bwselect

Select objects in binary image

Syntax

BW2 = bwselect(BW,c,r)

BW2 = bwselect(BW,c,r,n)

[BW2,idx] = bwselect(___)

[x,y,BW2,idx,xi,yi] = bwselect(___)

[___] = bwselect(x,y,BW,xi,yi,n)

[___] = bwselect(BW,n)

[___] = bwselect

Description

BW2 = bwselect(BW,c,r) returns a binary image containing the objects that overlap the pixel (r, c). Objects are connected sets of on pixels, that is, pixels having a value of 1.

BW2 = bwselect(BW,c,r,n) also specifies the object connectivity, n, as 4-connected or 8-connected.

[BW2,idx] = bwselect(___) returns the linear indices of the pixels belonging to the selected objects.

[x,y,BW2,idx,xi,yi] = bwselect(___) returns the x and y extents of the image and the (xi, yi) coordinates of the pixels. By default, bwselect uses the intrinsic coordinate system so that x and y are the image XData and YData.

[___] = bwselect(x,y,BW,xi,yi,n) establishes a nondefault world coordinate system for BW from the vectors x and y. The arguments xi and yi specify pixel coordinates in the world coordinate system.

[___] = bwselect(BW,n) displays the image BW in a figure and lets you select the (r, c) coordinates interactively using the mouse. With this syntax and the other interactive syntax, bwselect blocks the MATLAB^® command line until you finish selecting points.

For more information about selecting points interactively, see Interactive Behavior.

[___] = bwselect without an input argument lets you select the (r, c) coordinates of the image in the current axes interactively.

Examples

Select Objects in Binary Image

Select objects in a binary image and create a new image containing only those objects.

Read binary image into the workspace.

BW = imread('text.png');

Specify the locations of objects in the image using row and column indices.

c = [43 185 212];
r = [38 68 181];

Create a new binary image containing only the selected objects. This example specifies 4-connected objects.

BW2 = bwselect(BW,c,r,4);

Display the original image and the new image side-by-side.

imshowpair(BW,BW2,'montage');

Input Arguments

BW — Binary image

2-D numeric matrix | 2-D logical matrix

Binary image, specified as a 2-D numeric matrix or 2-D logical matrix.

Example: BW = imread('text.png');

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

c — Column index

numeric scalar | numeric vector

Column index, specified as a numeric scalar or numeric vector. If r and c are equal-length vectors, then BW2 contains the sets of objects overlapping with any of the pixels (r(k),c(k)).

Example: c = [43 185 212];

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

r — Row index

numeric scalar | numeric vector

Row index, specified as a numeric scalar or numeric vector. If r and c are equal-length vectors, then BW2 contains the sets of objects overlapping with any of the pixels (r(k),c(k)).

Example: r = [38 68 181];

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

n — Connectivity

8 (default) | 4

Connectivity, specified as 4 or 8.

Value	Description
4	4-connected objects
8	8-connected objects

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

x — World x-axis coordinates

numeric scalar | numeric vector

World x-axis coordinates, specified as a numeric scalar or numeric vector of the same length as y. Use x and y to establish a nondefault spatial coordinate system. By default, if you do not specify x and y, then bwselect uses the intrinsic coordinate system in which x is [1, size(BW,2)].

Example: x = [19.5 23.5];

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

y — World y-axis coordinates

numeric scalar | numeric vector

World y-axis coordinates, specified as a numeric scalar or numeric vector of the same length as x. Use x and y to establish a nondefault spatial coordinate system. By default, if you do not specify x and y, then bwselect uses the intrinsic coordinate system in which y is [1, size(BW,1)].

Example: y = [8.0 12.0];

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

xi — x-coordinates of points

numeric scalar | numeric vector

x-coordinates of points in the world coordinate system, specified as a numeric scalar or numeric vector.

Example: x = [19.5 23.5];

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

yi — y-coordinates of points

numeric scalar | numeric vector

y-coordinates of points in the world coordinate system, specified as a numeric scalar or numeric vector.

Example: y = [8.0 12.0];

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

Output Arguments

BW2 — Binary image containing objects that overlap specified pixels

logical array

Binary image containing objects that overlap the specified pixels, returned as a logical array. BW2 contains the set of objects overlapping with any of the pixels specified by r and c or xi and yi.

If you do not specify an output argument, then bwselect displays the output image in a new figure.

idx — Linear indices of pixels belonging to selected objects

numeric vector

Linear indices of the pixels belonging to the selected objects, returned as a numeric vector.

More About

Interactive Behavior

When you run bwselect without specifying pixel coordinates, bwselect enables you to select points interactively from an image in a figure window. Select points using these commands.

Interactive Behavior	Description
Add points	Left-click points in the image.
Remove previous point	Press Backspace or Delete.
Add final point and complete selection	Right-click, double-click, or press Shift and left-click simultaneously.
Complete selection without adding final point	Press Return.

Extended Capabilities

C/C++ Code Generation

Generate C and C++ code using MATLAB® Coder™.

Usage notes and limitations:

• bwselect supports the generation of C code (requires MATLAB Coder™). Note that if you choose the generic MATLAB Host Computer target platform, bwselect generates code that uses a precompiled, platform-specific shared library. Use of a shared library preserves performance optimizations but limits the target platforms for which code can be generated. For more information, see Code Generation Using a Shared Library.

• When generating code, bwselect only supports the following syntaxes:

  • BW2 = bwselect(BW,c,r)

  • [BW2,idx] = bwselect(BW,c,r)

  • BW2 = bwselect(BW,c,r,n)

  • [BW2,idx] = bwselect(BW,c,r,n)

• In addition, the optional fourth input argument, n, must be a compile-time constant.

GPU Code Generation

Generate CUDA® code for NVIDIA® GPUs using GPU Coder™.

Usage notes and limitations:

• When generating code, bwselect supports only these syntaxes:

  • BW2 = bwselect(BW, c, r)

  • [BW2, idx] = bwselect(BW, c, r)

  • BW2 = bwselect(BW, c, r, n)

  • [BW2, idx] = bwselect(BW, c, r, n)

• In addition, the optional fourth input argument, n, must be a compile-time constant.