?Mat H = findHomography( obj, scene, RANSAC );

?//-- Get the corners from the image_1 ( the object to be "detected" )
?std::vector obj_corners(4);
?obj_corners[0] = Point(0,0); obj_corners[1] = Point( img_object.cols, 0 );
?obj_corners[2] = Point( img_object.cols, img_object.rows ); obj_corners[3] = Point( 0, img_object.rows );
?std::vector scene_corners(4);
?perspectiveTransform( obj_corners, scene_corners, H);
?//-- Draw lines between the corners (the mapped object in the scene - image_2 )
?Point2f offset( (float)img_object.cols, 0);
?line( img_matches, scene_corners[0] + offset, scene_corners[1] + offset, Scalar(0, 255, 0), 4 );
?line( img_matches, scene_corners[1] + offset, scene_corners[2] + offset, Scalar( 0, 255, 0), 4 );
?line( img_matches, scene_corners[2] + offset, scene_corners[3] + offset, Scalar( 0, 255, 0), 4 );
?line( img_matches, scene_corners[3] + offset, scene_corners[0] + offset, Scalar( 0, 255, 0), 4 );

?//-- Show detected matches
?imshow( "Good Matches & Object detection", img_matches );
?waitKey(0);
?return 0;
}

代码运行效果：

?

?

findHomography()函数直接通过两个平面上相匹配的特征点求出变换公式，之后代码又对原图的四个边缘点进行变换，在右图上画出对应的矩形。这个图也很好地解释了所谓透视变换的“Viewing Plane”。

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