Overlay smaller image in a larger image in OpenCV - opencv

Question

I would like to replace a part of the image with my image in Opencv
I used
cvGetPerspectiveMatrix() with a warpmatrix and using cvAnd() and cvOr()
but could not get it to work
This is the code that is currently displaying the image and a white polygon for the replacement image. I would like to replace the white polygon for a pic with any dimension to be scaled and replaced with the region pointed.
While the code is in javacv I could convert it to java even if c code is posted
grabber.start();
while(isDisp() && (image=grabber.grab())!=null){
if (dst_corners != null) {// corners of the image to be replaced
CvPoint points = new CvPoint((byte) 0,dst_corners,0,dst_corners.length);
cvFillConvexPoly(image,points, 4, CvScalar.WHITE, 1, 0);//white polygon covering the replacement image
}
correspondFrame.showImage(image);
}
Any pointers to this will be very helpful.
Update:
I used warpmatrix with this code and I get a black spot for the overlay image
cvSetImageROI(image, cvRect(x1,y1, overlay.width(), overlay.height()));
CvPoint2D32f p = new CvPoint2D32f(4);
CvPoint2D32f q = new CvPoint2D32f(4);
q.position(0).x(0);
q.position(0).y(0);
q.position(1).x((float) overlay.width());
q.position(1).y(0);
q.position(2).x((float) overlay.width());
q.position(2).y((float) overlay.height());
q.position(3).x(0);
q.position(3).y((float) overlay.height());
p.position(0).x((int)Math.round(dst_corners[0]);
p.position(0).y((int)Math.round(dst_corners[1]));
p.position(1).x((int)Math.round(dst_corners[2]));
p.position(1).y((int)Math.round(dst_corners[3]));
p.position(3).x((int)Math.round(dst_corners[4]));
p.position(3).y((int)Math.round(dst_corners[5]));
p.position(2).x((int)Math.round(dst_corners[6]));
p.position(2).y((int)Math.round(dst_corners[7]));
cvGetPerspectiveTransform(q, p, warp_matrix);
cvWarpPerspective(overlay, image, warp_matrix);
I get a black spot for the overlay image and even though the original image is a polygon with 4 vertices the overlay image is set as a rectangle. I believe this is because of the ROI. Could anyone please tell me how to fit the image as is and also why I am getting a black spot instead of the overlay image.

Answer 1

I think cvWarpPerspective(link) is what you are looking for.
So instead of doing
CvPoint points = new CvPoint((byte) 0,dst_corners,0,dst_corners.length);
cvFillConvexPoly(image,points, 4, CvScalar.WHITE, 1, 0);//white polygon covering the replacement image
Try
cvWarpPerspective(yourimage, image, M, image.size(), INTER_CUBIC, BORDER_TRANSPARENT);
Where M is the matrix you get from cvGetPerspectiveMatrix

Answer 2

One way to do it is to scale the pic to the white polygon size and then copy it to the grabbed image setting its Region of Interest (here is a link explaining the ROI).
Your code should look like this:
resize(pic, resizedImage, resizedImage.size(), 0, 0, interpolation); //resizedImage should have the points size
cvSetImageROI(image, cvRect(the points coordinates));
cvCopy(resizedImage,image);
cvResetImageROI(image);
I hope that helps.
Best regards,
Daniel