JIT bitmap text rendering for microcontrollers
Raph Levien unveils an astonishingly fast JIT bitmap text rendering engine for microcontrollers, breathing new life into 'racing the beam' displays on constrained hardware. This deep dive showcases ingenious low-level optimization, from custom assembly languages to novel kerning techniques, appealing directly to the Hacker News affinity for technical prowess and efficient computing. It's a masterclass in pushing hardware limits with clever software, proving that retro aesthetics can thrive with modern, highly optimized code.
The Lowdown
Raph Levien, known for his work in graphics and text rendering, presents a sophisticated JIT bitmap text rendering technique specifically designed for microcontrollers like the RP2350. His project, vic-2350, aims to re-envision an 80s home computer, necessitating efficient text display that offers proportional spacing and kerning without the overhead of modern antialiasing, embracing the crisp aesthetic of early bitmap fonts.
- Hardware Constraints & Solution: The RP2350's limited 520kB SRAM and the need for DVI video output led to a "racing the beam" approach, where video buffers are filled scanline-by-scanline. This imposes a tight performance budget of approximately 6 cycles per pixel.
- Custom Language & JIT: To meet these demanding performance requirements, much of the renderer is written in
symbolasm, a custom assembly language for Cortex-M33 processors. The core innovation is a JIT compiler that generates highly optimized code on the fly for each string, enabling extreme rendering speeds. - Efficient Font Packing: Fonts are stored in an incredibly compact format. Ink data for glyphs is packed into a wide bitmap, exploiting overlaps between characters (e.g., 'l' being part of 'U') to reduce memory footprint and allowing for single 32-bit loads per font row.
- JIT Micro-optimizations: The generated JIT code meticulously assigns registers and employs micro-optimizations, such as reordering instructions to fill CPU delay slots, reducing the cycle cost per glyph to as low as 3 cycles.
- Astonishing Performance: The JIT text rendering achieves remarkable speed, with the JIT compilation taking about 21k cycles and execution taking 42k cycles for a typical demo, consuming only about 2.6% of the RP2350's single-core throughput.
- Dot Product Kerning: Levien introduces an efficient "dot product kerning" method, using bitvectors per glyph to quickly calculate kerning amounts. This is computed during display list build time and is significantly faster than traditional OpenType methods.
- AI/LLM Assistance: The author transparently discloses the use of large language models (specifically Claude) for assistance in translating Rust code to
symbolasm, refining and optimizing code, and developing the compiler tools, highlighting the balance between AI acceleration and maintaining quality.
This work represents a compelling blend of retrocomputing enthusiasm, state-of-the-art low-level optimization, and modern AI-assisted development. By tackling fundamental challenges of text rendering on severely resource-constrained hardware with such ingenuity, Levien delivers a solution that is both highly performant and aesthetically pleasing, setting a new benchmark for embedded graphics.