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Check vehicle costmap for collision-free poses or points


The checkFree function checks whether vehicle poses or points are free from obstacles on the vehicle costmap. Path planning algorithms use checkFree to check whether candidate vehicle poses along a path are navigable.

To simplify the collision check for a vehicle pose, vehicleCostmap inflates obstacles according to the vehicle's InflationRadius, as specified by the CollisionChecker property of the costmap. The collision checker calculates the inflation radius by enclosing the vehicle in a set of overlapping circles of radius R, where the centers of these circles lie along the longitudinal axis of the vehicle. The inflation radius is the minimum R needed to fully enclose the vehicle in these circles.

A vehicle pose is collision-free when the following conditions apply:

  • None of the vehicle's circle centers lie on an inflated grid cell.

  • The cost value of each containing a circle center is less than the FreeThreshold of the costmap.

For more details, see the algorithm on the vehicleCostmap reference page.


free = checkFree(costmap,vehiclePoses) checks whether the vehicle poses are free from collision with obstacles on the costmap.

free = checkFree(costmap,xyPoints) checks whether (x, y) points in xyPoints are free from collision with obstacles on the costmap.

freeMat = checkFree(costmap) returns a logical matrix that indicates whether each cell of the costmap is free.


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Load a costmap from a parking lot.

data = load('parkingLotCostmap.mat');
parkMap = data.parkingLotCostmap;

Create vehicle poses following a straight-line path. x and y are the (x,y) coordinates of the rear axle of the vehicle. theta is the angle of the rear axle with respect to the x-axis. Note that the dimensions of the vehicle are stored in the CollisionChecker.VehicleDimensions property of the costmap, and that there is an offset between the rear axle of the vehicle and its center.

x = 4:0.25:6;
y = 3:0.25:5;
theta = repmat(45,size(x));
vehiclePoses = [x',y',theta'];
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The first few (x,y) coordinates of the rear axle are within the inflated area. However, this does not imply a collision because the center of the vehicle may be outside the inflated area. Check if the poses are collision-free.

free = checkFree(parkMap,vehiclePoses)
free = 9x1 logical array


All values of free are 1 (true), so all poses are collision-free. The center of the vehicle does not enter the inflated area at any pose.

Input Arguments

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Costmap, specified as a vehicleCostmap object.

Vehicle poses, specified as an m-by-3 matrix of [x, y, Θ] vectors. m is the number of poses.

x and y specify the location of the vehicle in world units, such as meters. This location is the center of the rear axle of the vehicle.

Θ specifies the orientation angle of the vehicle in degrees with respect to the x-axis. Θ is positive in the clockwise direction.

Example: [3.4 2.6 0] specifies a vehicle with the center of the rear axle at (3.4, 2.6) and an orientation angle of 0 degrees.

Points, specified as an M-by-2 real-valued matrix that represents the (x, y) coordinates of M points.

Example: [3.4 2.6] specifies a single point at (3.4, 2.6)

Example: [3 2;3 3;4 7] specifies three points: (3, 2), (3, 3), and (4, 7)

Output Arguments

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Vehicle pose or point is free, returned as an M-by-1 logical vector. An element of free is 1 (true) when the corresponding vehicle pose in vehiclePoses or point in xyPoints is collision-free.

Costmap cell is free, returned as a logical matrix of the same size as the costmap grid. This size is specified by the MapSize property of the costmap. An element of freeMat is 1 (true) when the corresponding cell in costmap is unoccupied and the cost value of the cell is below the FreeThreshold of the costmap.


  • If you specify a small value of InflationRadius that does not completely enclose the vehicle, then checkFree might report occupied poses as collision-free. To avoid this situation, the default value of InflationRadius completely encloses the vehicle.

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using MATLAB® Coder™.

Version History

Introduced in R2018a