AprilTags are fiducial markers which provide the position and orientation in 3D space relative to a camera. AprilTags are commonly used in robotics for localization and navigation.
Here, AprilTags are being used to rectify images. A skewed image taken with a normal camera can be made parallel to the view frame of the camera with the help of AprilTags. The homography of a planar surface, for example, a wall, can be calculated by detecting an AprilTag on the plane of the surface given the dimensions of the tag and the focal length of the camera. The homography matrix obtained from the detection is then used to warp the images and make them parallel to the wall. If the size of the AprilTag is known in real-world coordinates, the length between any two points in the image can be calculated after warping.
Depends on cmake and opencv for building.
$ mkdir build
$ cmake ..
$ make -j
$ apriltag_warp <filename.png>
Writes out the rectified image.