Detection of ArUco boards {#tutorial_aruco_board_detection}
=========================

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| -: | :- |
| Original authors | Sergio Garrido, Alexander Panov |
| Compatibility    | OpenCV >= 4.7.0 |

An ArUco board is a set of markers that acts like a single marker in the sense that it provides a
single pose for the camera.

The most popular board is the one with all the markers in the same plane, since it can be easily printed:

![](images/gboriginal.jpg)

However, boards are not limited to this arrangement and can represent any 2d or 3d layout.

The difference between a board and a set of independent markers is that the relative position between
the markers in the board is known a priori. This allows that the corners of all the markers can be used for
estimating the pose of the camera respect to the whole board.

When you use a set of independent markers, you can estimate the pose for each marker individually,
since you dont know the relative position of the markers in the environment.

The main benefits of using boards are:

- The pose estimation is much more versatile. Only some markers are necessary to perform pose estimation.
Thus, the pose can be calculated even in the presence of occlusions or partial views.
- The obtained pose is usually more accurate since a higher amount of point correspondences (marker
corners) are employed.

Board Detection
---------------

A board detection is similar to the standard marker detection. The only difference is in the pose estimation step.
In fact, to use marker boards, a standard marker detection should be done before estimating the board pose.

To perform pose estimation for boards, you should use `solvePnP()` function, as shown below
in the `samples/cpp/tutorial_code/objectDetection/detect_board.cpp`.

@snippet samples/cpp/tutorial_code/objectDetection/detect_board.cpp aruco_detect_board_full_sample


The parameters are:

- `objPoints`, `imgPoints` object and image points, matched with `cv::aruco::GridBoard::matchImagePoints()`
   which, in turn, takes as input `markerCorners` and `markerIds` structures of detected markers from
   `cv::aruco::ArucoDetector::detectMarkers()` function.
- `board` the `cv::aruco::Board` object that defines the board layout and its ids
- `cameraMatrix` and `distCoeffs`: camera calibration parameters necessary for pose estimation.
- `rvec` and `tvec`: estimated pose of the board. If not empty then treated as initial guess.
- The function returns the total number of markers employed for estimating the board pose.

The drawFrameAxes() function can be used to check the obtained pose. For instance:

![Board with axis](images/gbmarkersaxis.jpg)

And this is another example with the board partially occluded:

![Board with occlusions](images/gbocclusion.jpg)

As it can be observed, although some markers have not been detected, the board pose can still be
estimated from the rest of markers.

Sample video:

@youtube{Q1HlJEjW_j0}

A full working example is included in the `detect_board.cpp` inside the `samples/cpp/tutorial_code/objectDetection/`.

The samples now take input via command line via the `cv::CommandLineParser`. For this file the example
parameters will look like:
@code{.cpp}
    -w=5 -h=7 -l=100 -s=10
    -v=/path_to_opencv/opencv/doc/tutorials/objdetect/aruco_board_detection/gboriginal.jpg
    -c=/path_to_opencv/opencv/samples/cpp/tutorial_code/objectDetection/tutorial_camera_params.yml
    -cd=/path_to_opencv/opencv/samples/cpp/tutorial_code/objectDetection/tutorial_dict.yml
@endcode
Parameters for `detect_board.cpp`:
@snippet samples/cpp/tutorial_code/objectDetection/detect_board.cpp aruco_detect_board_keys

Grid Board
----------

Creating the `cv::aruco::Board` object requires specifying the corner positions for each marker in the environment.
However, in many cases, the board will be just a set of markers in the same plane and in a grid layout,
so it can be easily printed and used.

Fortunately, the aruco module provides the basic functionality to create and print these types of markers
easily.

The `cv::aruco::GridBoard` class is a specialized class that inherits from the `cv::aruco::Board`
class and which represents a Board with all the markers in the same plane and in a grid layout,
as in the following image:

![Image with aruco board](images/gboriginal.jpg)

Concretely, the coordinate system in a grid board is positioned in the board plane, centered in the bottom left
corner of the board and with the Z pointing out, like in the following image (X:red, Y:green, Z:blue):

![Board with axis](images/gbaxis.jpg)

A `cv::aruco::GridBoard` object can be defined using the following parameters:

- Number of markers in the X direction.
- Number of markers in the Y direction.
- Length of the marker side.
- Length of the marker separation.
- The dictionary of the markers.
- Ids of all the markers (X*Y markers).

This object can be easily created from these parameters using the `cv::aruco::GridBoard` constructor:

@snippet samples/cpp/tutorial_code/objectDetection/detect_board.cpp aruco_create_board

- The first and second parameters are the number of markers in the X and Y direction respectively.
- The third and fourth parameters are the marker length and the marker separation respectively.
  They can be provided in any unit, having in mind that the estimated pose for this board will be
  measured in the same units (in general, meters are used).
- Finally, the dictionary of the markers is provided.

So, this board will be composed by 5x7=35 markers. The ids of each of the markers are assigned, by default,
in ascending order starting on 0, so they will be 0, 1, 2, ..., 34.

After creating a grid board, we probably want to print it and use it.
There are two ways to do this:
1. By using the script `apps/pattern-tools/generate_pattern.py`, see @subpage tutorial_camera_calibration_pattern.
2. By using the function `cv::aruco::GridBoard::generateImage()`.

The function `cv::aruco::GridBoard::generateImage()` is provided in cv::aruco::GridBoard class and
can be called by using the following code:

@snippet samples/cpp/tutorial_code/objectDetection/create_board.cpp aruco_generate_board_image

- The first parameter is the size of the output image in pixels. In this case 600x500 pixels. If this is not proportional
to the board dimensions, it will be centered on the image.
- `boardImage`: the output image with the board.
- The third parameter is the (optional) margin in pixels, so none of the markers are touching the image border.
In this case the margin is 10.
- Finally, the size of the marker border, similarly to `generateImageMarker()` function. The default value is 1.

A full working example of board creation is included in the `samples/cpp/tutorial_code/objectDetection/create_board.cpp`

The output image will be something like this:

![](images/board.png)

The samples now take input via commandline via the `cv::CommandLineParser`. For this file the example
parameters will look like:
@code{.cpp}
    "_output_path_/aboard.png" -w=5 -h=7 -l=100 -s=10 -d=10
@endcode

Refine marker detection
-----------------------

ArUco boards can also be used to improve the detection of markers. If we have detected a subset of the markers
that belongs to the board, we can use these markers and the board layout information to try to find the
markers that have not been previously detected.

This can be done using the `cv::aruco::refineDetectedMarkers()` function, which should be called
after calling `cv::aruco::ArucoDetector::detectMarkers()`.

The main parameters of this function are the original image where markers were detected, the board object,
the detected marker corners, the detected marker ids and the rejected marker corners.

The rejected corners can be obtained from the `cv::aruco::ArucoDetector::detectMarkers()` function and
are also known as marker candidates. This candidates are square shapes that have been found in the
original image but have failed to pass the identification step (i.e. their inner codification presents
too many errors) and thus they have not been recognized as markers.

However, these candidates are sometimes actual markers that have not been correctly identified due to high
noise in the image, very low resolution or other related problems that affect to the binary code extraction.
The `cv::aruco::ArucoDetector::refineDetectedMarkers()` function finds correspondences between these
candidates and the missing markers of the board. This search is based on two parameters:

- Distance between the candidate and the projection of the missing marker. To obtain these projections,
it is necessary to have detected at least one marker of the board. The projections are obtained using the
camera parameters (camera matrix and distortion coefficients) if they are provided. If not, the projections
are obtained from local homography and only planar board are allowed (i.e. the Z coordinate of all the
marker corners should be the same). The `minRepDistance` parameter in `refineDetectedMarkers()`
determines the minimum euclidean distance between the candidate corners and the projected marker corners
(default value 10).

- Binary codification. If a candidate surpasses the minimum distance condition, its internal bits
are analyzed again to determine if it is actually the projected marker or not. However, in this case,
the condition is not so strong and the number of allowed erroneous bits can be higher. This is indicated
in the `errorCorrectionRate` parameter (default value 3.0). If a negative value is provided, the
internal bits are not analyzed at all and only the corner distances are evaluated.

This is an example of using the `cv::aruco::ArucoDetector::refineDetectedMarkers()` function:

@snippet samples/cpp/tutorial_code/objectDetection/detect_board.cpp aruco_detect_and_refine

It must also be noted that, in some cases, if the number of detected markers in the first place is
too low (for instance only 1 or 2 markers), the projections of the missing markers can be of bad
quality, producing erroneous correspondences.

See module samples for a more detailed implementation.
