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you need 4 (2d) points from your 3d view, and 4 2d points where you want them projected. make sure they're in the same order (e.g. starting top-left and going clockwise), then it's similar to:

 Mat img = imread("3dview.jpg");
vector<Point2f> pts;  // hardcoded, in clockwise order
pts.push_back(Point2f(895,204)); 
pts.push_back(Point2f(995,195));
pts.push_back(Point2f(1222,292));
pts.push_back(Point2f(1099,307));

Size sz(250,500);    // desired output size
vector<Point2f> rct; // desired coords, same order
rct.push_back(Point2f(0,0));
rct.push_back(Point2f(sz.width,0));
rct.push_back(Point2f(sz.width,sz.height));
rct.push_back(Point2f(0,sz.height));

Mat P = getPerspectiveTransform(pts,rct);
Mat persp;
warpPerspective(img, persp, P, sz, INTER_LINEAR);