# Hit-or-miss transform

In mathematical morphology, hit-or-miss transform is an operation that detects a given configuration (or pattern) in a binary image, using the morphological erosion operator and a pair of disjoint structuring elements. The result of the hit-or-miss transform is the set of positions, where the first structuring element fits in the foreground of the input image, and the second structuring element misses it completely.

## Mathematical definition

In binary morphology, an image is viewed as a subset of an Euclidean space $\mathbb{R}^d$ or the integer grid $\mathbb{Z}^d$, for some dimension d. Let us denote this space or grid by E.

A structuring element is a simple, pre-defined shape, represented as a binary image, used to probe another binary image, in morphological operations such as erosion, dilation, opening, and closing.

Let $C$ and $D$ be two structuring elements satisfying $C\cap D=\emptyset$. The pair (C,D) is sometimes called a composite structuring element. The hit-or-miss transform of a given image A by B=(C,D) is given by:

$A\odot B=(A\ominus C)\cap(A^c\ominus D)$,

where $A^c$ is the set complement of A.

That is, a point x in E belongs to the hit-or-miss transform output if C translated to x fits in A, and D translated to x misses A (fits the background of A).

## Some applications

### Thinning

Let $E=Z^2$, and consider the eight composite structuring elements, composed by:

$C_1=\{(0,0),(-1,-1),(0,-1),(1,-1)\}$ and $D_1=\{(-1,1),(0,1),(1,1)\}$,
$C_2=\{(-1,0),(0,0),(-1,-1),(0,-1)\}$ and $D_2=\{(0,1),(1,1),(1,0)\}$

and the three rotations of each by $90^o$, $180^o$, and $270^o$. The corresponding composite structuring elements are denoted $B_1,\ldots,B_8$.

For any i between 1 and 8, and any binary image X, define

$X\otimes B_i=X\setminus (X\odot B_i)$,

where $\setminus$ denotes the set-theoretical difference.

The thinning of an image A is obtained by cyclically iterating until convergence:

$A\otimes B_1\otimes B_2\otimes\ldots\otimes B_8\otimes B_1\otimes B_2\otimes\ldots$.

### Other applications

• Pattern detection. By definition, the hit-or-miss transform indicates the positions where a certain pattern (characterized by the composite structuring element B) occurs in the input image.
• Pruning. The hit-or-miss transform can be used to identify the end-points of a line to allow this line to be shrunk from each end to remove unwanted branches.

## Bibliography

• An Introduction to Morphological Image Processing by Edward R. Dougherty, ISBN 0-8194-0845-X (1992)