OpenCV测量矩形图像大小

Question

I have an app that finds an object in a frame and uses warpPerspective to correct the image to be square. In the course of doing so you specify an output image size. However, I want to know how to do so without harming its apparent size. How can I unwarp the 4-corners of the image without changing the size of the image? I don't need the image itself, I just want to measure its height and width in pixels within the original image.

Get a transform matrix that will square up the corners.

std::vector<cv::Point2f> transformedPoints;
cv::Mat M = cv::getPerspectiveTransform(points, objectCorners);
cv::perspectiveTransform(points, transformedPoints, M);

This will square up the image, but in terms of the objectCorners coordinate system. Which is -0.5f to 0.5f not the original image plane.

BoundingRect almost does what I want.

cv::Rect boundingRectangle = cv::boundingRect(points);

But as the documentation states

The function calculates and returns the minimal up-right bounding rectangle for the specified point set.

And what I want is the bounding rectangle after it has been squared-up, not without squaring it up.

Answer 1

According to my understanding to your post, here is something which should help you. OpenCV perspective transform example .

Update if it still doesn't help you out in finding the height and width within the image

Answer 2

Minimum bounding rect of the points

cv::RotatedRect box = cv::minAreaRect(cv::Mat(points));

As the minAreaRect reference on OpenCV's website states

Finds a rotated rectangle of the minimum area enclosing the input 2D point set.

You can call box.size and get the width and height.