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ndk/README.md

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+# Native codecs (libvpx, Opus, Speex)
+
+The app loads `libandroidcast_codecs.so` for availability probes and future software encode/decode paths.
+
+## Current status
+
+| Component | Java | Native | Notes |
+|-----------|------|--------|-------|
+| FEC 3/4 + RS | Yes | — | v2 **per-shard** on wire (`FecShardWire`); v1 monolithic decode still accepted |
+| NACK + retransmit cache | Yes | — | UDP `MSG_NACK`, combined with FEC when enabled |
+| libvpx VP8/VP9 | Native | Encode + decode | Screen cast via ImageReader; receiver libvpx (decoder uses `VPX_CODEC_USE_ERROR_CONCEALMENT` when supported) or MediaCodec fallback |
+| Opus / Speex | Probe + static link when `lib*.a` built | Encode/decode JNI sinks | `scripts/build-native-codecs.sh` → CMake autolink |
+
+## Enable native codecs (developer)
+
+1. Initialize all third-party submodules:
+
+   ```bash
+   bash scripts/init-third-party-submodules.sh
+   ```
+
+2. Build static libs per ABI (NDK auto-detected via `scripts/android-ndk.sh`):
+
+   ```bash
+   ./scripts/build-native-codecs.sh arm64-v8a
+   ./scripts/build-native-codecs.sh armeabi-v7a   # if shipped in APK
+   ```
+
+   If the repo path has **spaces**, the script stages under `/tmp/androidcast-libvpx-$USER`.
+
+3. Gradle / CMake link `build/native/<abi>/lib*.a` when present and set `ANDROIDCAST_HAVE_LIBVPX`, `ANDROIDCAST_HAVE_OPUS`, `ANDROIDCAST_HAVE_SPEEX` accordingly.
+
+4. `./gradlew assembleDebug`
+
+Optional override: `ANDROIDCAST_LIBVPX_LIB` only applies when its path contains the current ABI name (e.g. `.../arm64-v8a/libvpx.a`).
+
+ABIs without matching `.a` files still build; missing codecs are disabled on those architectures.
+
+## Stream protection defaults
+
+- Default: **None** (no FEC/NACK)
+- Enable in **Global settings → Stream protection (UDP)**
+- After handshake, sender and receiver negotiate the effective mode (no protocol version bump)
+- Until negotiation completes, UDP protection stays **off**; peers without FEC support **ignore** protected frames
+
+Full checklist: [docs/ROADMAP.md](../docs/ROADMAP.md)