Optimizing a 6502 image decoder, from 70 minutes to 1 minute
10 hours ago
- #Apple II
- #Quicktake Camera
- #6502 Optimization
- The author aimed to develop a digital photography program for the Apple II using Quicktake cameras due to their compatibility.
- Initial focus was on decoding Quicktake 100 photos, but scope expanded to include Quicktake 150 and 200, delving deeper into image processing.
- The Quicktake 150 format is proprietary and undocumented, with existing decoders in the dcraw project being complex and hard to understand.
- The first functional decoder took seventy minutes to decode a single picture, prompting a series of optimizations.
- Key optimizations included dropping color decoding, understanding and minimizing buffer usage, optimizing divisions, and improving Huffman decoding.
- Algorithmic optimizations significantly reduced the number of instructions needed, from 301 million to under 20 million.
- Assembly optimizations further improved performance, including lookup tables, self-modifying code, and specialized functions for common operations.
- The final implementation reduced decoding time to under one minute, with detailed commits and a repository tracking the progress.
- The author highlights the importance of algorithmic optimization over hand-optimizing assembler for significant speed gains.