gegl:sharpen

 My work involved porting a legacy, foundational 3x3 Sharpen matrix over to GIMP's modern core engine, GEGL (Generic Graphics Library).

Gimp's older sharpen filter was removed from GIMP 3.0 due to it not being maintained and only working on 8 bit images and it often operated in a restrictive environment. It took an image, looked at a tiny grid of pixels, applied  some fast math, and spitted out the result. While this worked fine years ago, it has a massive drawback for modern creators which is that it destroys data. Legacy filters usually force images into 8-bit precision (only 256 levels of color per channel) and bake the changes permanently into your pixels. If you sharpen an image and change your mind later, you're out of luck.

By rebuilding this Sharpen filter as a native GEGL operation, we are giving a classic tool a massive performance and fidelity upgrade. The new implementation focuses on two main pillars which are High Bit-Depth (Instead of squeezing colors into a 8-bit precision box, the new filter processes images at 32-bit floating-point precision. This means absolutely zero color degradation), and Non-Destructive Editing (because GEGL views image processing as a continuous pipeline (a graph), this sharpening can happen live. You can tweak, toggle, or adjust the intensity at any point in your workflow without ever altering your original source pixels).

For the working part, it was quite straight forward: Imagine a tiny 3x3 grid of pixels sliding across your image. For every single pixel, the filter looks at its immediate 8 neighbors (surrounding pixels in the grid), calculates the contrast between them, and amplifies the difference. By boosting the contrast (amplifying lights and darks) your eyes perceive the image as significantly sharper. Also the legacy 8-bit filter used integer scaling factors (like multiplying by 800) and rounding biases (like +4). The goal of my derivation was to strip away the integer scaling and find the effective fractional weights applied to the center pixel and its 8 neighbors.

As https://gitlab.gnome.org/GNOME/gegl/-/merge_requests/269 has been merged, you can expect this new Sharpen filter in the upcoming GIMP and GEGL release.

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