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reduce

Reduce density of points in ROI

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Syntax

reduce(ROI)
reduce(ROI,tolerance)

Description

reduce(ROI) reduces the number of points that define the region-of-interest ROI. The ROI object stores the point array in the Position property. reduce replaces the original value of the Position property with the reduced value.

The reduce method calls the reducepoly function which uses the Ramer–Douglas–Peucker line simplification algorithm. This algorithm removes points along a straight line and leaves only knickpoints (points where the line curves).

example

reduce(ROI,tolerance) reduces the number of points that define the ROI, where tolerance specifies the sensitivity of the reduction. Specify the tolerance value in the range [0, 1].

example

Examples

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Open Live Script

Read an image into the workspace. 

I = imread('cameraman.tif');

Display the image. 

imshow(I);

Draw a Freehand ROI on the image.

roi = drawfreehand;

View the number of points in the Position property after completing the shape.

disp(['Original Size of Position property: ' mat2str(size(roi.Position))]);
Original Size of Position property: [272 2]

Use the reduce object function to reduce the number of points required to define the shape.

reduce(roi)

View the reduced number of points in the Position property. 

disp(['Reduced Size of Position property: ' mat2str(size(roi.Position))]);
Reduced Size of Position property: [100 2]
Open Live Script

Read an image into the workspace.

I = imread("cameraman.tif");

Display the image. 

imshow(I)

Draw a Polyline ROI on the image.

roi = drawpolyline;

View the number of points in the Position property after completing the shape.

disp("Original Size of Position property: "+mat2str(size(roi.Position)))
Original Size of Position property: [12 2]

Use the reduce object function to reduce the number of points required to define the shape.

reduce(roi)

View the reduced number of points in the Position property. 

disp("First try at reducing the number of points: "+mat2str(size(roi.Position)))
First try at reducing the number of points: [12 2]

Note that the number of points is not changed. To improve the result, change the Tolerance parameter. By default, tolerance is set to .01. Increase the value and try it again.

reduce(roi,0.3)

View the size of the Position property again. Changing the tolerance resulted in a reduction.

disp("Reduction after resetting tolerance parameter: "+mat2str(size(roi.Position)))
Reduction after resetting tolerance parameter: [4 2]

Input Arguments

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ROI object, specified as one of the following ROI objects: AssistedFreehand, Freehand, Polygon, and Polyline.

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

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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