C++ 在 OpenCV 的 findContours () 中使用层次结构？
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Using hierarchy in findContours () in OpenCV?
提问by fdh
When finding contours, I used the CV_RETR_CCOMP argument. This is supposed to create a two level hierarchy - the the first level are for outer contours, the second level are for boundaries of the holes. However, I have never used a hierarchy before so I am not familiar with this.
查找轮廓时，我使用了 CV_RETR_CCOMP 参数。这应该创建一个两级层次结构 - 第一级用于外轮廓，第二级用于孔的边界。但是，我之前从未使用过层次结构，所以我对此并不熟悉。
Could someone instruct my on how to access the boundaries of the holes only? I want to disregard the outer contours and only draw the hole boundaries. Code examples will be appreciated. I am using the C++ interface not the C, so please don't suggest C functions (i.e. use findContours () instead of cvFindContours ()).
有人可以指导我如何仅访问孔的边界吗？我想忽略外部轮廓，只绘制孔边界。代码示例将不胜感激。我使用的是 C++ 接口而不是 C，所以请不要建议使用 C 函数（即使用 findContours () 而不是 cvFindContours ()）。
回答by mathematical.coffee
The hierarchy returned by findContourshas the following form:
hierarchy[idx][{0,1,2,3}]={next contour (same level), previous contour (same level), child contour, parent contour}
返回的层次结构findContours具有以下形式：
hierarchy[idx][{0,1,2,3}]={next contour (same level), previous contour (same level), child contour, parent contour}
CV_RETR_CCOMP, returns a hierarchy of outer contours and holes.
This means elements 2 and 3 of hierarchy[idx]have at most one of these not equal to -1: that is, each element has either no parent or child, or a parent but no child, or a child but no parent.
CV_RETR_CCOMP, 返回外部轮廓和孔的层次结构。这意味着元素 2 和 3 中hierarchy[idx]至多有一个不等于 -1：也就是说，每个元素要么没有父元素，要么没有子元素，或者有父元素但没有子元素，或者有子元素但没有父元素。
An element with a parent but no child would be a boundary of a hole.
有父元素但没有子元素的元素将是孔的边界。
That means you basically go through hierarchy[idx]and draw anything with hierarchy[idx][3]>-1.
这意味着你基本上经历hierarchy[idx]和画任何东西hierarchy[idx][3]>-1。
Something like (works in Python, but haven't tested the C++. Idea is fine though.):
类似的东西（在 Python 中工作，但还没有测试 C++。虽然想法很好。）：
findContours( image, contours, hierarchy, CV_RETR_CCOMP, CV_CHAIN_APPROX_SIMPLE );
if ( !contours.empty() && !hierarchy.empty() ) {
// loop through the contours/hierarchy
for ( int i=0; i<contours.size(); i++ ) {
// look for hierarchy[i][3]!=-1, ie hole boundaries
if ( hierarchy[i][3] != -1 ) {
// random colour
Scalar colour( (rand()&255), (rand()&255), (rand()&255) );
drawContours( outImage, contours, i, colour );
}
}
}
回答by Shmuel Fine
AFAIK when using CV_RETR_CCOMP, all the holes are on the same level.
AFAIK 使用 CV_RETR_CCOMP 时，所有孔都在同一水平上。
int firstHoleIndex = hierarchy[0][2];
for (int i = firstHoleIndex; i >= 0 ; i = hierarchy[i][0])
// contours.at(i) is a hole. Do something with it.
