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pcfitsphere

Fit sphere to 3-D point cloud

Syntax

model = pcfitsphere(ptCloudIn,maxDistance)
[model,inlierIndices,outlierIndices] = pcfitsphere(ptCloudIn,maxDistance)
[___,meanError] = pcfitsphere(ptCloudIn,maxDistance)
[___] = pcfitsphere(___,Name,Value)

Description

example

model = pcfitsphere(ptCloudIn,maxDistance) fits a sphere to a point cloud tha 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.

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

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

[___] = pcfitsphere(___,Name,Value) uses additional options specified by one or more Name,Value pair arguments.

Examples

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Load data file.

load('object3d.mat');

Display original point cloud.

figure
pcshow(ptCloud)
xlabel('X(m)')
ylabel('Y(m)')
zlabel('Z(m)')
title('Original Point Cloud')

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

maxDistance = 0.01;

Set the roi to constrain the search.

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

Detect the sphere, a globe, and extract it from the point cloud.

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

Plot the globe.

hold on
plot(model)

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

Input Arguments

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Point cloud, specified as a pointCloud object.

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 Pair Arguments

Specify optional comma-separated pairs of Name,Value arguments. Name is the argument name and Value is the corresponding value. Name must appear inside single quotes (' '). You can specify several name and value pair arguments in any order as Name1,Value1,...,NameN,ValueN.

Example: 'SampleIndices',[].

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Linear indices of points to sample in the input point cloud, specified as the comma-separated pair consisting of 'SampleIndices' and 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 method of the pointCloud object.

Maximum number of random trials for finding inliers, specified as the comma-separated pair consisting of 'MaxNumTrials' and a positive integer. Increasing this value makes the output more robust but adds additional computations.

Confidence percentage for finding maximum number of inliers, specified as the comma-separated pair consisting of 'Confidence' and a numeric scalar representing percentage, in the range [0,100]. Increasing this value makes the output more robust but adds additional computations.

Output Arguments

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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.

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

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

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.

Introduced in R2015b