Here is a mathematician's domain. Most of filters are using convolution
matrix. With the Convolution Matrix filter, if the fancy takes you, you
can build a custom filter.
Delving Deep into Rectifiers: Surpassing Human-Level Performance on ImageNet Classification
by Kaiming He, Xiangyu Zhang, Shaoqing Ren, Jian Sun
(original: https://arxiv.org/pdf/1502.01852.pdf)