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
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.
figure 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
I2 = imfilter(I,h);
Display the filtered image.
figure 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); figure imshow(rgb2)