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poly2mask

Convert region of interest (ROI) polygon to region mask

Description

example

BW = poly2mask(xi,yi,m,n) computes a binary region of interest (ROI) mask, BW, of size m-by-n, from an ROI polygon with vertices at coordinates xi and yi. If the polygon is not already closed, then poly2mask closes the polygon automatically.

The poly2mask function sets pixels that are inside the polygon to 1 and sets pixels outside the polygon to 0. For more information about classifying pixels on the ROI boundary, see Classify Pixels That Are Partially Enclosed by ROI.

Examples

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Specify the x- and y-coordinates of the polygon.

x = [63 186 54 190 63];
y = [60 60 209 204 60];

Create the mask specifying the size of the image.

bw = poly2mask(x,y,256,256);

Display the mask, drawing a line around the polygon.

imshow(bw)
hold on
plot(x,y,'b','LineWidth',2)
hold off

Figure contains an axes object. The axes object contains 2 objects of type image, line.

Define two sets of random points for the x- and y-coordinates.

x = 256*rand(1,4);
y = 256*rand(1,4);
x(end+1) = x(1);
y(end+1) = y(1);

Create the mask.

bw = poly2mask(x,y,256,256);

Display the mask and draw a line around the polygon.

imshow(bw)
hold on
plot(x,y,'b','LineWidth',2)
hold off

Figure contains an axes object. The axes object contains 2 objects of type image, line.

Input Arguments

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x-coordinate of polygon vertices, specified as a numeric vector. The length of xi and yi must match.

Data Types: double

y-coordinate of polygon vertices, specified as a numeric vector. The length of xi and yi must match.

Data Types: double

Number of rows in the mask, specified as a nonnegative integer.

Data Types: double

Number of columns in the mask, specified as a nonnegative integer.

Data Types: double

Output Arguments

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Binary image, returned as an m-by-n logical matrix.

Data Types: logical

Tips

  • To specify a polygon that includes a given rectangular set of pixels, make the edges of the polygon lie along the outside edges of the bounding pixels, instead of the center of the pixels.

    For example, to include pixels in columns 4 through 10 and rows 4 through 10, you might specify the polygon vertices like this:

    x = [4 10 10 4 4];
    y = [4 4 10 10 4];
    mask = poly2mask(x,y,12,12)
    mask =
    
         0     0     0     0     0     0     0     0     0     0     0     0
         0     0     0     0     0     0     0     0     0     0     0     0
         0     0     0     0     0     0     0     0     0     0     0     0
         0     0     0     0     0     0     0     0     0     0     0     0
         0     0     0     0     1     1     1     1     1     1     0     0
         0     0     0     0     1     1     1     1     1     1     0     0
         0     0     0     0     1     1     1     1     1     1     0     0
         0     0     0     0     1     1     1     1     1     1     0     0
         0     0     0     0     1     1     1     1     1     1     0     0
         0     0     0     0     1     1     1     1     1     1     0     0
         0     0     0     0     0     0     0     0     0     0     0     0
         0     0     0     0     0     0     0     0     0     0     0     0

    In this example, the polygon goes through the center of the bounding pixels, with the result that only some of the desired bounding pixels are determined to be inside the polygon (the pixels in row 4 and column 4 and not in the polygon). To include these elements in the polygon, use fractional values to specify the outside edge of the 4th row (3.5) and the 10th row (10.5), and the outside edge of the 4th column (3.5) and the outside edge of the 10th column (10.5) as vertices, as in the following example:

    x = [3.5 10.5 10.5 3.5 3.5];
    y = [3.5 3.5 10.5 10.5 3.5];
    mask = poly2mask(x,y,12,12)
    mask =
    
         0     0     0     0     0     0     0     0     0     0     0     0
         0     0     0     0     0     0     0     0     0     0     0     0
         0     0     0     0     0     0     0     0     0     0     0     0
         0     0     0     1     1     1     1     1     1     1     0     0
         0     0     0     1     1     1     1     1     1     1     0     0
         0     0     0     1     1     1     1     1     1     1     0     0
         0     0     0     1     1     1     1     1     1     1     0     0
         0     0     0     1     1     1     1     1     1     1     0     0
         0     0     0     1     1     1     1     1     1     1     0     0
         0     0     0     1     1     1     1     1     1     1     0     0
         0     0     0     0     0     0     0     0     0     0     0     0
         0     0     0     0     0     0     0     0     0     0     0     0

Extended Capabilities

Version History

Introduced before R2006a