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perf(install): build rpi-rgb-led-matrix with a faster SetImage (#736)

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first_time_install.sh applies patches/rpi-rgb-led-matrix/0001-bulk-setimage.patch just before building the Python binding and reverts it afterwards (and from the EXIT trap), so the submodule stays at its pinned commit. The patch copies each image row with one bulk FrameCanvas::SetPixels call and writes the bit planes branch-free: on a 512x64 Pi 4 the frame copy went from 6.57 ms to 2.21 ms. A patch that no longer applies is reported and skipped. Existing installs get it on a rebuild (RPI_RGB_FORCE_REBUILD=1).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
This commit is contained in:
Chuck
2026-10-03 13:48:27 -04:00
committed by GitHub
co-authored by Claude Opus 5.5
parent 05deb1ee7d
commit ee789775b4
4 changed files with 347 additions and 4 deletions
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CHANGELOG.md
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@@ -19,6 +19,30 @@ accepts both, but the store flags the old spelling as deprecated
## Unreleased
### Faster frame copy into the panel (library patch, applied at build time)
- Copying each frame into the panel buffer (`SetImage`) was the biggest CPU
cost LEDMatrix owns on large panels: 6-7.5 ms per frame on a 512x64 Pi 4 at
~85 fps, about 60% of a core. The library's binding walked the image column
by column and set one pixel at a time, and each pixel rewrote a word in
every PWM bit plane, 2KB apart, so nearly every write missed the cache.
`patches/rpi-rgb-led-matrix/0001-bulk-setimage.patch` copies row by row in
one bulk call per row, with the colour lookup done once and branch-free
bit-plane writes. The panel buffer is byte-identical to before (882 checks
across image types, offsets, PWM bits, brightness, inverse colours and a
pixel mapper).
- Measured on hdpi (Pi 4, 4x128x64): frame copy 6.57 -> 2.21 ms, the display
process 139% -> 103% of a core, late frames 7.8 -> 5.4 per 1,000.
- `first_time_install.sh` applies the patch to `rpi-rgb-led-matrix-master`
just before building the binding and takes it back out straight after (and
on any exit), so the submodule stays at its pinned commit with no local
changes. A patch that no longer applies after a submodule bump is reported
and skipped; the unpatched library still builds.
- Existing installs keep the library they have until it is rebuilt:
`sudo RPI_RGB_FORCE_REBUILD=1 ./first_time_install.sh`.
`scripts/build_rgbmatrix_nogil.sh` builds from an unpatched copy and is
unchanged.
### Install
- Raspberry Pi OS **Bookworm** (Debian 12, Python 3.11) is supported,