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Raph Levien’s November 7, 2026 report describes a just-in-time (JIT) method for rendering bitmap text on an RP2350 microcontroller. The technique generates optimized code for a string to help a scanline-based renderer meet a budget of about six processor cycles per pixel while conserving memory. The report presents an implementation in an experimental home-computer project, not a benchmarked general-purpose library.
Software developer Raph Levien has described a just-in-time bitmap text renderer for the RP2350 microcontroller, generating specialized code on the fly to draw a string. The implementation is part of vic-2350, Levien’s recreation of an 1980s home computer, where rendering each scanline must fit a tight time budget.
Levien says the renderer is designed for a system that outputs DVI video using the RP2350’s HSTX peripheral but relies on software to fill the video data. His project renders as the display is scanned, rather than keeping a full image in a conventional framebuffer. The stated budget is about six processor cycles per pixel; time saved on text can be used for other graphics or application work.
The JIT’s central idea is to create optimized code for the string being rendered, rather than repeatedly using a general-purpose text-rendering path. Levien says he implemented the approach in his race-the-beam renderer and that it could also be used when drawing into a framebuffer. The report does not provide comparative benchmark results or measurements of the JIT’s speedup.
Text objects hold the string, a font pointer, and foreground and background colors. Levien says an earlier design stored an expanded form of the text in the display list, but that used too much memory. The display list is double-buffered, and the RP2350 has 520 kilobytes of SRAM, which must also serve the application and other rendering needs. The report describes the memory-saving motivation, though the supplied material does not quantify the final text object’s RAM use.
Fitting Flexible Text Into Each Scanline
The technique addresses a practical trade-off in small display systems: flexible typography takes processing time, while fixed character cells can be faster but constrain font design and spacing. Levien wants proportional spacing and kerning as well as crisp bitmap glyphs, choices that add demands to a renderer working against a per-scanline deadline.
For RP2350 developers, the approach could make it more practical to combine readable, individually spaced text with other graphics without reserving a full framebuffer. That could leave more of the chip’s limited SRAM available for application data and reduce the time before changes appear on screen. Those are the design aims described in the report; wider performance benefits have not been established by independent testing.
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From Character Cells to Scanline Rendering
Older home computers commonly displayed text through fixed character cells, with hardware reading glyphs from font ROM as the picture was sent to a monitor. That method was responsive and economical, but generally limited text to monospaced layouts. Bitmap framebuffers brought more visual flexibility, but required more memory and computing work to update.
Levien’s RP2350 project takes a different route: it generates each scanline for video output rather than maintaining a complete framebuffer. Its display list is double-buffered, with one list being rendered while the application prepares the next. Before presentation, objects are sorted by their vertical position; the renderer tracks which objects intersect each scanline and calls their render methods. This structure makes the available time for drawing text particularly constrained.
Levien says the renderer and much of vic-2350 use Symbolasm, a language he created for high-performance Cortex-M33 programming, alongside ARM assembly. He writes that Rust did not meet the project’s performance requirements. These are descriptions of his own implementation and development choices, not a general comparison of languages across microcontroller projects.
“There’s a budget of approximately 6 cycles per pixel.”
— Raph Levien, in the Linebender report
Performance Results Still Unreported
The report describes the implementation and its design goals, but does not give benchmark figures showing how much faster JIT rendering is than another method, or how many cycles a rendered character or string consumes. It also does not report the JIT’s code-generation time, its code-memory requirements, or the final memory footprint of text objects.
It is not clear from the supplied report material how the technique behaves across different string lengths, fonts, or display configurations, or whether it has been packaged for use outside vic-2350. Levien says the same approach could work with a framebuffer, but that possibility is not accompanied by a separate implementation or test results here. The report also mentions future renderer optimization, including tracking opaque spans, without presenting that work as complete.
More Renderer Optimization Planned
Levien says he plans to optimize the renderer further by tracking opaque spans and skipping pixels that will later be overwritten by other display objects. He describes that work as a separate improvement, rather than a requirement for the text technique.
The report does not give a release schedule for vic-2350 or a date for public performance tests. The next useful evidence would be measurements of rendering time and memory use, along with tests showing how the generated code performs for different strings and display workloads. Until those details are published, the technique is best understood as an implementation report from a specific microcontroller project.
Key Questions
What is JIT bitmap text rendering?
It is a method that generates code while a program is running to draw a particular string of bitmap text, rather than relying only on a general-purpose rendering routine.
Which microcontroller is the technique designed for?
Levien describes using it on the RP2350, in a project that outputs DVI video through the chip’s HSTX peripheral.
Why render one scanline at a time?
Levien’s project generates video as the display is scanned instead of storing a complete framebuffer. He says this approach can save memory and reduce latency, but it places a strict time limit on drawing each line.
Does the report prove that JIT rendering is faster?
No benchmark comparison is included in the supplied report material. Levien presents the technique as a way to improve rendering performance, but its speedup and memory costs remain unquantified.
Can the method work with a framebuffer?
Levien says the technique could also be used when rendering into a framebuffer. The report does not provide a separate framebuffer implementation or results.
Source: linebender.org
