McalGrid

This function uses an image of a user-defined grid of circles or squares and the world description of this grid to establish calibration points and calibrate your camera setup. The mapping is stored with the specified calibration context. The specified calibration context determines the calibration mode used.

MIL is distributed with four different calibration grids that you can print and use: ChessboardCalibrationGrid_15x16_Letter.pdf, ChessboardCalibrationGrid_18x22_Letter.pdf, CircleCalibrationGrid_15x16_Letter.pdf, CircleCalibrationGrid_18x22_Letter.pdf. These files are located in the Matrox Imaging\Images directory. Each file includes the world description of the grid.

Typically, you specify the world description of the grid in the absolute coordinate system. However, for 3D-based calibration modes (M_TSAI_BASED or M_3D_ROBOTICS), it is possible to use a grid that is not in the XY-plane (Z=0) of the absolute coordinate system. In this case, use the GridOffsetZ parameter to specify the height of the grid being used.

When working in M_3D_ROBOTICS calibration mode, you must call this function at least three times to accumulate data with different images and orientations (each time with M_ACCUMULATE) before calling this function a fourth time (with M_FULL_CALIBRATION and no image) to perform the full calibration. Before each data accumulation call, you must change the position and orientation of the tool holding the camera with respect to the robot base. More specifically, you must rotate the tool along at least two non-parallel axes. You must also use McalSetCoordinateSystem() to set the position of the tool coordinate system with respect to the robot base coordinate system before each of these calls. You must call McalGrid() with M_ACCUMULATE more than three times before performing a full calibration improves the accuracy of your calibration.

You can inquire about the pixel coordinates and associated real-world coordinates of each calibration point of one particular call using McalInquireSingle().

For 3D-based calibration modes, McalGrid() performs two types of calculations. It calculates the camera's internal attributes, and it calculates the orientation and distance between the camera and camera calibration plane. Initially, the latter is used to set the camera coordinate system. If you move the camera or the grid (not both), you can have McalGrid() recalculate only the orientation and distance between them. In this case, depending on what you moved, you can specify that McalGrid() displace either the camera coordinate system or the relative coordinate system, using M_DISPLACE_CAMERA_COORD or M_DISPLACE_RELATIVE_COORD, respectively. When using M_DISPLACE_RELATIVE_COORD, the camera calibration plane is considered to be the XY-plane of the relative coordinate system, regardless of the M_CALIBRATION_PLANE setting.

Also, for 3D-based calibration modes, a successful call to McalGrid() with M_FULL_CALIBRATION creates a rigid link between the camera coordinate system and the tool coordinate system. This link ensures that moving either the tool or the camera coordinate system will affect both. This link can be broken using McalControl() with M_LINK_TOOL_AND_CAMERA.

Note that the internal attributes calculated for the camera are not those of the physical camera, but those of the ideal pinhole-camera used to model the physical camera.

After calling McalGrid(), you can inquire about the success of the calibration using McalInquire() with M_CALIBRATION_STATUS. Also, you can inquire about the pixel coordinates and associated real-world coordinates of each calibration point using McalInquire() with M_CALIBRATION_IMAGE_POINTS_X, M_CALIBRATION_IMAGE_POINTS_Y, M_CALIBRATION_WORLD_POINTS_X, M_CALIBRATION_WORLD_POINTS_Y, and M_CALIBRATION_WORLD_POINTS_Z. These coordinates correspond to the center of circles in a circle grid or to points connecting four corners in a chessboard grid.

When the image of the calibration grid is rotated, for instance when the image of the grid has been taken by a second camera at a different angle, you might need to specify where the top-left corner of the real-world grid is in the rotated image. This is done using McalGrid() set to M_GRID_CORNER_HINT_X and M_GRID_CORNER_HINT_Y. Note that RowNumber and ColumnNumber specify the number of rows and columns of the calibration grid as it is in the world, and not as it appears in the rotated image.

For more information on performing camera calibration, see the Steps to performing a calibration section of Chapter 25: Calibrating your camera setup.