Low-Frequency Filtering in the Spatial Domain

These are enhancements that block the high spatial frequency detail. They are called low-pass filters. This filter evaluates a particular input pixel brightness value and the pixel surrounding the input pixel and output is a new brightness value (BY, j) for the central pixel, that is the mean of this convolution. The size of the kernel ( ) is usually 3x3, 5x5 and 7x7. 

The coefficients c., in the mask are multiplied by the following individual brightness values (BV.) in the input image 

The primary pixel under investigation is BV5=BV,. . The original data which will result in a low frequency image is given by expression 

The weighted low-pass mask carries an option of giving a center weight to an individual pixel. It can be used to have selective smoothing. 

The edge preserving median filter is where the median value of the black pixels and gray pixels is computed in a 5x5 array, these two values and the central original brightness value are ranked in ascending order. A final median value is selected to replace the central pixel. This filter preserves edges and comers. 

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