Linear filtering: Low- and High-pass filters

The first choice is a mean filter, which can be considered as a low-pass filter in the frequency domain.
You can specify the size of the kernel. For example, a 3x3 kernel will give: 

1   |1 1 1|
- * |1 1 1|
9   |1 1 1|

Gaussian filters are defined by their parameter sigma in the equation:

h(x,y) = exp[-(x²+y²)/(2*sigma²)]

Increased value of sigma will augment the low-pass effect.

A high-pass filter can be defined by the kernel

|-1 -1 -1|
|-1  c -1|, c>=8
|-1 -1 -1|

Increasing the value of c will decrease the high pass effect, as the center coefficient will dominate.