Video: Convolution filters

1.Foreword

This filter family deals with convolution filters : Mean, median, sharpen, ....

The convolution filters take a 3x3 matrix and apply it to the pixel neighbours. They can be applied on luma/chroma or both.

Median and Mean filters applied on chroma only are very efficient to get rid of chroma noise from VCR capture

2.Convolution filters

Mean 

The filter replaces a pixel by the average value of its neighboors. It is a crude low pass filter.

Median 

The filter replaces a pixel by median value of its neighbor. It is a bit like mean, except it removes very different value. Efficient to remove high fq noise (like spot of white in old movies). It blurs a lot.

Sharpen 

This filter is interesting as it has negative values in the convolution matrix. It sharpens the image by enhancing difference between near pixels. It can lead to artefacts if the source has a lot of contrast already.

Gaussian smooth 

It is also an averaging filter, like mean. But coefficients are different and the result is less brutal. Still strong a blurring, but less than mean.