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CalibrateWorldPlane_Labeled


Module: Calibration

Finds the image to world plane transformation parameters using sparse world coordinate information, i.e. world coordinates are known for only a few points of the grid.

Applications

Image to world coordinates transformations, also useful for image rectification where exact bounds of output image are important, e.g. stitching.
Name Type Range Description
Input value inImageGrid AnnotatedPoint2DArray Annotated calibration grid.
Input value inLabeledWorldPoints AnnotatedPoint2DArray Sparse array of world coordinate points. Annotations need to correspond to those in the inImageGrid input.
Input value inCameraModel AnyCameraModel* For undistortion of inImageGrid. If not supplied, the filter will assume that grid came from undistorted image.
Input value inGridSpacing Real* 0.000001 - World distance between grid indices. Used when spacing cannot be computed from supplied inLabeledWorldPoints.
Input value inGridThickness Real The world plane will be shifted by given amount in direction perpendicular to the grid to compensate for grid thickness.
Input value inInvertedWorldY Bool Set to true if world coordinate system has right-handed orientation, also known as mathematical or standard.
Output value outTransform RectificationTransform
Output value outGridWorldPoints AnnotatedPoint2DArray Array of world coordinates of the calibration grid points.
Output value outComputedGridSpacing Real? World distance between grid indices. NIL when there is no enough information to compute the spacing.
Output value outRmsImageError Real RMS reprojection error of inImageGrid onto the image plane, in pixels. This is a partial error characterizing inaccuracies in perspective estimation, excluding the influence of world point labeling.
Output value outRmsWorldError Real RMS reprojection error of inLabeledWorldPoints onto the world plane, in world units. This is a partial error characterizing inaccuracies with labeling of world coordinate system, excluding perspective estimation.
Output value outMaxReprojectionError Real Maximum reprojection error of inImageGrid onto the image plane, in pixels. This is a partial error characterizing inaccuracies in perspective estimation, excluding the influence of world point labeling.
Output value outReprojectionErrorSegments Segment2DArray Array of segments connecting inImageGrid points to their reprojections. Note that these segments depict only inaccuracies in perspective estimation - excluding inaccuracies due to the world point labeling.

Description

The filter estimates the correspondence between the image plane and a "world plane" – a given planar surface in observed space. It is capable of using a sparse world coordinate information - a calibration grid for which only a few world plane coordinates are known.

The image plane, and thus inImageGrid points are assumed to be distorted, and to correct for the distortion the inCameraModel (calibration data) needs to be provided. The calculated result – outTransform contains all the information for transforming the distorted image plane to the world plane.

The inCameraModel is also used to define the type of the planar correspondence. For a standard projective camera (pinhole camera), the planar correspondence is a homography. If the inCameraModel is a telecentric camera, the planar correspondence is affine (as there are no perspective parameters in orthographic projection). If no inCameraModel is provided, the filter defaults to homography.

To uniquely define the outTransform, the algorithm requires at least four inImageGrid points (the affine relation requires at least three), and at least two inLabeledWorldPoints.

The filter provides a few methods for judging the feasibility of calculated solution.

  • The outRmsImageError and outRmsWorldError could show if the error is due to the inImageGrid or inLabeledWorldPoints errors. Model mismatch (i.e. trying to calibrate a non-planar object, e.g. wavy surface) will also result in increased reprojection errors. Large difference between outRmsImageError and outMaxReprojectionError could be a sign of presence of outliers in inImageGrid input data.
  • The outReprojectionErrorSegments consists of segments connecting input image points to reprojected world points, and thus it can be readily used for visualization of per-point reprojection errors (excluding the errors due to incorrect world plane labeling).

Hints

  • Image to world plane coordinate transform can be obtained from as few as 4 image points (inImageGrid points), however high accuracy calculations need a considerable amount of high quality calibration points. The calibration plane should contain about hundred points, spanning whole area of interest. Take care of proper conditions when taking the calibration images: reduce vibrations that may yield motion blur, prevent reflections from the calibration surface (ideally use diffusion lighting).
  • It is often assumed that calibration grid is perfect in terms of 2D world coordinates of its points. If such an assumption is made then 2 world coordinates are enough for labeling of grid world point coordinates (inLabeledWorldPoints).
  • The output reprojection errors are useful tool for assessing the feasibility of computed solution. Note, that the filter outputs two RMS metrics: outRmsImageError and outRmsWorldError. The outRmsImageError characterizes inaccuracies in perspective estimation, it is not influenced by world coordinate labeling. On the other hand, the outRmsWorldError characterizes inaccuracies with labeling of grid 2D world coordinate system, excluding perspective estimation. In order for a solution to be feasible, both these errors must be low.

Examples

Description of usage of this filter can be found in examples and tutorial: Hand-Eye Calibration - On Filters.

Good case of image to world plane calibration using high amount of calibration points. Calibration resulted in RMS and maximum reprojection errors less than 1.0, as expected. The outReprojectionErrorSegments are not visible at that scale.

Errors

This filter can throw an exception to report error. Read how to deal with errors in Error Handling.

List of possible exceptions:

Error type Description
DomainError inGridSpacing needs to be positive

Complexity Level

This filter is available on Advanced Complexity Level.

Filter Group

This filter is member of CalibrateWorldPlane filter group.

See Also

  • CalibrateWorldPlane_Default – Finds the image to world plane transformation parameters from world plane calibration grid. World plane origin and axes are unspecified.
  • CalibrateWorldPlane_Multigrid – Finds the image to world plane transformation parameters using multiple grids, using sparse world coordinate information.
  • CalibrateCamera_Pinhole – Finds the camera intrinsic parameters from calibration grids. Uses pinhole camera model (perspective camera).
  • CreateRectificationMap_PixelUnits – Computes a spatial map for transforming distorted images to rectified images defined in world coordinate plane. Defines the output geometry in pixels.
  • CreateRectificationMap_WorldUnits – Computes a spatial map for transforming distorted images to rectified images defined in world coordinate plane. Defines the output geometry in world units.