denise-video

Hardware video decode for Denise, a direct-rendering UI toolkit in Rust for embedded Linux and systems without a desktop environment.

A kiosk playing a promo loop is a set-top box with a different sticker, and this crate does what a set-top box does: compressed bytes go to the SoC's decoder over V4L2 memory-to-memory, decoded frames come back as dmabufs, and each dmabuf is flipped onto a DRM plane the display controller composites during scanout. The frame never touches the rasteriser and is never copied. No ffmpeg, no GStreamer, no C library — the V4L2 uapi is spoken directly, the same single-static-binary discipline as denise-drm.

The format menu

Two elementary streams, chosen so every Raspberry Pi hardware-plays at least one: H.264 (.h264, Pi Zero–4 and most embedded SoCs) and HEVC (.h265, Pi 4 and 5). Both yuv420, at most 1080p30. The board picks — a V4L2 decoder enumerates what it accepts, so detection is a question, not a guess:

# #[cfg(target_os = "linux")] {
use denise_video::{Asset, Decoders};
let assets = [Asset::h264("promo.h264"), Asset::h265("promo.h265")];
if let Some((asset, node)) = Decoders::detect().pick(&assets) {
    println!("playing {} via {}", asset.path.display(), node.path.display());
}
# }

A kiosk ships both files; the asset pipeline is two ffmpeg lines at build time:

ffmpeg -i in.mp4 -c:v libx264 -profile:v main -pix_fmt yuv420p -an -bsf:v h264_mp4toannexb out.h264
ffmpeg -i in.mp4 -c:v libx265 -pix_fmt yuv420p -an -bsf:v hevc_mp4toannexb out.h265

No container, no demuxer, no seeking — play, loop and stop, which is what a promo loop is. Audio is a different subsystem and deliberately absent.

Playing

Player drives the transport from the application's own event loop, against the same DRM card the surface owns — one process is DRM master, and DrmSurface::card() is the seam:

let mut player = Player::open(&assets, surface.card(), surface.crtc(), rect)?;
loop {
    player.pump(surface.card())?;   // feeds, flips; never blocks
    // ... the UI keeps drawing itself; the plane composes over it
}

In the tree, the Video widget in denise-ui is the rectangle the plane sits in: it reserves the space, paints the letterbox ground, and the application hands its bounds to Player::set_dst. The frames never come through the tree.

Status

The stateful decoder path: bcm2835-codec on the Pi and its equivalents on i.MX, Rockchip and Amlogic. The stateless HEVC path (rpivid, the price of the Pi 5) is detected and reported but not yet driven — it is tracked as its own issue, being an order of magnitude more work: slice parsing, reference management, the media request API.

Linux only; on anything else the crate compiles to the pure Annex-B module and nothing more. The probe example prints what a board can do, and is the same enumeration Decoders::detect runs at runtime.

MIT licensed.