# bwmorph

Morphological operations on binary images

## Syntax

``BW2 = bwmorph(BW,operation)``
``BW2 = bwmorph(BW,operation,n)``

## Description

````BW2 = bwmorph(BW,operation)` applies a specific morphological operation to the binary image `BW`. NoteTo perform morphological operations on a 3-D volumetric image, use `bwmorph3`. ```

example

````BW2 = bwmorph(BW,operation,n)` applies the operation `n` times. `n` can be `Inf`, in which case the operation is repeated until the image no longer changes.```

## Examples

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Read binary image and display it.

```BW = imread('circles.png'); imshow(BW);```

Remove interior pixels to leave an outline of the shapes.

```BW2 = bwmorph(BW,'remove'); figure imshow(BW2)```

Get the image skeleton.

```BW3 = bwmorph(BW,'skel',Inf); figure imshow(BW3)```

## Input Arguments

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Binary image, specified as a 2-D numeric matrix or 2-D logical matrix. For numeric input, any nonzero pixels are considered to be `1` (`true`).

Morphological operation to perform, specified as one of the following.

Operation

Description

`"bothat"`

Perform the morphological bottom hat operation, returning the image minus the morphological closing of the image.

The `bwmorph` function performs morphological closing using the neighborhood `ones(3)`. If you want to perform a morphological bottom hat operation with a different neighborhood, then use the `imbothat` function.

`"branchpoints"`

Find branch points of skeleton. For example:

```0 0 1 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 1 1 1 1 becomes 0 0 1 0 0 0 0 1 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0```

Note: To find branch points, the image must be skeletonized. To create a skeletonized image, use `bwmorph(BW,"skel")`.

`"bridge"`

Bridge unconnected pixels, that is, sets `0`-valued pixels to `1` if they have two nonzero neighbors that are not connected. For example:

```1 0 0 1 1 0 1 0 1 becomes 1 1 1 0 0 1 0 1 1 ```

`"clean"`

Remove isolated pixels (individual `1`s that are surrounded by `0`s), such as the center pixel in this pattern.

```0 0 0 0 1 0 0 0 0 ```

`"close"`

Perform morphological closing (dilation followed by erosion).

The `bwmorph` function performs morphological closing using the neighborhood `ones(3)`. If you want to perform a morphological closing operation with a different neighborhood, then use the `imclose` function.

`"diag"`

Use diagonal fill to eliminate 8-connectivity of the background. For example:

```0 1 0 0 1 0 1 0 0 becomes 1 1 0 0 0 0 0 0 0 ```

`"endpoints"`

Find end points of skeleton. For example:

```1 0 0 0 1 0 0 0 0 1 0 0 becomes 0 0 0 0 0 0 1 0 0 0 1 0 0 0 0 0 0 0 0 0```

Note: To find end points, the image must be skeletonized. To create a skeletonized image, use `bwmorph(BW,"skel")`.

`"fill"`

Fill isolated interior pixels (individual `0`s that are surrounded by `1`s), such as the center pixel in this pattern.

```1 1 1 1 0 1 1 1 1 ```

`"hbreak"`

Remove H-connected pixels. For example:

```1 1 1 1 1 1 0 1 0 becomes 0 0 0 1 1 1 1 1 1 ```

`"majority"`

Set a pixel to `1` if five or more pixels in its 3-by-3 neighborhood are `1`; otherwise, set the pixel to `0`.

`"open"`

Perform morphological opening (erosion followed by dilation).

The `bwmorph` function performs morphological opening using the neighborhood `ones(3)`. If you want to perform a morphological opening operation with a different neighborhood, then use the `imopen` function.

`"remove"`

Remove interior pixels. This option sets a pixel to `0` if all its 4-connected neighbors are `1`, thus leaving only the boundary pixels on.

`"shrink"`

With `n = Inf`, shrink objects to points by removing pixels from the boundaries of objects. Objects without holes shrink to a point, and objects with holes shrink to a connected ring halfway between each hole and the outer boundary. This option preserves the Euler number (also known as the Euler characteristic).

`"skel"`

With `n = Inf`, remove pixels on the boundaries of objects without allowing objects to break apart. The pixels remaining make up the image skeleton. This option preserves the Euler number.

When working with 3-D volumes, or when you want to prune a skeleton, use the `bwskel` function.

`"spur"`

Remove spur pixels. For example:

```0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 becomes 0 0 0 0 0 1 0 0 0 1 0 0 1 1 0 0 1 1 0 0 ```

`"thicken"`

With `n = Inf`, thicken objects by adding pixels to the exterior of objects until doing so would result in previously unconnected objects being 8-connected. This option preserves the Euler number.

`"thin"`

With `n = Inf`, thin objects to lines by removing pixels from the boundary of objects. An object without holes shrinks to a minimally connected stroke, and an object with holes shrinks to a connected ring halfway between each hole and the outer boundary. This option preserves the Euler number. See Algorithms for more detail.

`"tophat"`

Perform the morphological top hat operation, returning the image minus the morphological opening of the image.

The `bwmorph` function performs morphological opening using the neighborhood `ones(3)`. If you want to perform a morphological top hat operation with a different neighborhood, then use the `imtophat` function.

Tip

To perform morphological erosion or dilation, use the `imerode` or `imdilate` function, respectively. If you want to replicate the dilation or erosion performed by the `bwmorph` function, then specify the neighborhood as `ones(3)`.

Data Types: `char` | `string`

Number of times to perform the operation, specified as a positive integer or `Inf`. When you specify `n` as `Inf`, the `bwmorph` function repeats the operation until the image no longer changes.

Example: `100`

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

## Output Arguments

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Image after morphological operations, returned as a 2-D logical matrix.

Data Types: `logical`

## Algorithms

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When used with the `"thin"` option, `bwmorph` uses the following algorithm [3]:

1. In the first subiteration, delete pixel p if and only if the conditions G1, G2, and G3 are all satisfied.

2. In the second subiteration, delete pixel p if and only if the conditions G1, G2, and ${G}_{3}\prime$ are all satisfied.

### Condition G1:

`${X}_{H}\left(p\right)=1$`

where

`${X}_{H}\left(p\right)=\sum _{i=1}^{4}{b}_{i}$`

x1, x2, ..., x8 are the values of the eight neighbors of p, starting with the east neighbor and numbered in counter-clockwise order.

### Condition G2:

`$2\le \mathrm{min}\left\{{n}_{1}\left(p\right),{n}_{2}\left(p\right)\right\}\le 3$`

where

`${n}_{1}\left(p\right)=\sum _{k=1}^{4}{x}_{2k-1}\vee {x}_{2k}$`
`${n}_{2}\left(p\right)=\sum _{k=1}^{4}{x}_{2k}\vee {x}_{2k+1}$`

### Condition G3:

`$\left({x}_{2}\vee {x}_{3}\vee {\overline{x}}_{8}\right)\wedge {x}_{1}=0$`

### Condition G3':

`$\left({x}_{6}\vee {x}_{7}\vee {\overline{x}}_{4}\right)\wedge {x}_{5}=0$`

The two subiterations together make up one iteration of the thinning algorithm. When the user specifies an infinite number of iterations (`n=Inf`), the iterations are repeated until the image stops changing. The conditions are all tested using `applylut` with precomputed lookup tables.

## References

[1] Haralick, Robert M., and Linda G. Shapiro, Computer and Robot Vision, Vol. 1, Addison-Wesley, 1992.

[2] Kong, T. Yung and Azriel Rosenfeld, Topological Algorithms for Digital Image Processing, Elsevier Science, Inc., 1996.

[3] Lam, L., Seong-Whan Lee, and Ching Y. Suen, "Thinning Methodologies-A Comprehensive Survey," IEEE Transactions on Pattern Analysis and Machine Intelligence, Vol 14, No. 9, September 1992, page 879, bottom of first column through top of second column.

[4] Pratt, William K., Digital Image Processing, John Wiley & Sons, Inc., 1991.

## Version History

Introduced before R2006a