LibreMediaConverter

A free and open-source media converter for Android — batch video transcoding and compression, audio extraction and conversion, GIF and frame export, and file merging.

Android 13+ (API 33). Built with Jetpack Compose and Material 3.

Status: working, unreleased. Both conversion engines, the router, the background job queue and the join flow are implemented and building. The FFmpeg format tests run green on a physical Pixel 10 Pro XL (API 37) and on local emulators at API 33–36, and CI runs the instrumented suite at API 33–36 on every pull request.

Correction (R18 / #27, 2026-08-22): this line used to say the FFmpeg format tests had "been written but not yet executed on a device" — true when written, false from the first device pass, and never updated. FFmpegEngineTest's nine format tests were in every run named above and none of them failed; the runs are recorded under "Verified on real API 37 hardware" in docs/api-37-emulator-crash.md and "The sweep, run" in docs/local-emulator.md. API 37 has no CI row — that emulator image is broken — so it stays a manual Pixel check before each release.

Licensing at a glance

• Source code: MIT
• Distributed APK: GPL-3.0 — because it bundles FFmpeg built with x264/x265

That split is deliberate, not an oversight. See LICENSES/README.md for the reasoning and the corresponding-source obligations.

Architecture

Two conversion engines behind an explicit router, because neither one covers the job alone.

AndroidX Media3 Transformer — the hardware path

Handles the common cases: H.264/HEVC, resolution and frame-rate changes, rotation, overlays, and audio to AAC. Fully hardware accelerated end to end — MediaCodec decodes to a GL surface and MediaCodec re-encodes, so frames never round-trip through the CPU. Roughly 7–8× realtime on 720p.

It writes MP4 and nothing else. media3-muxer ships WebM, Ogg, WAV and AAC muxers too, but none can be driven by Transformer — they throw from addMetadataEntry, which the muxer wrapper calls for every metadata entry a real recording carries. It reads far more than it writes, Matroska included, which is what makes MKV → MP4 a hardware remux.

FFmpeg — the long tail

Everything Media3 structurally cannot do:

• Containers outside MP4/WebM/Ogg/WAV/AAC — MKV, MOV, AVI, FLV, MPEG-TS, WMV/ASF
• MP3 output — Android has no MP3 encoder at any version; this is a platform gap
• GIF and image sequences
• Input codecs with no platform decoder on the device
• CRF and 2-pass rate control, for the quality tier
• Codecs Media3's muxers decline even on a stream copy — its MP4 muxer carries AAC, Opus, Vorbis and PCM, but neither MP3 nor FLAC

Quality tiers

The router is surfaced to users as a quality choice rather than hidden:

Tier	Engine	Rate control	Trade-off
Fast (default)	Media3 / MediaCodec	bitrate-targeted	~7–8× realtime, low battery cost
Best quality	FFmpeg + x264/x265	CRF or 2-pass	~realtime or slower, better quality per byte

A note on "GPU acceleration"

Android has no GPU video codec path. There are three distinct tiers, and conflating them causes a lot of confusion:

1. Fixed-function video codec silicon — reached through MediaCodec. This is what "hardware accelerated" means for encode and decode. It is not the GPU.
2. GPU shader cores — genuinely used, but only for filters, scaling, and color effects on already-decoded frames, via OpenGL ES. Never for entropy coding.
3. CPU — x264, x265, and software decoders.

FFmpeg's -hwaccel is meaningful on Android only as mediacodec, and even then it targets direct-to-Surface playback rather than file-to-file transcoding. Vulkan Video exists in FFmpeg 8.0+ but no shipping Android GPU driver exposes it — no VK_KHR_video_* extension appears in any Android Vulkan Profile tier.

So this app is hardware accelerated via MediaCodec, and GPU accelerated for effects via GL shaders. Both are real; neither is "the GPU decoding video."

Remuxing

Changing the container without touching the streams. Copying an H.264 track from MKV into MP4 moves the same samples into a different wrapper: it finishes in seconds instead of minutes, costs no quality, and needs no encoder — which is why it stays on the hardware path even on a device that cannot encode the codec in question.

Copy is a codec choice like any other, so it can be mixed: copy the video and re-encode only the audio, or the reverse. Picking a codec the source already uses is upgraded to a copy automatically when the container is changing — if container and codec both already match, the only reason to run the job is to re-encode it, so it does.

A copy is never attempted on a stream whose codec could not be identified. A needless re-encode costs time; a wrong stream copy costs a file that will not play.

Features

	Formats
Video out	MP4, MOV, MKV, WebM, MPEG-TS, AVI, FLV, WMV/ASF
Video codecs	H.264, H.265, VP9, or copy the source stream
Audio out	MP3, AAC/M4A, FLAC, Opus, WAV, MKA
Audio codecs	AAC, Opus, MP3, FLAC, PCM, or copy the source stream
Images	GIF, PNG frame sequences
Other	Remux without re-encoding; join several files into one

Presets cover the common combinations in one tap. The Advanced picker exposes the full container × codec matrix — including combinations that cannot work, which it explains and offers alternatives for rather than hiding.

Conversions run as durable background work, so they survive leaving the app and are restored after a restart.

Building

Requires the Android SDK with API 37. Do not pick a JDK — the repo does. gradle/gradle-daemon-jvm.properties pins the daemon to Java 25 and carries foojay download URLs per platform, so Gradle finds an installed Java 25 or downloads one on the first build, whatever JAVA_HOME points at. JAVA_HOME only chooses the launcher, which Gradle 9.7.1 will run on Java 8 or newer. Everything the build actually compiles is Java 25, the app's own bytecode included. ./gradlew --version prints the launcher and the daemon separately, and they routinely differ.

FFmpeg is committed as a prebuilt archive under bin/, so a clone builds without a cross-compile. That is deliberate: rebuilding it per CI run made test results ambiguous, because a red build could mean broken code or a build that hiccuped. See bin/README.md for its provenance and how to regenerate it.

./gradlew :app:assembleDebug          # debug APK
./gradlew :app:testDebugUnitTest      # JVM tests
./gradlew :app:connectedDebugAndroidTest   # device tests, needs a running device
./gradlew :app:assembleRelease        # R8-minified release

See tools/ffmpeg/README.md for why the build is containerised and which flags matter. That recipe remains the authority — the committed archive is its output, and is also what satisfies the GPL corresponding-source obligation.

Testing

Unit tests cover the parts that decide correctness without needing hardware: the routing matrix, the container × codec capability matrix, the FFmpeg argument builder, and both stream-copy-versus-re-encode planners. They run against fabricated device profiles, so branches like "this device cannot encode HEVC" are reachable regardless of what the test machine is.

Instrumented tests cover the parts that only a device can prove: real hardware transcoding, the foreground service type, and each FFmpeg output format asserted against the produced file rather than the exit code. The remux tests additionally assert which engine ran — a stream copy produces an identical file either way, so an output-only assertion cannot tell a hardware transmux from FFmpeg's -c copy.

Privacy

The app has no INTERNET permission, so it cannot open a network connection at all. Nothing is uploaded, and there is no analytics or advertising. See PRIVACY.md, which also explains the permissions WorkManager adds automatically.

Contributing

Contributions are welcome. Note that contributions to the source are under MIT, while the distributed binary remains GPL-3.0 for the reasons described in LICENSES/README.md.