Image Filtering

Convolution and correlation, predefined and custom filters, nonlinear filtering, edge-preserving filters

Filtering is a technique for modifying or enhancing an image. For example, you can filter an image to emphasize certain features or remove other features. Image processing operations implemented with filtering include smoothing, sharpening, and edge enhancement.

Apps

Image Region Analyzer

Browse and filter connected components in an image

Functions

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Design Image Filters

`fspecial`	创建预定义的二维滤波器
`fspecial3`	Create predefined 3-D filter
`convmtx2`	2-D convolution matrix

Basic Image Filtering in the Spatial Domain

`imfilter`	N-D filtering of multidimensional images
`roifilt2`	Filter region of interest (ROI) in image
`nlfilter`	General sliding-neighborhood operations
`imgaussfilt`	2-D Gaussian filtering of images
`imgaussfilt3`	3-D Gaussian filtering of 3-D images
`wiener2`	2-D adaptive noise-removal filtering
`medfilt2`	2-D median filtering
`medfilt3`	3-D median filtering
`modefilt`	2-D and 3-D mode filtering
`ordfilt2`	2-D order-statistic filtering
`stdfilt`	Local standard deviation of image
`rangefilt`	Local range of image
`entropyfilt`	Local entropy of grayscale image
`imboxfilt`	2-D box filtering of images
`imboxfilt3`	3-D box filtering of 3-D images
`fibermetric`	Enhance elongated or tubular structures in image
`maxhessiannorm`	Maximum of Frobenius norm of Hessian of matrix
`padarray`	Pad array

Edge-Preserving Filtering

`imbilatfilt`	Bilateral filtering of images with Gaussian kernels
`imdiffuseest`	Estimate parameters for anisotropic diffusion filtering
`imdiffusefilt`	Anisotropic diffusion filtering of images
`imguidedfilter`	Guided filtering of images
`imnlmfilt`	Non-local means filtering of image
`burstinterpolant`	Create high-resolution image from set of low-resolution burst mode images

Texture Filtering

`gabor`	Create Gabor filter or Gabor filter bank
`imgaborfilt`	Apply Gabor filter or filter bank to 2-D image

Filtering By Property Characteristics

`bwareafilt`	Extract objects from binary image by size
`bwpropfilt`	Extract objects from binary image using properties

Integral Image Domain Filtering

`integralImage`	Calculate 2-D integral image
`integralImage3`	计算3 -D integral image
`integralBoxFilter`	2-D box filtering of integral images
`integralBoxFilter3`	3-D box filtering of 3-D integral images

Frequency Domain Filtering

`freqspace`	Frequency spacing for frequency response
`freqz2`	2-D frequency response
`fsamp2`	2-D FIR filter using frequency sampling
`ftrans2`	2-D FIR filter using frequency transformation
`fwind1`	2-D FIR filter using 1-D window method
`fwind2`	2-D FIR filter using 2-D window method

Topics

Getting Started with Image Filtering in the Spatial Domain

What Is Image Filtering in the Spatial Domain?

In a spatially filtered image, the value of each output pixel is the weighted sum of neighboring input pixels. The weights are provided by a matrix called the convolution kernel or filter.

Filter Grayscale and Truecolor (RGB) Images using imfilter Function

This example shows how to filter an image with a 5-by-5 averaging filter containing equal weights.

Filter Images Using Predefined Filter

This example shows how to create a type of special filter called an unsharp masking filter, which makes edges and detail in an image appear sharper.

imfilter Boundary Padding Options

When a portion of the convolution or correlation kernel extends past the edge of an image, you can extrapolate image values by zero-padding the image or by replicating boundary pixels.

Denoising Filtering

Noise Removal

Noise refers to random error in pixel values acquired during image acquisition or transmission. Removing noise can improve image quality.

Apply Gaussian Smoothing Filters to Images

This example shows how to blur an image using Gaussian smoothing filters of different strengths. The example includes isotropic and anisotropic Gaussian filtering.

Reduce Noise in Image Gradients

This example shows how to reduce noise associated with computing image gradients.

Edge-Preserving Filtering

What is Guided Image Filtering?

Guided image filtering performs edge-preserving smoothing on an image. It uses the content of a second image, called a guidance image, to influence the filtering.

Perform Flash/No-flash Denoising with Guided Filter

This example shows how to reduce noise from an image while using a guidance image to preserve the sharpness of edges.

Segment Thermographic Image after Edge-Preserving Filtering

This example shows how to segment a hot object from the background in a thermographic image.

Integral Image Domain Filtering

Integral Image

Integral images are a quick way to represent images for filtering. In an integral image, the value of each pixel is the summation of the pixels above and to the left of it.

Apply Multiple Filters to Integral Image

This example shows how to smooth an image by different amounts by applying box filters of varying sizes to the integral image.

Frequency Domain Filtering

Design Linear Filters in the Frequency Domain

You can design filters that modify the frequency content of images. Filtering in the frequency domain is often faster than filtering in the spatial domain.