The ability to create custom filters is a mainstay of any quality graphics application.  A robust matrix-based filter engine can generate hundreds of unique effects by simply adjusting the matrices that get passed into the engine.

In this project, I’ve provided a 5x5 custom filter engine with support for scaling/weighting and biasing/offsetting.  This is identical to the custom filter engine provided by Photoshop, and mine is even slightly faster (at least for the screen-sized images I’ve been testing).

Besides being fast, the engine is also smart.  It is clever enough to estimate edge pixels correctly - even for scaled/weighted filters - and it will automatically validate all input values to make sure they’re appropriate.

Finally, I’ve included 7 sample filters for you to play with, including blur, sharpen, emboss, engrave, grease, unfocus, and vibrate. Below, you can see how each filter looks on a promotional image from the TV show Castle.

Original:

castle

Blur:

castle_blur

Sharpen:

castle_sharpen

Emboss:

castle_emboss

Engrave:

castle_engrave

Grease:

castle_grease

Unfocus:

castle_unfocus

Vibrate:

castle_vibrate

Download the sample app and source code from GitHub

For a more advanced version of this tool, including auto-normalization for the divisor and offset values, you can see the source code for this function in my open-source photo editor, PhotoDemon. Here’s a screenshot of its Effects > Custom tool:

PD_custom_filter_screenshot.png