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# Erosion

Find local minima in binary or intensity images

## Library

Morphological Operations

visionmorphops

## Description

The Erosion block slides the neighborhood or structuring element over an image, finds the local minima, and creates the output matrix from these minimum values. If the neighborhood or structuring element has a center element, the block places the minima there, as illustrated in the following figure.

If the neighborhood or structuring element does not have an exact center, the block has a bias toward the upper-left corner and places the minima there, as illustrated in the following figure.

This block uses flat structuring elements only.

PortInput/OutputSupported Data TypesComplex Values Supported

I

Vector or matrix of intensity values

• Double-precision floating point

• Single-precision floating point

• Fixed point

• Boolean

• 8-, 16-, and 32-bit signed integer

• 8-, 16-, and 32-bit unsigned integer

No

Nhood

Matrix or vector of 1s and 0s that represents the neighborhood values

Boolean

No

Output

Vector or matrix of intensity values that represents the eroded image

Same as I port

No

The output signal is the same data type as the input to the I port.

Use the Neighborhood or structuring element source parameter to specify how to enter your neighborhood or structuring element values. If you select Specify via dialog, the Neighborhood or structuring element parameter appears in the dialog box. If you select Input port, the Nhood port appears on the block. Use this port to enter your neighborhood values as a matrix or vector of 1s and 0s. You can only specify a structuring element using the dialog box.

Use the Neighborhood or structuring element parameter to define the neighborhood or structuring element that the block applies to the image. Specify a neighborhood by entering a matrix or vector of 1s and 0s. Specify a structuring element with the strel function from the Image Processing Toolbox™. If the structuring element is decomposable into smaller elements, the block executes at higher speeds due to the use of a more efficient algorithm. If you enter an array of STREL objects, the block applies each object to the entire matrix in turn.

## Dialog Box

The Erosion dialog box appears as shown in the following figure.

Neighborhood or structuring element source

Specify how to enter your neighborhood or structuring element values. Select Specify via dialog to enter the values in the dialog box. Select Input port to use the Nhood port to specify the neighborhood values. You can only specify a structuring element using the dialog box.

Neighborhood or structuring element

If you are specifying a neighborhood, this parameter must be a matrix or vector of 1s and 0s. If you are specifying a structuring element, use the strel function from the Image Processing Toolbox. This parameter is visible if, for the Neighborhood or structuring element source parameter, you select Specify via dialog.

## References

[1] Soille, Pierre. Morphological Image Analysis. 2nd ed. New York: Springer, 2003.