将旋转的矩形绘制成图像

Question

I am trying to draw rotatec rectangle using openCV and python so far my code:

rect = cv2.minAreaRect(boxes[i][1:])
box = cv2.boxPoints(rect)
box = np.int0(box)
cv2.drawContours(image,[box],0,(0,0,255),2)

where boxes[i][1:] is array of coordinations [xmin,ymin,xmax,ymax]

and image is image to draw recrangle on.

However this keeps throwing

   rect = cv2.minAreaRect(boxes[i][1:])
cv2.error: OpenCV(3.4.1) C:\bld\opencv_1520732670222\work\opencv-3.4.1\modules\imgproc\src\convhull.cpp:137: error: (-215) total >= 0 && (depth == 5 || depth == 4) in function cv::convexHull 

Wha does it mean and how can i fix it?

Answer 1

According to the doc of cv2.minAreaRect , the function requires a pointset as input. So if you give a (xmin, ymin, xmax, ymax) tuple, it will be considered as a pointset composed of 2 single points (xmin, ymin) and (xmax, ymax) . Thus finding the rotated rectangle with minimum area, will lead to a null-area rectangle (the line segment between the 2 points), which is likely to generate the error you obtain. I'm not 100% confident with that answer, but I guess that it might be right.

So I suggest to replace [(xmin, ymin), (xmax, ymax)] by [(xmin, ymin), (xmin, ymax), (xmax, ymin), (xmax, ymax)] and test if it solves the problem...