TransAVormer

https://github.com/yahweasel/transavormer
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TransAVormer is a stream-based frontend for libav.js and WebCodecs designed to transcode, transmux,...
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### Build a Muxer with TransAVormer

Source: https://github.com/yahweasel/transavormer/blob/master/docs/API.md

Demonstrates initializing a muxer transformation using a libav.js instance and an input file.

```javascript
const libav = await LibAV.LibAV();
const out = await TransAVormer.build(libav, {
    type: "muxer",
    input: inputFile
});
const rdr = out.stream.getReader();
while (true) {
    const rd = await rdr.read();
    if (rd.done) break;
    // Do something with rd.value
}
libav.terminate();
```

--------------------------------

### Initialize Transcoding and Playback

Source: https://github.com/yahweasel/transavormer/blob/master/demo/player.html

Sets up the AudioContext, initializes the TransAVormer instance, and configures the seek bar for media playback.

```javascript
(function() { const tav = TransAVormer; const fileBox = document.getElementById("file"); const canvas = document.getElementById("playerCanvas"); const seekBar = document.getElementById("seekBar"); fileBox.onchange = async function() { const file = fileBox.files[0]; if (!file) return; const ac = new AudioContext(); if (ac.state !== "running") ac.resume(); ac.destination.channelCount = ac.destination.maxChannelCount; const playback = await Weasound.createAudioPlayback(ac); (playback.unsharedNode() || playback.sharedNode()).connect(ac.destination); const ctx = canvas.getContext("2d"); const la = await LibAV.LibAV(); const out = await tav.build(la, { type: "play-normalizer", sampleRate: ac.sampleRate, channels: ac.destination.channelCount, input: { type: "demuxer", chunkSize: 1, input: file } }); // Set up the seek bar let seekTo = null; let seeking = false; let duration = 2; for (const stream of await out.streams) { if (stream.duration > duration) duration = stream.duration; } seekBar.max = Math.round(duration * 1000); seekBar.value = 0; seekBar.oninput = async () => { seekTo = seekBar.value / 1000; if (!seeking) { seeking = true; while (seekTo !== null) { const to = seekTo; seekTo = null; await out.sendCommands([{ c: "seek", time: to }]); await new Promise(res => setTimeout(res, 100)); } seeking = false; } }; const streams = out.stream.tee(); let videoOffset = null; let audioOffset = null; // Video playback (async function() { const rdr = streams[0].getReader(); while (true) { const rd = await rdr.read(); if (rd.done) break; for (const sf of rd.value) { const frame = sf.frame; if (frame.codedWidth || frame.width) { let image, frameTime; if (frame.codedWidth) { // WebCodecs image = frame; frameTime = frame.timestamp / 1000; } else { // libav.js image = frame.data; frameTime = frame.pts * frame.time_base_num / frame.time_base_den * 1000; } const now = performance.now(); let targetTime = now; if (audioOffset !== null) { targetTime = frameTime + audioOffset; } else if (videoOffset !== null) { targetTime = frameTime + videoOffset; } else { videoOffset = now - frameTime; } const delay = targetTime - now; if (delay > 100 || delay < -100) { videoOffset = now - frameTime; } if (delay > 0) { await new Promise(res => setTimeout( res, Math.min(delay, 100) )); } canvas.width = frame.codedWidth || frame.width; canvas.height = frame.codedHeight || frame.height; ctx.drawImage(image, 0, 0); image.close(); seekBar.value = Math.round(frameTime); } } } })(); // Audio playback (async function() { const rdr = streams[1].getReader(); while (true) { const rd = await rdr.read(); if (rd.done) break; for (const sf of rd.value) { const frame = sf.frame; const now = performance.now(); if (frame.sample_rate) { const frameTime = frame.pts * frame.time_base_num / frame.time_base_den * 1000; const delay = playback.play(frame.data); audioOffset = performance.now() + delay - frameTime; seekBar.value = Math.round(frameTime - delay); if (delay > 200) await new Promise(res => setTimeout(res, delay - 100)); } } } })(); } })();
```

--------------------------------

### Initialize a muxer

Source: https://github.com/yahweasel/transavormer/blob/master/docs/API.md

Create a muxer to output file streams. Set randomAccess to true for formats like mp4 that require it.

```javascript
const out = await TransAVormer.build(libav, {
    type: "muxer",
    format: "matroska", // libavformat format name
    randomAccess: false, // Optional, may be true
    input: inputData
});
```

--------------------------------

### TransAVormer.build (play-normalizer)

Source: https://github.com/yahweasel/transavormer/blob/master/docs/API.md

Initializes a playback normalizer to convert media frames into playback-optimized formats.

```APIDOC
## TransAVormer.build({ type: "play-normalizer" })

### Description
Creates a playback normalizer instance to convert video and audio frames into consistent formats suitable for playback.

### Parameters
- **type** (string) - Required - Must be "play-normalizer".
- **sampleRate** (number) - Required - The target sample rate for audio resampling.
- **channels** (number) - Optional - The target number of channels for audio mixing or splitting.
- **input** (object) - Required - The input data stream to be normalized.

### Request Example
```javascript
const out = await TransAVormer.build(libav, {
    type: "play-normalizer",
    sampleRate: ac.sampleRate,
    channels: ac.destination.channelCount,
    input: inputData
});
```

### Output Formats
- **WebCodecs VideoFrames**: Returned if supported; identified by a `codedWidth` field.
- **libav.js video frames**: Returned if WebCodecs is not supported; contains `ImageBitmap` data and identified by a `width` field.
- **libav.js audio frames**: Planar floating-point data; identified by a `sampleRate` field.
```

--------------------------------

### Initialize an Encoder

Source: https://github.com/yahweasel/transavormer/blob/master/docs/API.md

Creates an encoder instance using the 'encoder' initializer. Video and audio configurations can be provided via WebCodecs or lower-level libav settings.

```javascript
const out = await TransAVormer.build(libav, {
    type: "encoder",
    videoConfig: {
        /* This is a WebCodecs video configuration, but the `width` and `height`
         * fields are optional, as they will be found from the input. If you
         * don't intend to handle video input, `videoConfig` can be elided. */
        codec: "vp8"
    },
    audioConfig: {
        /* This is a WebCodecs audio configuration. If you don't intend to
         * handle audio input, `audioConfig` can be elided. */
        codec: "opus"
    },
    input: inputData
});
```

--------------------------------

### Initialize Playback Normalizer

Source: https://github.com/yahweasel/transavormer/blob/master/docs/API.md

Create a playback normalizer using the 'play-normalizer' type. Requires a sampleRate for audio resampling and optionally accepts channel counts.

```javascript
const out = await TransAVormer.build(libav, {
    type: "play-normalizer",
    sampleRate: ac.sampleRate,
    channels: ac.destination.channelCount,
    input: inputData
});
```

--------------------------------

### Execute TransAVormer Test Harness

Source: https://github.com/yahweasel/transavormer/blob/master/tests/web-test.html

Initializes the test harness, configures output redirection to the DOM, and executes the test suite with specific options.

```javascript
(function() { const runTests = document.getElementById("runTests"); runTests.onclick = async function() { runTests.style.display = "none"; const harness = TransAVormerTestHarness; harness.print = text => { document.getElementById("stdout").innerText += text + "\n"; }; harness.printErr = text => { document.getElementById("stderr").innerText += text + "\n"; }; harness.printStatus = text => { document.getElementById("status").innerText = text; }; let suite; { const resp = await fetch("suite.json"); suite = await resp.json(); } await harness.loadTests(suite); harness.options.includeSlow = document.getElementById("includeSlow").checked; await harness.runTests([ null, {yesthreads: true}, {nowasm: true} ]); runTests.style.display = ""; }; })();
```

--------------------------------

### TransAVormer.build (Demuxer)

Source: https://github.com/yahweasel/transavormer/blob/master/docs/API.md

Initializes a demuxer to process an input file into a packet stream.

```APIDOC
## TransAVormer.build (Demuxer)

### Description
Creates a demuxer instance to read packets from an input file. The demuxer outputs a stream of packet arrays.

### Parameters
- **type** (string) - Required - Must be "demuxer".
- **input** (File) - Required - The input file to be demuxed.
- **chunkSize** (number) - Optional - Size in bytes of chunks sent through the stream. Defaults to 65536.
- **libav** (Object) - Optional - A specific libav.js instance to use for the operation.

### Request Example
const out = await TransAVormer.build(libav, {
    type: "demuxer",
    chunkSize: 65536,
    input: inputFile,
    libav: libav
});
```

--------------------------------

### Transcoding to VP8 and Opus

Source: https://github.com/yahweasel/transavormer/blob/master/README.md

Shows how to nest an encoder configuration within the build function to transcode input media into specific video and audio codecs.

```javascript
const muxer = await TransAVormer.build(libav, {
    type: "muxer",
    randomAccess: true,
    input: {
        type: "encoder",
        videoConfig: {
            codec: "vp8"
        },
        audioConfig: {
            codec: "opus"
        },
        input: inputFile
    }
});
```

--------------------------------

### TransAVormer.build (Encoder)

Source: https://github.com/yahweasel/transavormer/blob/master/docs/API.md

Initializes an encoder instance to process video and audio streams.

```APIDOC
## TransAVormer.build (Encoder)

### Description
Creates an encoder instance using the provided libav context and configuration. It supports WebCodecs configurations or lower-level libav.js configurations.

### Method
Async Function

### Parameters
- **libav** (Object) - Required - The libav context.
- **options** (Object) - Required - Configuration object.
  - **type** (String) - Required - Must be "encoder".
  - **videoConfig** (Object) - Optional - WebCodecs video configuration.
  - **audioConfig** (Object) - Optional - WebCodecs audio configuration.
  - **libavVideoConfig** (Object) - Optional - Low-level libav video configuration.
  - **libavAudioConfig** (Object) - Optional - Low-level libav audio configuration.
  - **input** (Any) - Required - The input data source.
```

--------------------------------

### Transmuxing file input with TransAVormer

Source: https://github.com/yahweasel/transavormer/blob/master/demo/transmux.html

Handles file selection, initializes LibAV, performs transmuxing to Matroska format, and creates a downloadable link for the resulting blob.

```javascript
(function() { const tav = TransAVormer; const fileBox = document.getElementById("file"); fileBox.onchange = async function() { const file = fileBox.files[0]; if (!file) return; const la = await LibAV.LibAV(); const out = await tav.build(la, { type: "muxer", format: "matroska", input: file }); const outParts = []; const rdr = out.stream.getReader(); while (true) { const rd = await rdr.read(); if (rd.done) break; outParts.push(rd.value.data); } la.terminate(); const outBlob = new File(outParts, "out.mkv", {type: "video/x-matroska"}); const outURL = URL.createObjectURL(outBlob); const a = document.createElement("a"); a.style.display = "block"; a.href = outURL; a.innerText = "Output"; document.body.appendChild(a); } })();
```

--------------------------------

### TransAVormer.build (Filter)

Source: https://github.com/yahweasel/transavormer/blob/master/docs/API.md

Initializes a libavfilter component to apply filterchains to audio or video streams.

```APIDOC
## TransAVormer.build (type: "la-filter")

### Description
Creates a libav filter instance. Supports filterchains for video and audio streams.

### Parameters
- **type** (string) - Required - Must be "la-filter".
- **input** (object) - Required - The input data.
- **videoFilters** (string) - Optional - Filterchain string for video.
- **videoIOSettings** (object) - Optional - FilterIOSettings for video output.
- **audioFilters** (string) - Optional - Filterchain string for audio.
- **audioIOSettings** (object) - Optional - FilterIOSettings for audio output.

### Request Example
const out = await TransAVormer.build(libav, {
    type: "la-filter",
    videoFilters: "crop=320:240",
    videoIOSettings: {},
    audioFilters: "volume=0.5",
    audioIOSettings: {},
    input: inputData
});
```

--------------------------------

### TransAVormer.build(libav, initializer)

Source: https://github.com/yahweasel/transavormer/blob/master/docs/API.md

The build function initializes a transformation pipeline. It requires a libav.js instance and an initializer object defining the transformation type and input source.

```APIDOC
## TransAVormer.build(libav, initializer)

### Description
Initializes a media transformation pipeline. TransAVormer automatically discovers necessary intermediate stages (like demuxing or decoding) based on the provided input and desired output type.

### Parameters
- **libav** (LibAV instance) - Required - An instance of libav.js.
- **initializer** (Object) - Required - A descriptor object containing:
  - **type** (string) - The type of transformation (e.g., "muxer").
  - **input** (Object|string) - The input source or another initializer stage.

### Request Example
```javascript
const libav = await LibAV.LibAV();
const out = await TransAVormer.build(libav, {
    type: "muxer",
    input: inputFile
});
```

### Response
- **out** (Object) - Returns an object containing a `stream` property, which is a `ReadableStream` of the transformed data.
```

--------------------------------

### Modular Transcoding Pipeline Implementation

Source: https://github.com/yahweasel/transavormer/blob/master/demo/modular-transcode.html

Handles file input, format detection, and the construction of a transcoding pipeline using LibAV and TransAVormer.

```javascript
(function() { const tav = TransAVormer; const fileBox = document.getElementById("file"); fileBox.onchange = async function() { const file = fileBox.files[0]; if (!file) return; const header = await file.slice(0, 12).arrayBuffer(); const type = await AVGuesser.guess(header); const demuxer = await LibAV.LibAV({ variant: `demuxer-${type.format}` }); const la = await LibAV.LibAV(); const out = await tav.build(la, { type: "muxer", format: "matroska", input: { type: "encoder", videoConfig: { codec: "vp8" }, audioConfig: { codec: "opus" }, input: { type: "decoder", libav: demuxer, LibAV: LibAV, input: { type: "demuxer", libav: demuxer, input: file } } } }); const outParts = []; const rdr = out.stream.getReader(); while (true) { const rd = await rdr.read(); if (rd.done) break; outParts.push(rd.value.data); } la.terminate(); const outBlob = new File(outParts, "out.mkv", {type: "video/x-matroska"}); const outURL = URL.createObjectURL(outBlob); const a = document.createElement("a"); a.style.display = "block"; a.href = outURL; a.innerText = "Output"; document.body.appendChild(a); } })();
```

--------------------------------

### TransAVormer.build (muxer)

Source: https://github.com/yahweasel/transavormer/blob/master/docs/API.md

Creates a muxer for output file streams, supporting various libavformat formats.

```APIDOC
## TransAVormer.build (muxer)

### Description
Initializes a muxer to create an output file stream. Supports random access for formats like mp4.

### Parameters
- **type** (string) - Required - Must be "muxer"
- **format** (string) - Required - The libavformat format name
- **randomAccess** (boolean) - Optional - Set to true for formats requiring random access (e.g., mp4)
- **input** (any) - Required - The input data source
```

--------------------------------

### Initialize a LibAV Filter

Source: https://github.com/yahweasel/transavormer/blob/master/docs/API.md

Creates a filter instance using the 'la-filter' type. Supports defining filterchains for video and audio streams.

```javascript
const out = await TransAVormer.build(libav, {
    type: "la-filter",
    videoFilters: "crop=320:240",
    videoIOSettings: {
        // Optional output parameters for video streams
    },
    audioFilters: "volume=0.5"
    audioIOSettings: {
        // Optional output parameters for audio streams
    },
    input: inputData
});
```

--------------------------------

### Initialize a packet stream

Source: https://github.com/yahweasel/transavormer/blob/master/docs/API.md

Create a packet stream by providing codec parameters and a ReadableStream of packets.

```javascript
const out = await TransAVormer.build(libav, {
    type: "packet-stream",
    streams: streams /* array of StreamParameters */,
    input: packetStream /* ReadableStream of Packet[] */
});
```

--------------------------------

### Initialize a Decoder

Source: https://github.com/yahweasel/transavormer/blob/master/docs/API.md

Creates a decoder instance using the 'decoder' type. The libav and LibAV fields are optional and support modular builds.

```javascript
const out = await TransAVormer.build(libav, {
    type: "decoder",
    input: inputData,
    libav: libav,
    LibAV: LibAV
});
```

--------------------------------

### TransAVormer.build (packet-stream)

Source: https://github.com/yahweasel/transavormer/blob/master/docs/API.md

Initializes a packet stream by providing codec parameters and a ReadableStream of packets.

```APIDOC
## TransAVormer.build (packet-stream)

### Description
Creates a new packet stream instance using the provided libav.js codec parameters and a stream of packets.

### Parameters
- **libav** (object) - Required - The libav.js instance.
- **options** (object) - Required - Configuration object.
  - **type** (string) - Required - Must be "packet-stream".
  - **streams** (array) - Required - Array of StreamParameters.
  - **input** (ReadableStream) - Required - ReadableStream of Packet[] or number[] (pointers).
```

--------------------------------

### TransAVormer.build (Decoder)

Source: https://github.com/yahweasel/transavormer/blob/master/docs/API.md

Initializes a decoder component to process packet streams or input files.

```APIDOC
## TransAVormer.build (type: "decoder")

### Description
Creates a decoder instance to process input data. It accepts packet streams or input files as input.

### Parameters
- **type** (string) - Required - Must be "decoder".
- **input** (object) - Required - The input data (packet streams or input files).
- **libav** (object) - Optional - A specific libav.js instance.
- **LibAV** (object) - Optional - Entry point for loading modular decoder variants.

### Request Example
const out = await TransAVormer.build(libav, {
    type: "decoder",
    input: inputData,
    libav: libav,
    LibAV: LibAV
});
```

--------------------------------

### Transmuxing an MP4 file to Matroska

Source: https://github.com/yahweasel/transavormer/blob/master/README.md

Demonstrates using the build function to create a muxer for converting an input file to a different container format.

```javascript
const libav = await LibAV.LibAV();
const muxer = await TransAVormer.build(libav, {
    type: "muxer",
    randomAccess: true,
    input: inputFile
});
const rdr = muxer.stream.getReader();
const out = new FileWriter("out.mkv"); // Hypothetical output class
while (true) {
    const rd = await rdr.read();
    if (rd.done) {
        out.close();
        break;
    }
    out.write(rd.value.data, rd.value.position);
}
libav.terminate();
```

--------------------------------

### TransAVormer.build (Packet Selector)

Source: https://github.com/yahweasel/transavormer/blob/master/docs/API.md

Initializes a packet selector to filter specific streams from the input.

```APIDOC
## TransAVormer.build (Packet Selector)

### Description
Creates a packet selector to filter and keep specific streams from a demuxer input.

### Method
Async Function

### Parameters
- **libav** (Object) - Required - The libav context.
- **options** (Object) - Required - Configuration object.
  - **type** (String) - Required - Must be "packet-selector".
  - **selection** (Number|String|Object) - Required - Defines which streams to keep (e.g., "v0", "a", or index).
  - **input** (Object) - Required - The input source, typically a demuxer.
```

--------------------------------

### Transcoding media files with TransAVormer

Source: https://github.com/yahweasel/transavormer/blob/master/demo/filter.html

Initializes a file input handler that processes a selected file through a LibAV-based filter pipeline and generates a Matroska output file.

```javascript
(function() { const tav = TransAVormer; const fileBox = document.getElementById("file"); fileBox.onchange = async function() { const file = fileBox.files[0]; if (!file) return; const la = await LibAV.LibAV(); const out = await tav.build(la, { type: "muxer", format: "matroska", input: { type: "encoder", videoConfig: { codec: "vp8" }, audioConfig: { codec: "opus" }, input: { type: "la-filter", videoFilters: "crop=320:240:0:0", audioFilters: "volume=0.25", input: file } } }); const outParts = []; const rdr = out.stream.getReader(); while (true) { const rd = await rdr.read(); if (rd.done) break; outParts.push(rd.value.data); } la.terminate(); const outBlob = new File(outParts, "out.mkv", {type: "video/x-matroska"}); const outURL = URL.createObjectURL(outBlob); const a = document.createElement("a"); a.style.display = "block"; a.href = outURL; a.innerText = "Output"; document.body.appendChild(a); } })();
```

--------------------------------

### sendCommands(commands: Command[])

Source: https://github.com/yahweasel/transavormer/blob/master/docs/STREAM-COMMANDS.md

Sends an array of commands to the output stream, which are forwarded backwards through the pipeline. Returns an array of command results.

```APIDOC
## sendCommands(commands: Command[])

### Description
Sends an array of commands to the output stream to alter pipeline behavior. Commands are forwarded backwards through the entire pipeline.

### Parameters
- **commands** (Array) - Required - An array of command objects, each containing at least a 'c' field identifying the command type.

### Response
- **results** (Array) - An array of objects containing:
  - **c** (string) - The command name.
  - **ran** (boolean) - True if the command was interpreted by any element in the pipeline.
  - **success** (boolean) - True if the command executed successfully or was not interpreted.
  - **diagnostic** (any[]) - Optional diagnostic information if execution failed.
```

--------------------------------

### Initialize a Packet Selector

Source: https://github.com/yahweasel/transavormer/blob/master/docs/API.md

Creates a packet selector using the 'packet-selector' initializer to filter specific streams from an input source.

```javascript
const out = await TransAVormer.build(libav, {
    type: "packet-selector",
    selection: "v0",
    input: {
        type: "demuxer",
        ...
    }
});
```

--------------------------------

### Initialize a Demuxer

Source: https://github.com/yahweasel/transavormer/blob/master/docs/API.md

Creates a demuxer instance to process input files. The chunkSize parameter controls the byte size of packet chunks sent through the stream.

```javascript
const out = await TransAVormer.build(libav, {
    type: "demuxer",
    chunkSize: 65536,
    input: inputFile,
    libav: libav
});
```

--------------------------------

### Initialize a custom frame stream

Source: https://github.com/yahweasel/transavormer/blob/master/docs/API.md

Use the frame-stream type to provide custom codec parameters and frames in libav.js or WebCodecs format.

```javascript
const out = await TransAVormer.build(libav, {
    type: "frame-stream",
    streams: streams /* array of StreamParameters */,
    input: packetStream /* ReadableStream of StreamFrame[] */
});
```

--------------------------------

### TransAVormer.build (frame-stream)

Source: https://github.com/yahweasel/transavormer/blob/master/docs/API.md

Creates a custom frame stream using provided codec parameters and frames.

```APIDOC
## TransAVormer.build (frame-stream)

### Description
Allows creation of custom packet frames using libav.js or WebCodecs formats.

### Parameters
- **type** (string) - Required - Must be "frame-stream"
- **streams** (array) - Required - Array of StreamParameters
- **input** (ReadableStream) - Required - ReadableStream of StreamFrame[]
```

--------------------------------

### TransAVormer.build (frame-selector)

Source: https://github.com/yahweasel/transavormer/blob/master/docs/API.md

Creates a stream selector for frames to avoid extra processing steps.

```APIDOC
## TransAVormer.build (frame-selector)

### Description
Creates a stream selector for frames. The selection field follows the same logic as the packet selector.

### Parameters
- **type** (string) - Required - Must be "frame-selector"
- **selection** (string) - Required - The selection identifier (e.g., "v0")
- **input** (object) - Required - The input configuration, typically a decoder
```

--------------------------------

### Streaming Input File Interface

Source: https://github.com/yahweasel/transavormer/blob/master/docs/API.md

Interface for providing input data that is read sequentially from beginning to end without seeking.

```APIDOC
## Streaming Input Interface

### Description
Provides a mechanism for sequential data input. Can be implemented as a ReadableStream or an object with a read method.

### Interface
```typescript
{
    read: (len: number) => Promise<Uint8Array | null>
}
```

### Parameters
- **len** (number) - The requested length of data to read.

### Return Value
- **Promise<Uint8Array | null>** - Returns a Uint8Array containing the data, or null to indicate EOF.
```

--------------------------------

### Command and Result Structures

Source: https://github.com/yahweasel/transavormer/blob/master/docs/STREAM-COMMANDS.md

Defines the base structure for sending commands and the expected format of the returned results.

```typescript
{
    c: string
}
```

```typescript
{
    c: string,
    ran: boolean,
    success: boolean,
    diagnostic: any[]
}
```

--------------------------------

### Initialize a frame stream selector

Source: https://github.com/yahweasel/transavormer/blob/master/docs/API.md

Use the frame-selector type to select frames directly. The selection field follows the same syntax as packet selectors.

```javascript
const out = await TransAVormer.build(libav, {
    type: "frame-selector",
    selection: "v0",
    input: {
        type: "decoder",
        ...
    }
});
```

--------------------------------

### Random-Access Input File Interface

Source: https://github.com/yahweasel/transavormer/blob/master/docs/API.md

Interface for providing input data that requires seeking to specific positions within the file.

```APIDOC
## Random-Access Input Interface

### Description
Provides a mechanism for random-access data input, required for formats that do not support streaming. Can be provided as a Blob/File or an object with size and read fields.

### Interface
```typescript
{
    size: number,
    read: (pos: number, len: number) => Promise<Uint8Array | null>
}
```

### Parameters
- **size** (number) - The total size of the file in bytes.
- **pos** (number) - The position in the file to start reading from.
- **len** (number) - The requested length of data to read.

### Return Value
- **Promise<Uint8Array | null>** - Returns a Uint8Array containing the data, or null to indicate EOF.
```

--------------------------------

### Seek Command

Source: https://github.com/yahweasel/transavormer/blob/master/docs/STREAM-COMMANDS.md

Performs a seek operation in the input file.

```APIDOC
## Seek Command

### Description
Seeks to a specific time in the input file. Seeking is generally inexact and depends on keyframe availability.

### Command Structure
- **c** (string) - Required - Must be "seek".
- **time** (number) - Required - The time to seek to (seconds by default).
- **min** (number) - Optional - Floor time for the seek.
- **max** (number) - Optional - Ceiling time for the seek (defaults to time).
- **stream** (number) - Optional - Index of the stream to base timing decisions on.
- **streamTimebase** (boolean) - Optional - If true, interprets times in units of the selected stream's timebase (or microseconds if no stream is selected).
```

--------------------------------

### Random-access input file interface

Source: https://github.com/yahweasel/transavormer/blob/master/docs/API.md

Required structure for random-access input, including file size and a read function supporting positional access.

```typescript
{
    size: number,
    read: (pos: number, len: number) => Promise<Uint8Array | null>
}
```

--------------------------------

### Reselect Command

Source: https://github.com/yahweasel/transavormer/blob/master/docs/STREAM-COMMANDS.md

Changes the selection mapping for a packet or frame selector.

```APIDOC
## Reselect Command

### Description
Updates the selection mapping for a packet or frame selector. Use with caution as changing mappings that affect codecs or stream counts may cause pipeline failure.

### Command Structure
- **c** (string) - Required - Must be "reselect".
- **selection** (Object) - Required - The new selection mapping (same form as selector initializers).
```

--------------------------------

### Reselect Command Structure

Source: https://github.com/yahweasel/transavormer/blob/master/docs/STREAM-COMMANDS.md

Updates the selection mapping for packet or frame selectors. Ensure the new mapping does not cause pipeline failures due to codec or stream count changes.

```typescript
{
    c: "reselect",
    selection: <same form as selector initializers>
}
```

--------------------------------

### Seek Command Structure

Source: https://github.com/yahweasel/transavormer/blob/master/docs/STREAM-COMMANDS.md

Performs a seek operation in the input file. Time values default to seconds unless streamTimebase is enabled.

```typescript
{
    c: "seek",
    time: number,
    min?: number,
    max?: number,
    stream?: number,
    streamTimebase?: boolean
}
```

--------------------------------

### Streaming input file interface

Source: https://github.com/yahweasel/transavormer/blob/master/docs/API.md

An alternative to ReadableStream for streaming input, requiring a read function that returns a Promise of Uint8Array or null for EOF.

```typescript
{
    read: (len: number) => Promise<Uint8Array | null>
}
```

--------------------------------

### Muxer stream wrapper structure

Source: https://github.com/yahweasel/transavormer/blob/master/docs/API.md

The object returned when creating a muxer stream, containing component metadata and command handling.

```typescript
{
    component: "muxer",
    streamType: "file",
    stream: ReadableStream<{position: number, data: Uint8Array}>,
    sendCommands: (cmds: Command[]) => Promise<CommandResult[]>
}
```

--------------------------------

### Define output file stream structure

Source: https://github.com/yahweasel/transavormer/blob/master/docs/API.md

Represents the structure of an output file stream, supporting random access write operations.

```typescript
{
    streamType: "file",
    stream: ReadableStream<{position: number, data: Uint8Array}>,
    sendCommands: (cmds: Command[]) => Promise<CommandResult[]>
}
```

--------------------------------

### Define frame stream structure

Source: https://github.com/yahweasel/transavormer/blob/master/docs/API.md

The interface definition for frame streams, supporting multiple frame types and command execution.

```typescript
{
    streamType: "frame" | "libav-frame" | "webcodecs-frame",
    streams: Promise<StreamParameters[]>,
    stream: ReadableStream<StreamFrame[]>,
    sendCommands: (cmds: Command[]) => Promise<CommandResult[]>
}
```

--------------------------------

### Define StreamFrame structure

Source: https://github.com/yahweasel/transavormer/blob/master/docs/API.md

The wrapper object structure for frames, identifying the stream index and the frame data.

```typescript
{
    streamIndex: number,
    frame: Frame | number | VideoFrame | AudioData
}
```

--------------------------------

### Packet Stream Structure

Source: https://github.com/yahweasel/transavormer/blob/master/docs/API.md

The interface definition for a packet stream object, including stream metadata and command handling.

```typescript
{
    streamType: "packet",
    streams: Promise<StreamParameters[]>,
    stream: ReadableStream<Packet[]>,
    sendCommands: (cmds: Command[]) => Promise<CommandResult[]>
}
```
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