A kernel is a (usually) smallish matrix of numbers that is used in image convolutions. Differently sized kernels containing different patterns of numbers give rise to different results under convolution. For instance, Figure 1 shows a 3×3 kernel that implements a mean filter.

Figure 1Convolution kernel for a mean filter with 3×3 neighborhood.

The word `kernel' is also commonly used as a synonym for
`structuring element', which is a similar object
used in mathematical morphology. A structuring
element differs from a kernel in that it also has a specified
*origin*. This sense of the word `kernel' is not used in HIPR.