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reducepoly

Reduce density of points in ROI using Ramer–Douglas–Peucker algorithm

Since R2019b

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Syntax

P_reduced = reducepoly(P)
P_reduced = reducepoly(P,tolerance)

Description

example

P_reduced = reducepoly(P) reduces the density of points in array P. The reducepoly function uses the Ramer-Douglas-Peucker line simplification algorithm, removing points along straight lines and leaving only knickpoints (points where the line curves).

P_reduced = reducepoly(P,tolerance) reduces the density of points in array P, where tolerance specifies how much a point can deviate from a straight line.

Examples

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Open Live Script

Read an image into the workspace. 

I = imread('coins.png');
imshow(I)

Convert the image from grayscale to binary.

bw = imbinarize(I);

Obtain the boundaries of all the coins in the binary image.

[B,L] = bwboundaries(bw,'noholes');

Select the boundary of the first detected coin.

coinNumber = 1;
boundary = B{coinNumber};

Plot the boundary for the first detected coin over the original image.

hold on
visboundaries({boundary})
xlim([min(boundary(:,2))-10 max(boundary(:,2))+10])
ylim([min(boundary(:,1))-10 max(boundary(:,1))+10])
hold off

Use reducepoly to reduce the number of points defining the coin boundary. Return a smaller number of points by increasing the tolerance from the default value of 0.001.

tolerance = 0.02;
p_reduced = reducepoly(boundary,tolerance);

To see how well the reduced polygon matches the original polygon, plot the reduced polygon vertices over the image.

line(p_reduced(:,2),p_reduced(:,1), ...
       'color','b','linestyle','-','linewidth',1.5,...
       'marker','o','markersize',5);
title('Original Polygon (Red) and Reduced Polygon (Blue)');

Input Arguments

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Points to be reduced, specified as an n-by-2 numeric matrix of the form [x1 y1; ...; xn yn]. Each row in the array defines a vertex in an ROI shape, such as a polyline, polygon, or freehand.

For example, you can draw a freehand ROI by using the drawfreehand function. Then, get the ROI vertices from the Position property of the freehand ROI object.

roi = drawfreehand;
P = roi.Position;

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64

Sensitivity of the reduction algorithm, specified as a numeric scalar in the range [0, 1]. Increasing the tolerance increases the number of points removed. A tolerance value of 0 has a minimum reduction in points. A tolerance value of 1 results in maximum reduction in points, leaving only the end points of the line.

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64

Output Arguments

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Reduced data set, returned as an m-by-2 numeric matrix. The number of reduced points is usually smaller than the number of original points in P.

Data Types: double

Algorithms

The Ramer-Douglas-Peucker line simplification algorithm recursively subdivides a shape looking to replace a run of points with a straight line. The algorithm checks that no point in the run deviates from the straight line by more than the value specified by tolerance.

Version History

Introduced in R2019b

See Also

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