imclearborder

Suppress light structures connected to image border

Syntax

J = imclearborder(I)

J = imclearborder(I,conn)

Description

J = imclearborder(I) suppresses structures in image I that are lighter than their surroundings and that are connected to the image border. Use this function to clear the image border. For grayscale images, imclearborder tends to reduce the overall intensity level in addition to suppressing border structures. The output image, J, is grayscale or binary, depending on the input.

J = imclearborder(I,conn) specifies the pixel connectivity, conn.

Examples

Impact of Connectivity on Clearing the Border

Create a simple binary image.

BW = [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     0     0     1     1     1     0     0     0
      0     1     0     1     1     1     0     0     0
      0     0     0     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     0     0     0     0];

Clear pixels on the border of the image using 4-connectivity. Note that imclearborder does not clear the pixel at (5,2) because, with 4-connectivity, it is not considered connected to the border pixel at (4,1).

BWc1 = imclearborder(BW,4)

BWc1 = 9×9

     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     0     0     0
     0     1     0     1     1     1     0     0     0
     0     0     0     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     0     0     0     0

Now clear pixels on the border of the image using 8-connectivity. imclearborder clears the pixel at (5,2) because, with 8-connectivity, it is considered connected to the border pixel (4,1).

BWc2 = imclearborder(BW,8)

BWc2 = 9×9

     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     0     0     0
     0     0     0     1     1     1     0     0     0
     0     0     0     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     0     0     0     0

Input Arguments

I — Grayscale or binary image

numeric array | logical array

Grayscale or binary image, specified as a numeric or logical array.

Example: I = imread('pout.tif');

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

conn — Pixel connectivity

4 | 8 | 6 | 18 | 26 | 3-by-3-by- ... -by-3 matrix of 0s and 1s

Pixel connectivity, specified as one of the values in this table. The default connectivity is 8 for 2-D images, and 26 for 3-D images.

Value	Meaning
Two-Dimensional Connectivities
4	Pixels are connected if their edges touch. The neighborhood of a pixel are the adjacent pixels in the horizontal or vertical direction.	Current pixel is shown in gray.
8	Pixels are connected if their edges or corners touch. The neighborhood of a pixel are the adjacent pixels in the horizontal, vertical, or diagonal direction.	Current pixel is shown in gray.
Three-Dimensional Connectivities
6	Pixels are connected if their faces touch. The neighborhood of a pixel are the adjacent pixels in: One of these directions: in, out, left, right, up, and down	Current pixel is shown in gray.
18	Pixels are connected if their faces or edges touch. The neighborhood of a pixel are the adjacent pixels in: One of these directions: in, out, left, right, up, and down A combination of two directions, such as right-down or in-up	Current pixel is center of cube.
26	Pixels are connected if their faces, edges, or corners touch. The neighborhood of a pixel are the adjacent pixels in: One of these directions: in, out, left, right, up, and down A combination of two directions, such as right-down or in-up A combination of three directions, such as in-right-up or in-left-down	Current pixel is center of cube.

For higher dimensions, imclearborder uses the default value conndef(ndims(I),'maximal').

Connectivity can also be defined in a more general way for any dimension by specifying a 3-by-3-by- ... -by-3 matrix of 0s and 1s. The 1-valued elements define neighborhood locations relative to the center element of conn. Note that conn must be symmetric about its center element. See Specifying Custom Connectivities for more information.

Note

A pixel on the edge of the input image might not be considered to be a border pixel if you specify a nondefault connectivity. For example, if conn = [0 0 0; 1 1 1; 0 0 0], elements on the first and last row are not considered to be border pixels because, according to that connectivity definition, they are not connected to the region outside the image.

Data Types: double | logical

Output Arguments

J — Processed image

numeric array | logical array

Processed grayscale or binary image, returned as numeric or logical array, depending on the input image you specify.

Algorithms

imclearborder uses morphological reconstruction where:

• Mask image is the input image.

• Marker image is zero everywhere except along the border, where it equals the mask image.

References

[1] Soille, P., Morphological Image Analysis: Principles and Applications, Springer, 1999, pp. 164-165.

Extended Capabilities

C/C++ Code Generation

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

Usage notes and limitations:

• imclearborder supports the generation of C code (requires MATLAB^® Coder™). Note that if you choose the generic MATLAB Host Computer target platform, imclearborder 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 Types of Code Generation Support in Image Processing Toolbox.

• Supports only up to 3-D inputs.

• The optional second input argument, conn, must be a compile-time constant.

Version History

Introduced before R2006a