轮廓发现与绘制

本节涉及到两个API:

轮廓发现

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

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.

• 注意以下几个参数的意思
  RETR_EXTERNAL 展示最大的轮廓
  RETR_TREE 包括所有轮廓，以TREE结构（有层次感那种）。。。
  RETR_LIST 包括所有结构 ，以list结构。。。。。

轮廓记录方式

CHAIN_APPROX_SIMPLE 四点（左上，左下，右下，右上）表示轮廓
CHAIN_APPROX_NONE 所有的轮廓上的像素点

绘制轮廓

CV_EXPORTS_W void drawContours( InputOutputArray image, InputArrayOfArrays contours,
int contourIdx, const Scalar& color,
int thickness = 1, int lineType = LINE_8,
InputArray hierarchy = noArray(),
int maxLevel = INT_MAX, Point offset = Point() );

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.

可以通过contourIdx 来绘制我们想要的轮廓

实验效果：
RETR_EXTERNAL

RETR_TREE

RETR_LIST

我的示范代码地址：

https://github.com/cyssmile/openCV_learning_notes/blob/master/opencv_test/opencv_027/opencv_027.cpp