Car License Plate Detection Using Morphological Operators

Elements of Morphology

Types of Morphological Operations

Morphology is a broad set of image processing operations that process images based on shapes. Morphological operations apply a structuring element to an input image, creating an output image of the same size. In a morphological operation, the value of each pixel in the output image is based on a comparison of the corresponding pixel in the input image with its neighbors.

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Morphological Reconstruction

Morphological reconstruction can be thought of conceptually as repeated dilations of an image, called the marker image, until the contour of the marker image fits under a second image, called the mask image. In morphological reconstruction, the peaks in the marker image “spread out,” or dilate.

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Structuring Elements

An essential part of the morphological dilation and erosion operations is the structuring element used to probe the input image. A structuring element is a matrix that identifies the pixel in the image being processed and defines the neighborhood used in the processing of each pixel. You typically choose a structuring element the same size and shape as the objects you want to process in the input image. For example, to find lines in an image, create a linear structuring element.

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Border Padding for Morphology

Morphological functions position the origin of the structuring element, it’s center element, over the pixel of interest in the input image. For pixels at the edge of an image, parts of the neighborhood defined by the structuring element can extend past the border of the image.

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Pixel Connectivity

Morphological processing starts at the peaks in the marker image and spreads throughout the rest of the image based on the connectivity of the pixels. Connectivity defines which pixels are connected to other pixels. A set of pixels in a binary image that forms a connected group is called an object or a connected component.

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Lookup Table Operations

A lookup table is a vector in which each element represents the different permutations of pixels in a neighborhood. Lookup tables are useful for custom erosion and dilation operations.

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Applications of Morphology

Dilate an Image to Enlarge a Shape

Dilation adds pixels to boundary of an object. Dilation makes objects more visible and fills in small holes in the object.

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Remove Thin Lines Using Erosion

Erosion removes pixels from the boundary of an object. Erosion removes islands and small objects so that only substantive objects remain.

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Use Morphological Opening to Extract Large Image Features

You can use the morphological opening to remove small objects from an image while preserving the shape and size of larger objects in the image.

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Flood-Fill Operations

A flood fill operation assigns a uniform pixel value to connected pixels, stopping at object boundaries.

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