cv::Mat src;
cv::Mat dst;

// Replaces each pixel with the average of its neighborhood
cv::blur(src, dst, cv::Size(5,5)); // simple average
cv::boxFilter(src, dst, -1, cv::Size(5,5), cv::Point(-1,-1), true);

// Replaces each pixel with the median in its neighborhood
// Very effective for removing salt-and-pepper noise
cv::medianBlur(src, dst, 5);

// Weights depend on both spatial distance and intensity difference
cv::bilateralFilter(src, dst, 9, 75, 75);


cv::fastNlMeansDenoising(src, dst, 10, 7, 21);
cv::fastNlMeansDenoisingColored(src, dst, 10, 7, 21);

// Custom Convolution
cv::Mat kernel = (1.0/9.0) * (cv::Mat_<float>(3,3) <<
    1,1,1,
    1,1,1,
    1,1,1);
cv::filter2D(src, dst, -1, kernel);

cv::pyrDown(src, dst);    // smooth + resize