pcfitsphere

Fit sphere to 3-D point cloud

Syntax

model = pcfitsphere(ptCloudIn,maxDistance)

[___,inlierIndices,outlierIndices] = pcfitsphere(___)

[___,meanError] = pcfitsphere(___)

[___] = pcfitsphere(___,Name=Value)

Description

model = pcfitsphere(ptCloudIn,maxDistance) fits a sphere to a point cloud that has a maximum allowable distance from an inlier point to the sphere. The function returns a geometrical model that describes the sphere.

This function uses the M-estimator SAmple Consensus (MSAC) algorithm to find the sphere. The MSAC algorithm is a variant of the RANdom SAmple Consensus (RANSAC) algorithm.

[___,inlierIndices,outlierIndices] = pcfitsphere(___) additionally returns linear indices to the inlier and outlier points in the point cloud input.

[___,meanError] = pcfitsphere(___) additionally returns the mean error of the distance of inlier points to the model, using any of the preceding syntaxes.

[___] = pcfitsphere(___,Name=Value) specifies options using one or more name-value arguments in addition to any combination of arguments from previous syntaxes. For example, MaxNumTrials=1000 sets the maximum number of random trials to 1000.

Examples

Detect Sphere in Point Cloud

Load a point cloud into the workspace.

load("object3d.mat");

Display the point cloud and label the figure.

figure
pcshow(ptCloud)
title("Detect a sphere in a point cloud")

Set the maximum point-to-sphere distance for sphere fitting to 1cm.

maxDistance = 0.01;

Set the region of interest to constrain the search.

roi = [-inf,0.5;0.2,0.4;0.1,inf];
sampleIndices = findPointsInROI(ptCloud,roi);

Detect the globe in the point cloud and extract it.

[model,inlierIndices] = pcfitsphere(ptCloud,maxDistance,SampleIndices=sampleIndices);
globe = select(ptCloud,inlierIndices);

Plot the extracted globe.

figure
pcshow(globe)
title("Globe Point Cloud")

Input Arguments

ptCloudIn — Point cloud

pointCloud object

Point cloud, specified as a pointCloud object.

maxDistance — Maximum distance from an inlier point to the sphere

scalar value

Maximum distance from an inlier point to the sphere, specified as a scalar value. Specify the distance in units that are consistent with the units you are using for the point cloud.

Data Types: single | double

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.

Example: MaxNumTrials=1000 sets the maximum number of random trials to 1000.

SampleIndices — Linear indices of points to be sampled

[] (default) | column vector

Linear indices of points to sample in the input point cloud, specified as a column vector. An empty vector means that all points are candidates to sample in the RANSAC iteration to fit the sphere. When you specify a subset, only points in the subset are sampled to fit a model. Providing a subset of points can significantly speed up the process and reduce the number of trials. You can generate the indices vector using the findPointsInROI object function of the pointCloud object.

MaxNumTrials — Maximum number of random trials

1000 (default) | positive integer

Maximum number of random trials for finding inliers, specified as a positive integer. Increasing this value makes the output more robust but adds additional computations.

Confidence — Confidence percentage for finding maximum number of inliers

99 (default) | numeric scalar in the range (0 100)

Confidence percentage for finding maximum number of inliers, specified as a numeric scalar representing percentage, in the range (0 100). Increasing this value makes the output more robust but adds additional computations.

Output Arguments

model — Geometric model of sphere

sphereModel object

Geometric model of sphere, returned as a sphereModel object.

When the input point cloud does not contain enough valid points, or when the function cannot find enough inlier points, the coefficients for the output model are set to zero.

inlierIndices — Linear indices of inlier points

column vector

Linear indices of inlier points within the input point cloud, returned as a column vector.

outlierIndices — Linear indices of outlier points

column vector

Linear indices of outlier points within the input point cloud, returned as a column vector.

meanError — Mean square error

scalar value

Mean error of the distance of inlier points to the model, returned as a scalar value.

References

[1] Torr, P. H. S. and A. Zisserman. “MLESAC: A New Robust Estimator with Application to Estimating Image Geometry.” Computer Vision and Image Understanding. 2000.

Extended Capabilities

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

Version History

Introduced in R2015b