContours得到了复杂轮廓往往不适合特征的检测，这里再介绍一个点集凸包络的提取函数convexHull，输入参数就可以是contours组中的一个轮廓，返回外凸包络的点集还可以得到轮廓的外包矩形，使用函数boundingRect，如果想得到旋转的外包络矩形，使用函数minAreaRect，返回值为RotatedRect；也可以得到轮廓的外包络圆，对应的函数为minEnclosingCircle；想得到轮廓的外包络椭圆，对应的函数为fitEllipse，返回值也是RotatedRect，可以用ellipse函数画出对应的椭圆
    如果想获得一点与多边形封闭轮廓的信息，可以调用pointPolygonTest函数，这个函数返回值为该点距离轮廓最近边界的距离，为正值为在轮廓内部，负值为在轮廓外部，0表示在边界上。