findNeighborsInRadius

Find neighbors within a radius of a point in the point cloud

Syntax

[indices,dists] = findNeighborsInRadius(ptCloud,point,radius)

[indices,dists] = findNeighborsInRadius(ptCloud,point,radius,camMatrix)

[indices,dists] = findNeighborsInRadius(___,Name,Value)

Description

[indices,dists] = findNeighborsInRadius(ptCloud,point,radius) returns the indices of neighbors within a radius of a query point in the input point cloud. ptCloud can be an unorganized or organized point cloud. The neighbors within a radius of the query point are computed by using the Kd-tree based search algorithm.

[indices,dists] = findNeighborsInRadius(ptCloud,point,radius,camMatrix) returns the neighbors within a radius of a query point in the input point cloud. The input point cloud is an organized point cloud generated by a depth camera. The neighbors within a radius of the query point are determined using fast approximate neighbor search algorithm.

The function uses the camera projection matrix camMatrix to know the relationship between adjacent points and hence, speeds up the search. However, the results have lower accuracy as compared to the Kd-tree based approach.

Note

• This syntax only supports organized point cloud data produced by RGB-D sensors.

• You can use estimateCameraMatrix to estimate camera projection matrix for the given point cloud data.

[indices,dists] = findNeighborsInRadius(___,Name,Value) specifies options using one or more name-value pair arguments in addition to the input arguments in the preceding syntaxes.

Examples

Find Radial Neighbors in Point Cloud

Load a set of 3-D coordinate points into the workspace.

load('xyzPoints.mat');

Create a point cloud object.

ptCloud = pointCloud(xyzPoints);

Specify a query point and the radius within which the neighbors are to be identified.

point = [0,0,3];
radius = 0.5;

Get the indices and the distances of points that lie within the specified radius.

[indices,dists] = findNeighborsInRadius(ptCloud,point,radius);

Get the point cloud data of radial neighbors.

ptCloudB = select(ptCloud,indices);

Display the point cloud. Plot the query point and the corresponding radial neighbors.

figure
pcshow(ptCloud)
hold on
plot3(point(1),point(2),point(3),'*')
pcshow(ptCloudB.Location,'r')
legend('Point Cloud','Query Point','Radial Neighbors','Location','southoutside','Color',[1 1 1])
hold off

Find Radial Neighbors in Organized Point Cloud

Find radial neighbors of a query point in the organized point cloud data by using the camera projection matrix. Compute the camera projection matrix from sampled point cloud data points and their corresponding image point coordinates.

Load an organized point cloud data into the workspace. The point cloud is generated by using the Kinect depth sensor.

ld = load('object3d.mat');
ptCloud = ld.ptCloud;

Specify the step size for sampling the point cloud data.

stepSize = 100;

Sample the input point cloud and store the sampled 3-D point coordinates as a point cloud object.

indices = 1:stepSize:ptCloud.Count;
tempPtCloud = select(ptCloud,indices);

Remove invalid points from the sampled point cloud.

[tempPtCloud,validIndices] = removeInvalidPoints(tempPtCloud);

Define the 3-D world point coordinates of input point cloud.

worldPoints = tempPtCloud.Location;

Find the 2-D image coordinates corresponding to the 3-D point coordinates of input point cloud.

[Y,X] = ind2sub([size(ptCloud.Location,1),size(ptCloud.Location,2)],indices);
imagePoints = [X(validIndices)' Y(validIndices)'];

Estimate camera projection matrix from the image and the world point coordinates.

camMatrix = estimateCameraMatrix(imagePoints,worldPoints);

Specify a query point and the radius within which the neighbors are to be identified.

point = [0.4 0.3 0.2];
radius = 0.05;

Get the indices and the distances of radial neighbors. Use the point cloud method select to get the point cloud data of neighboring points.

[indices,dists] = findNeighborsInRadius(ptCloud,point,radius,camMatrix);
ptCloudB = select(ptCloud,indices);

Display the point cloud and the radial neighbors found around a query point.

figure
pcshow(ptCloud);
hold on;
pcshow(ptCloudB.Location, 'b');
legend('Point Cloud','Radial Neighbors','Location','southoutside','Color',[1 1 1] )
hold off;

Input Arguments

ptCloud — Point cloud

pointCloud object

Point cloud, specified as a pointCloud object.

point — Query point

three-element vector of form [x,y,z]

Query point, specified as a three-element vector of form [x,y,z].

radius — Search radius

scalar

Search radius, specified as a scalar. The function finds the neighbors within the specified radius around a query point in the input point cloud.

camMatrix — Camera projection matrix

4-by-3 matrix

Camera projection matrix, specified as a 4-by-3 matrix that maps 3-D world points to 2-D image points. You can find camMatrix by using the estimateCameraMatrix function.

Name-Value Arguments

Specify optional pairs of arguments as Name1=Value1,...,NameN=ValueN, where Name is the argument name and Value is the corresponding value. Name-value arguments must appear after other arguments, but the order of the pairs does not matter.

Before R2021a, use commas to separate each name and value, and enclose Name in quotes.

Example: findNeighborsInRadius(ptCloud,point,radius,'Sort',true)

Sort — Sort indices

false (default) | true

Sort indices, specified as a comma-separated pair of 'Sort' and a logical scalar. When you set Sort to true, the returned indices are sorted in the ascending order based on the distance from a query point. To turn off sorting, set Sort to false.

MaxLeafChecks — Number of leaf nodes

Inf (default) | integer

Number of leaf nodes, specified as a comma-separated pair consisting of 'MaxLeafChecks' and an integer. When you set this value to Inf, the entire tree is searched. When the entire tree is searched, it produces exact search results. Increasing the number of leaf nodes to check increases accuracy, but reduces efficiency.

Output Arguments

indices — Indices of stored points

column vector

Indices of stored points, returned as a column vector. The vector contains the linear indices of the radial neighbors stored in the point cloud.

dists — Distances to query point

column vector

Distances to query point, returned as a column vector. The vector contains the Euclidean distances between the query point and its radial neighbors.

References

[1] Muja, M. and David G. Lowe. "Fast Approximate Nearest Neighbors with Automatic Algorithm Configuration". In VISAPP International Conference on Computer Vision Theory and Applications. 2009. pp. 331–340.

Extended Capabilities

C/C++ Code Generation

Generate C and C++ code using MATLAB® Coder™.

Usage notes and limitations:

• For code generation in non-host platforms, the value for 'MaxLeafChecks' must be set to the default value Inf. If you specify values other than Inf, the function generates a warning and automatically assigns the default value for 'MaxLeafChecks'.

GPU Code Generation

Generate CUDA® code for NVIDIA® GPUs using GPU Coder™.

Usage notes and limitations:

• For GPU code generation, the camMatrix input argument is ignored.

• For GPU code generation, the 'MaxLeafChecks' name-value pair option is ignored.

Version History

Introduced in R2015a