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Ah, ok. Create a vector of Point2f, one located at each corner of your model, or each relevant location in your model. Then use the transform function to map those onto your document. The output should be a vector of Point2f that are those points on the document.

So if your input is the corners of the model, then your output is the corners of the box in the image.

If you need to do the inverse, it's just H.inv(), I think.

So if your input is the corners of the model, then your output is the corners of the box in the image.

If you need to do the inverse, it's just H.inv(), I think.

EDIT: I'm really not sure what you're doing wrong, because I don't have enough of your code, but here. This code does what I think you want done.

Mat warped;
warpPerspective(scene, warped, h, Size(model.cols, model.rows), WARP_INVERSE_MAP);
imshow("Warped", warped);

Mat modifiedScene = scene.clone();
warpPerspective(model, modifiedScene, h, Size(scene.cols, scene.rows), 1, BORDER_TRANSPARENT);
imshow("Modified Scene", modifiedScene);

vector<Point2f> ptsModel, ptsScene;
vector<Point3f> tempPts;
ptsModel.push_back(Point2f(0, 0));
ptsModel.push_back(Point2f(model.cols, 0));
ptsModel.push_back(Point2f(0, model.rows));
ptsModel.push_back(Point2f(model.cols, model.rows));
transform(ptsModel, tempPts, h);
convertPointsHomogeneous(tempPts, ptsScene);

for (int i = 0; i < ptsScene.size(); ++i)
    std::cout << ptsScene[i] << "\n";
std::cout << "\n";

line(scene, ptsScene[0], ptsScene[1], Scalar(255, 0, 0));
line(scene, ptsScene[1], ptsScene[3], Scalar(255, 0, 0));
line(scene, ptsScene[2], ptsScene[3], Scalar(255, 0, 0));
line(scene, ptsScene[2], ptsScene[0], Scalar(255, 0, 0));
imshow("Scene", scene);

waitKey();