OpenCV rotate, distort and translate ROI in new image - c++

Question

I have an image from which I want to get a vertical ROI, apply some transformations and add to another image.
I read a lot of questions and answer on StackOverflow and other forums, but I'm still stuck with this problem. For the moment I'm using the C interface of OpenCV, but I could use the C++ one if needed (I would have to write a conversion function, since I'm working with CGImageRef in Cocoa).
To get from the top image (see below) to the bottom image, I guess I have to :
Get the ROI on the first image ;
Scale it down ;
Get the intersection points on the lines between the center and the 2 circles for my "width" angle (the angle is fixed) ;
Distort the image so the corners stick to my intersection points ;
Rotate around the center point and put it in the output image.
For the moment, I manage well to do this :
Getting the ROI ;
Scaling it with cvResize ;
Getting the intersection points shouldn't be too complicated, as it is pure geometry and I implemented it yet for another purpose.
But, I have no idea at all of how to distort the resulting image of my ROI, and I don't know if it is even possible in OpenCV. Would I have to use a kind of perspective correction ?
And, I've been trying the few good posts solutions I found by here to rotate with the rotated bounding box, but with no good results for the moment.
EDIT :
Well, I managed to do the first part of the work :
Getting a ROI in a basis image ;
Rotating and placing it at a fixed distance from the center.
I used the method explained and coded in this post : https://stackoverflow.com/a/16285286/1060921
I only added a variable to set the rotation point and get my inner circle.
NB : I set the ROI BEFORE to call the method, so the ROI in the post method is... the image size. Then I place it at the center of my final image with a cvAdd.
Here I get one pixel slices of my camera input. What I want to do now is to distort bigger slices, for example from 2 pixels on the inner circle to 5 pixels on the outer one.

Answer 1

See this tutorial which uses warpPerspective to correct perspective distortion.
EDIT: In your case warpAffine should be better and simpler solution.
So, you could do something like this, just use four points instead of three:
Point2f srcTri[3];
Point2f dstTri[3];
Mat rot_mat( 2, 3, CV_32FC1 );
Mat warp_mat( 2, 3, CV_32FC1 );
Mat src, warp_dst, warp_rotate_dst;
/// Load the image
src = imread( ... );
/// Set the dst image the same type and size as src
warp_dst = Mat::zeros( src.rows, src.cols, src.type() );
/// Set your 3 points to calculate the Affine Transform
srcTri[0] = Point2f( 0,0 );
srcTri[1] = Point2f( src.cols - 1, 0 );
srcTri[2] = Point2f( 0, src.rows - 1 );
dstTri[0] = Point2f( src.cols*0.0, src.rows*0.33 );
dstTri[1] = Point2f( src.cols*0.85, src.rows*0.25 );
dstTri[2] = Point2f( src.cols*0.15, src.rows*0.7 );
/// Get the Affine Transform
warp_mat = getAffineTransform( srcTri, dstTri );
/// Apply the Affine Transform just found to the src image
warpAffine( src, warp_dst, warp_mat, warp_dst.size() );