Add script for GIF optimisation

docs/notes/gif-optimisation.md

@@ -0,0 +1,42 @@
+---
+title: Optimising GIFs
+tags:
+  - Images
+sources:
+  - https://cassidy.codes/blog/2017/04/25/ffmpeg-frames-to-gif-optimization/
+  - https://blog.pkh.me/p/21-high-quality-gif-with-ffmpeg.html
+---
+
+Optimising images is a fairly well understood problem, with lots of different solutions and tools. Images can be resized, their pixels grouped, or converted to new, more efficient formats. GIFs however aren't as simple.
+
+Resizing images is fairly simple, and so long as the image isn't too large, not too resource intensive. Resizing GIFs, again, is far more complex.
+
+## 1. Reducing the framerate
+
+Using `ffmpeg`, it's possible to reduce the framerate of a GIF. This script reduces the framerate to 5FPS, which drastically drops the size, as GIFs have poor inter-frame compression.
+
+```bash
+#!/usr/bin/env bash
+
+palette="/tmp/palette.png"
+
+filters="fps=5"
+
+ffmpeg -v warning -i $1 -vf "$filters,palettegen=stats_mode=diff" -y $palette
+
+ffmpeg -i $1 -i $palette -lavfi "$filters,paletteuse=dither=bayer:bayer_scale=5:diff_mode=rectangle" -y $2
+```
+
+`stats_mode=diff` helps generate a more efficient colour palette, and `diff_mode=rectangle` makes areas which didn't change transparent in the subsequent frames, to further reduce image size.
+
+## 2. Mass optimisation
+
+[`gifsicle`](https://www.lcdf.org/gifsicle/) can optimise and resize a GIF.
+
+It can change the delay between frames, but not strip out frames (thus reducing the FPS), hence the need for `ffmpeg`.
+
+```bash
+$ gifsicle -i in.gif -o out.gif --lossy -O3 --resize-width 500
+```
+
+This optimises the GIF, without too much emphasis on the resulting quality, and resizes it to 500px wide (maintaining aspect ratio).