Filter Grayscale and Truecolor (RGB) Images Using imfilter Function

This example shows how to filter a 2-D grayscale image with a 5-by-5 filter containing equal weights (often called an averaging filter) using imfilter. The example also shows how to filter an truecolor (RGB) image with the same filter. A truecolor image is a 3-D array of size m-by-n-by-3, where the last dimension represents the three color channels. Filtering a truecolor image with a 2-D filter is equivalent to filtering each plane of the image individually with the same 2-D filter.

There are several MATLAB® functions that perform 2-D and multidimensional filtering that can be compared to imfilter. The function filter2 performs two-dimensional correlation, conv2 performs two-dimensional convolution, and convn performs multidimensional convolution. However, each of these filtering functions always converts the input to double, and the output is always double. Also, these MATLAB® filtering functions always assume the input is zero padded, and they do not support other padding options. In contrast, imfilter does not convert input images to double. The imfilter function also offers a flexible set of boundary padding options.

Filter 2-D Grayscale Image with an Averaging Filter

Read a grayscale image into the workspace.

I = imread("coins.png");

Display the original image.

imshow(I)
title("Original Image")

Create a normalized, 5-by-5, averaging filter.

h = ones(5,5)/25;

Apply the averaging filter to the grayscale image using imfilter.

I2 = imfilter(I,h);

Display the filtered image.

imshow(I2)
title("Filtered Image")

Filter Multidimensional Truecolor (RGB) Image Using imfilter

Read a truecolor image into the workspace.

rgb = imread("peppers.png");
imshow(rgb)

Create a filter. This averaging filter contains equal weights, and causes the filtered image to look more blurry than the original.

h = ones(5,5)/25;

Filter the image using imfilter and display it.

rgb2 = imfilter(rgb,h);
imshow(rgb2)