从列表中检索轮廓时，轮廓显示的是点而不是曲线，否则显示曲线 [英] Contour shows dots rather than a curve when retrieving it from the list, but shows the curve otherwise

问题描述

我正在找到一个阈值图像的轮廓并将其绘制为:

I'm finding the contour of a thresholded image and drawing it like so:

self.disc_contour = cv2.findContours(self.thresh.copy(), cv2.RETR_LIST,cv2.CHAIN_APPROX_NONE)[1] 
cv2.drawContours(self.original_image, self.disc_contour, -1, (0,255,0), 2)

然后我得到所需的轮廓:

and I get the contour as desired:

(忽略内圆.外部是上下文中的轮廓)

(ignore the inner circle. The outer part is the contour in context)

但是，如果我将drawContour函数中的self.disc_contour更改为self.disc_contour[0]，则会得到以下结果:

But if I change self.disc_contour in the drawContour function to self.disc_contour[0] I get the following result:

可能是什么原因?

推荐答案

NB:特定于OpenCV 3.x

cv2.findContours的第二个结果是轮廓列表. cv.drawContours的第二个参数应该是轮廓列表.

The second result from cv2.findContours is a list of contours. The second parameter of cv.drawContours should be a list of contours.

轮廓表示为点的列表(或数组).每个点都是一个坐标列表.

A contour is represented as a list (or array) of points. Each point is a list of coordinates.

有多种方法可以仅绘制单个轮廓:

There are multiple ways how to draw only a single contour:

import cv2

src_img = cv2.imread("blob.png")
gray_img = cv2.cvtColor(src_img, cv2.COLOR_BGR2GRAY)

contours = cv2.findContours(gray_img, cv2.RETR_LIST, cv2.CHAIN_APPROX_NONE)[1]

print(contours)

# Choose one:    

# Draw only first contour from the list
cv2.drawContours(src_img, contours, 0, (0,255,0), 2)
# Pass only the first contour (v1)
cv2.drawContours(src_img, [contours[0]], -1, (0,255,0), 2)
# Pass only the first contour (v2)
cv2.drawContours(src_img, contours[0:1], -1, (0,255,0), 2)


cv2.imshow("Contour", src_img)
cv2.waitKey()

示例输入图像:

当我们检查cv2.findContours的结果时，看到点的原因就很明显了-共有4级嵌套.

When we inspect the result of cv2.findContours, the reason why you were seeing dots becomes apparent -- there are 4 levels of nesting.

[
    array([
        [[ 95,  61]], # Point 0
        [[ 94,  62]], # Point 1
        [[ 93,  62]],
        ... <snip> ...
        [[ 98,  61]],
        [[ 97,  61]],
        [[ 96,  61]]
    ]) # Contour 0
]

根据此答案开头的定义，我们可以看到在这种情况下，这些点被包装在其他列表中，例如[[ 98, 61]]. OpenCV显然可以正确处理此问题-我想这是作为一项功能来实现的.

According to the definitions at the beginning of this answer, we can see that the points in this case are wrapped in an additional list, e.g. [[ 98, 61]]. OpenCV apparently deals with this correctly - I suppose this was intended as a feature.

如果仅通过使用contours的第一个元素来删除外部列表，则可以有效地将每个点转换为包含单个点的单独轮廓.

If we remove the outer list by using only the first element of contours, we effectively turn each point into a separate contour containing a single point.

array([
    [
        [ 95,  61] # Point 0
    ], # Contour 0
    [
        [ 94,  62] # Point 0
    ], # Contour 1
    ... and so on
])

这篇关于从列表中检索轮廓时，轮廓显示的是点而不是曲线，否则显示曲线的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！