Dawn Stream building for real media

Without CMake media gates, Dawn Stream uses a stub WebRTC adapter: signaling and input work, but the browser receives no real video. This chapter covers enabling real encode/send paths.

Third-party SDKs must stay outside the source tree. .gitignore guards Gems/DawnStream/3rdParty/—never copy SDK headers or binaries into the repo.

Runtime pick order

At run time the peer factory chooses: native > wrapper > stub.

Both A and B may be enabled together; native wins when present.

Route A: libwebrtc wrapper

Pinned package: webrtc-sdk/libwebrtc release libwebrtc.m144.7559.09. Deploy the unzipped package outside the repo, then configure:

cmake -B build/windows -S . -DLY_DAWNSTREAM_LIBWEBRTC_ENABLED=ON `
 -DLY_DAWNSTREAM_LIBWEBRTC_PACKAGE_PATH=C:/path/to/libwebrtc-package

Expected layout and ABI notes (including the mandatory RTC_DESKTOP_DEVICE alignment): Gems/DawnStream/Code/cmake/3rdParty/Findlibwebrtc.cmake. You can also point the env var DAWNSTREAM_LIBWEBRTC_PACKAGE at the package root.

After rebuild, confirm libwebrtc DLL/deps deploy next to the launcher and that ps_dumpStats shows non-zero capture/send FPS once a viewer is connected.

Route B: native libwebrtc + hardware encode

Resolve a native libwebrtc package from outside the tree. The first supported layout is the WebRTC drop inside a UE5 installation:

cmake -B build/windows -S . -DLY_DAWNSTREAM_WEBRTC_NATIVE_ENABLED=ON `
 -DLY_DAWNSTREAM_WEBRTC_NATIVE_PACKAGE_PATH=C:/path/to/UE/Engine/Source/ThirdParty

Layout and CRT constraints (profile/release, /MD): Findwebrtcnative.cmake.

NVENC (optional)

# …plus:
-DLY_DAWNSTREAM_NVENC_ENABLED=ON

Findnvenc.cmake resolves header-only NVENC (nvEncodeAPI.h) from outside the repo. At session start the runtime probes the driver and falls back (e.g. VP8/VP9 or other encode paths) when unavailable.

Encoder probe order when the corresponding gates/platforms are available:

NVENC → AMF → VAAPI (Linux) → VideoToolbox (macOS) → MediaFoundation (Windows) → libvpx VP8/VP9

Capture paths and zero-copy

Default capture is GPU/CPU readback (Atom FrameCapture), with optional GPU RGBA→I420 (Video.GpuColorConvert, default true; falls back to CPU).

Zero-copy (Video.ZeroCopyCapture, --dawnstream-zero-copy-capture) is off by default. It requires NVENC + DX12 and can accumulate a driver-side host memory pool that is not returned on session teardown—repeated viewer reconnects may grow process memory without bound. Prefer the default readback path for long-lived demos and reconnect loops. Any zero-copy failure falls back to readback automatically.

ps_dumpStats reports capPath=zerocopy|readback for the active path.

Default capture target: CapturePassHierarchy=["MainPipeline","CopyToSwapChain"], CaptureSlot="Output" (final composite including UI). Launcher window pipelines may rename the root pass; the runtime remaps the first hierarchy element when needed. If video stays black with a real adapter, verify this hierarchy—see Diagnostics.

Confirm real media

1. Rebuild with Route A and/or B enabled; restart the launcher with streaming flags from Getting started.
2. Connect a viewer; expect video in the browser.
3. Run ps_dumpStats:

• capFps / sendFps should be non-zero.
• Bitrate should climb within your min/max window.
• capPath should match your zero-copy setting (usually readback).

4. Optionally enable Diagnostics.LogStats or --dawnstream-stats-csv for a time series.

If the player connects but stays black, you are almost certainly still on stub—or capture/codec negotiation failed.

Next steps

• Signaling — production WSS / TURN alongside real media.
• Configuration — bitrate, resolution, capture pass keys.
• Deployment — NVENC concurrency and public checklist.