LEDMatrix/docs/SCROLL_PERFORMANCE.md

Scroll Performance

How scrolling is paced on this hardware, what was wrong with it, and how to configure a plugin so its marquee is smooth.

Measured on a Raspberry Pi 4 driving a 2×128×64 chain (256×64 logical) at limit_refresh_rate_hz: 100. Numbers below come from that panel.

	before	after
scroll frame rate	44–46 fps	100 fps, locked
frames ≥ 45 ms	14–17%	none observed
dominant frame time	20 ms	10 ms
disk cache write (~1 MB)	14.8 ms	5.4 ms

---

The one rule that matters

Motion is smooth when the strip advances a whole number of pixels per panel refresh.

Advancing one pixel per refresh on a 100 Hz panel gives 100 px/s. Slower crisp speeds come from holding each frame for several refreshes -- 50 px/s is one pixel every second refresh -- which is covered under Choosing a speed below. A speed that lands on no such combination has to do one of two bad things:

• blend two adjacent columns to render a half-step — on pixel-font text this alternates crisp and smeared frames and reads as shimmer, or as the text jumping a pixel ahead of itself;
• repeat a frame — the strip stands still, then jumps, which reads as judder.

Neither is tunable away. Pick a speed that divides evenly.

src.common.scroll_config solves this for you: configure() snaps a requested speed to the nearest one the panel can actually show in whole pixels, and scripts/scroll_speeds.py prints the full ladder for your hardware.

Choosing a speed

The crisp speeds are not a fixed list -- they depend on how fast your panel refreshes, which depends on its size, pwm_bits, gpio_slowdown and the Pi model. A Pi Zero driving a long chain has a completely different set of good speeds from a Pi 4 driving a short one.

# what can this panel do? (reads your configured refresh rate)
python3 scripts/scroll_speeds.py

# what does it ACTUALLY manage, rather than what is configured?
sudo systemctl stop ledmatrix
sudo python3 scripts/scroll_speeds.py --measure
sudo systemctl start ledmatrix

# highlight the closest option to the speed you want
python3 scripts/scroll_speeds.py --want 45

# try one on the panel
sudo systemctl stop ledmatrix
sudo python3 scripts/scroll_speeds.py --demo 50
sudo systemctl start ledmatrix

Sample ladder for a 100 Hz panel:

  20.0 px/s  (1px every 5 refreshes =  20.0 fps, slightly stepped)
  25.0 px/s  (1px every 4 refreshes =  25.0 fps, slightly stepped)
  33.3 px/s  (1px every 3 refreshes =  33.3 fps, smooth)
  50.0 px/s  (1px every 2 refreshes =  50.0 fps, smooth)
  66.7 px/s  (2px every 3 refreshes =  33.3 fps, smooth)
 100.0 px/s  (1px every 1 refresh  = 100.0 fps, smooth)

How a slow speed stays crisp

SwapOnVSync(canvas, framerate_fraction) holds each frame for N panel refreshes. The panel keeps refreshing at its full rate either way, so holding a frame costs nothing in flicker -- it only changes how often a new image is presented. That is what allows 50 px/s to be one whole pixel every second refresh, instead of half a pixel every refresh (which has no good rendering, only a choice between blur and judder).

scroll_config.configure() snaps the requested speed to the nearest entry on the ladder and applies the hold, provided it is given the display manager:

scroll_config.configure(
    self.scroll_helper,
    plugin_config=self.config,
    global_config=self.global_config,
    display_manager=self.display_manager,   # required for the hold to apply
)

Without display_manager a sub-refresh speed still resolves, but the hold is never applied and the motion falls back to fractional pixels -- so configure logs a warning rather than failing quietly. Pass snap_to_crisp=False to keep an exact requested speed and accept the artefacts.

Speeds slower than about 20 px/s are stepped no matter what, because a 1-pixel advance at 20 fps is simply a coarse increment. That is the pixel pitch, not a software limit; the only way to move in smaller increments is sub-pixel blending, which this display does not tolerate (see above).

Configuring a plugin

Use the shared resolver rather than reading config keys yourself:

from src.common import scroll_config

settings = scroll_config.configure(
    self.scroll_helper,
    plugin_config=self.config,
    global_config=self.global_config,
    refresh_hz=scroll_config.refresh_hz_from_config(self.global_config),
    plugin_logger=self.logger,
)

It resolves every config shape in one place, applies the speed, and returns what it did. Precedence, highest first:

1. display_options.scroll_speed + scroll_delay — the recommended form
2. display.scroll_speed + scroll_delay — deprecated shape
3. scroll_speed + scroll_delay at the root — legacy flat
4. scroll_pixels_per_second — deprecated
5. the global display block
6. the built-in default (100 px/s)

scroll_speed is pixels per frame and scroll_delay is the frame period in seconds, so the pair means scroll_speed / scroll_delay px/s. The recommended config for a 100 Hz panel:

"display_options": { "scroll_speed": 1.0, "scroll_delay": 0.01 }

Why the deprecated key ranks below the explicit pair

Because some plugins give scroll_pixels_per_second a schema default, and schema defaults are merged into plugin config. Ranking it above the pair means it is always present and always wins, so the documented settings become unreachable. That is a real, shipped bug — see ledmatrix-plugins#408.

If you are writing a plugin: do not give a deprecated key a schema default.

What was actually wrong

Four independent faults, each found by measurement.

1. The frame loop slept on top of a wait it had already done

display_controller.py ran the high-FPS loop as render → SwapOnVSync (blocks to the panel's refresh) → time.sleep(0.008) → plugin ticks. The sleep was unconditional and added to a wait that had already happened. Render work measured ~4 ms, so each iteration cost ~12 ms against a 10 ms refresh grid — every swap missed a refresh and landed on the next one. The loop settled at exactly 50 fps while asking for 125, with no headroom, so ~14% of frames slipped a further refresh.

Now the loop sleeps only the remainder of the frame budget, with a 1 ms floor so plugin threads still get the GIL.

2. SwapOnVSync held the GIL while blocking

The rgbmatrix binding declares it without nogil (unlike SetPixel, Clear and Fill immediately above it in cppinc.pxd), so the render thread held the GIL for the entire vsync wait — most of every frame. Background threads were starved into long uninterruptible bursts; a 1.5 MB API response costs ~17 ms to parse and ~18 ms to re-encode for the cache, and json.raw_decode cannot be preempted mid-document. Those bursts are what the render loop then waited on.

Fixed by rebuilding the binding: scripts/build_rgbmatrix_nogil.sh.

3. Sub-pixel blending was wrong for this display

Enabling it made things worse, not better — see the rule at the top. It is off by default and only Vegas mode opts in via set_sub_pixel_scrolling(True).

4. Frame-based stepping raced the vsync clock

Frame-based mode gated motion on a wall clock at 1/scroll_delay steps per second. Plugins set scroll_delay to the frame period, which puts that comparison exactly on its own threshold: a frame arriving a hair early moved zero pixels and rendered an identical frame, which dirty-tracking skipped, so it returned in ~2 ms and the beat repeated. No scroll_delay value tunes this out — a shorter delay just trades stalled frames for periodic double-steps.

ScrollHelper now accumulates elapsed time in both modes at the same configured speed, so position stays proportional to real time.

Diagnosing a juddery scroller

Avg FPS will lie to you. It is a 100-frame moving average, and a 2 ms duplicate frame plus a 21 ms double-wait average to exactly 10 ms. A ticker that is stalling on half its frames still reports a healthy 100.0.

Look at the distribution instead:

journalctl -u ledmatrix --since "-10min" --no-pager \
  | grep -oE "Frame time: [0-9.]+ms" | awk '{print $3}' | sed 's/ms//' \
  | awk '{printf "%.0f\n", $1}' | sort -n | uniq -c

Reading it, on a 100 Hz panel:

you see	it means
everything at 10 ms	healthy
a mode at ~2 ms	duplicate frames — the swap was skipped because the image did not change. The scroller is advancing less than one pixel per frame.
a mode at 20/30/50 ms	frames missing refreshes — per-frame work is overrunning, or a background thread is holding the GIL
Avg FPS above 100	duplicates present, unless the scroll cycle has completed and is idling

Then confirm what the plugin actually loaded — config edits do not always reach the running code:

journalctl -u ledmatrix --since "-5min" --no-pager | grep -iE "px/s|px/frame"

If a plugin logs its scroll config twice with different modes, the second line is what is running.

Rebuilding the binding

bash scripts/build_rgbmatrix_nogil.sh              # build into a scratch dir
sudo bash scripts/build_rgbmatrix_nogil.sh --install
sudo bash scripts/build_rgbmatrix_nogil.sh --rollback

The build never touches the installed module. --install backs up the original to ~/rgbmatrix-core.so.ORIGINAL first, and rolls back automatically if the service does not come back healthy. Requires build-essential; Cython is installed into a cached venv under ~/.cache/ledmatrix-cython.

Re-run it after upgrading rpi-rgb-led-matrix, since a library upgrade replaces the patched binding.

Faster JSON

src/cache/disk_cache.py uses orjson when it is importable and falls back to the stdlib otherwise, so it is optional:

sudo pip3 install --break-system-packages orjson

Encoding is where it pays — about 7× on this hardware. Decoding gains far less (~1.3× on large payloads) because the cost there is building Python objects, not scanning text. That is also why moving parsing to a subprocess does not help: pickle.loads of the same payload costs 8.1 ms against json.loads at 10.9 ms, so the work just moves rather than disappearing.