Remove Noise from Color Image Using Pretrained Neural Network

This example shows how to remove Gaussian noise from an RGB image using a denoising convolutional neural network.

Read a color image into the workspace and convert the data to data type double. Display the pristine color image.

pristineRGB = imread("lighthouse.png");
pristineRGB = im2double(pristineRGB);
imshow(pristineRGB)
title("Pristine Image")

Add zero-mean Gaussian white noise with a variance of 0.01 to the image. The imnoise function adds noise to each color channel independently. Display the noisy color image.

noisyRGB = imnoise(pristineRGB,"gaussian",0,0.01);
imshow(noisyRGB)
title("Noisy Image")

The pretrained denoising convolutional neural network, DnCNN, operates on single-channel images. Split the noisy RGB image into its three individual color channels.

[noisyR,noisyG,noisyB] = imsplit(noisyRGB);

Load the pretrained DnCNN network.

net = denoisingNetwork("dncnn");

Use the DnCNN network to remove noise from each color channel.

denoisedR = denoiseImage(noisyR,net);
denoisedG = denoiseImage(noisyG,net);
denoisedB = denoiseImage(noisyB,net);

Recombine the denoised color channels to form the denoised RGB image. Display the denoised color image.

denoisedRGB = cat(3,denoisedR,denoisedG,denoisedB);
imshow(denoisedRGB)
title("Denoised Image")

Calculate the peak signal-to-noise ratio (PSNR) for the noisy and denoised images. A larger PSNR indicates that noise has a smaller relative signal, and is associated with higher image quality.

noisyPSNR = psnr(noisyRGB,pristineRGB);
fprintf("\n The PSNR value of the noisy image is %0.4f.",noisyPSNR);

 The PSNR value of the noisy image is 20.6395.

denoisedPSNR = psnr(denoisedRGB,pristineRGB);
fprintf("\n The PSNR value of the denoised image is %0.4f.",denoisedPSNR);

 The PSNR value of the denoised image is 29.6857.

Calculate the structural similarity (SSIM) index for the noisy and denoised images. An SSIM index close to 1 indicates good agreement with the reference image, and higher image quality.

noisySSIM = ssim(noisyRGB,pristineRGB);
fprintf("\n The SSIM value of the noisy image is %0.4f.",noisySSIM);

 The SSIM value of the noisy image is 0.7393.

denoisedSSIM = ssim(denoisedRGB,pristineRGB);
fprintf("\n The SSIM value of the denoised image is %0.4f.",denoisedSSIM);

 The SSIM value of the denoised image is 0.9507.

In practice, image color channels frequently have correlated noise. To remove correlated image noise, first convert the RGB image to a color space with a luminance channel, such as the L*a*b* color space. Remove noise on the luminance channel only, then convert the denoised image back to the RGB color space.