How do I use warpPerspective correctly?

1

image
image941×788 145 KB
. The blue positions are from paint. Output is the output from the code.

I have attached the image I am talking about. I want to get the perspective transformation so I can get the card in its canonical form. It does not work and I get a bad output.

This is my code. The points I got from point and they are correct (checked with the function circle() )

void main(){
cv::Mat begin = cv::imread("\cards.jpg");
cv::Mat output;
cv::Point2f Poly2[4] = {
			  cv::Point2f(222,93),
			  cv::Point2f(430,134),
			  cv::Point2f(368,426),
			  cv::Point2f(162,378),

};

cv::Point2f Points[4] = {
			  cv::Point2f(0,0),
			  cv::Point2f(300,0),
			  cv::Point2f(0,600),       //The picture I want to transform to. 
			  cv::Point2f(600,300),

};

for (int i = 0; i < 4; i++) {
	cv::circle(begin, cv::Point2d(Poly2[i].x, Poly2[i].y), 2, cv::Scalar(244, 233, 44), 3, 8, 0);
}

cv::Mat Matrix = cv::getPerspectiveTransform( Points,Poly2);

cv::warpPerspective(begin, output, Matrix, cv::Size(300, 600));

cv::imshow("begin", begin);
cv::imshow("Output", output);
cv::waitKey(0);
     }

I hope you guys can help me.

2

your Points are in the wrong order. if Poly2 is clockwise, starting from tl, the Points have to be so, too.

(you have to swap the last 2 points)

3

It seems that you switched the parameters of getPerspectiveTransform. Try with:

cv::Mat Matrix = cv::getPerspectiveTransform( Poly2,Points); // (source,destination)