miniav 0.5.2

A cross-platform capture library for audio and video providing a standard buffer with GPU path

miniav

A Flutter library for cross-platform audio, video, and input capture with high-performance buffer management and GPU integration support.

Try it out at miniav.practicalxr.com!

Three Things to Know

1. Native assets compilation can take time, especially on first build. Run with -v to see build progress and errors.

2. This package uses dart native assets. For flutter, you must be on the master channel and run flutter config --enable-native-assets For dart, each run must contain the --enable-experiment=native-assets flag.

3. Platform-specific permissions are required for camera, microphone, and screen capture. See the Permissions section below for detailed setup instructions.

Platform Support

Module	Windows	Linux	macOS	Web	Android	iOS
Camera	✅	✅	✅	✅	🚧	🚧
Screen Capture	✅	✅	✅	✅	🚧	🚧
Audio Input	✅	✅	✅	✅	🚧	🚧
Audio Loopback	✅	✅	✅ 15+	❌	❌	❌
Input Capture	✅	🚧	🚧	🚧	❌	❌

Legend: ✅ Supported • ❌ Not Available • 🚧 Planned

(Maybe) Planned Features

• Android/iOS: Full support on mobile platforms
• GPU Interop: Helpers to easily manage handles and shared fences for GPU processing
• Permission Management: Simplified APIs for handling platform-specific permissions
• macOS/Linux context-lost wiring: Per-context device-loss callbacks for non-Windows platforms (currently handled via the polling watcher)

Installation

Add the following to your pubspec.yaml:

dependencies:
  miniav: ^0.5.0

Then run:

dart pub get

Getting Started

 git clone https://github.com/practicalxr/miniav.git
 cd miniav_ffi
 dart --enable-experiment=native-assets test

 dart:
 cd miniav
 dart --enable-experiment=native-assets example/miniav_example.dart

 flutter:
 cd miniav/example/flutter_example
 flutter config --enable-native-assets
 flutter run -d chrome/windows/linux

Example

import 'package:miniav/miniav.dart';

Future<void> captureCamera() async {
  // Initialize MiniAV
  MiniAV.setLogLevel(MiniAVLogLevel.info);
  
  // Enumerate camera devices
  final cameras = await MiniCamera.enumerateDevices();
  if (cameras.isEmpty) {
    print('No cameras found');
    return;
  }
  
  // Use first camera
  final selectedCamera = cameras.first;
  final format = await MiniCamera.getDefaultFormat(selectedCamera.deviceId);
  
  print('Using camera: ${selectedCamera.name}');
  print('Format: ${format.width}x${format.height} @ ${format.frameRateNumerator}/${format.frameRateDenominator} FPS');
  
  // Create and configure context
  final context = await MiniCamera.createContext();
  await context.configure(selectedCamera.deviceId, format);
  
  // Start capture with callback
  int frameCount = 0;
  await context.startCapture((buffer, userData) {
    frameCount++;
    print('Camera frame #$frameCount - ${buffer.dataSizeBytes} bytes');
    
    // Process video data here
    if (buffer.type == MiniAVBufferType.video) {
      final videoBuffer = buffer.data as MiniAVVideoBuffer;
      final rawData = videoBuffer.planes[0];
      // Use raw pixel data for computer vision, GPU upload, etc.
    }
    
    // IMPORTANT: Release buffer when done
    MiniAV.releaseBuffer(buffer);
  });
  
  // Capture for 10 seconds
  await Future.delayed(Duration(seconds: 10));
  
  // Stop and cleanup
  await context.stopCapture();
  await context.destroy();
  MiniAV.dispose();
}

Features

Multi-Stream Capture

MiniAV supports simultaneous capture from multiple sources:

• Camera: Access webcams and external cameras
• Screen: Capture displays and windows with optional audio
• Audio Input: Record from microphones and audio devices
• Loopback Audio: Capture system audio output (platform dependent)
• Input Capture: Keyboard, mouse, and gamepad events with configurable throttling

Device Change Subscriptions

Subscribe to device add/remove events without polling. Each subscription returns a disposer function — call it to unsubscribe.

// Camera devices
final cancel = MiniCamera.addDeviceChangeListener((notification) {
  print('Camera ${notification.event.name}: ${notification.device.name}');
});

// Microphone devices
final cancel = MiniAudioInput.addDeviceChangeListener((notification) {
  print('Mic ${notification.event.name}: ${notification.device.name}');
});

// Loopback (audio output) targets
final cancel = MiniLoopback.addDeviceChangeListener((notification) {
  print('Loopback target ${notification.event.name}: ${notification.device.name}');
});

// Display monitors
final cancel = MiniScreen.addDisplayChangeListener((notification) {
  print('Display ${notification.event.name}: ${notification.device.name}');
});

// Windows (visible windows list)
final cancel = MiniScreen.addWindowChangeListener((notification) {
  print('Window ${notification.event.name}: ${notification.device.name}');
});

// Gamepads
final cancel = MiniInput.addGamepadChangeListener((notification) {
  print('Gamepad ${notification.event.name}: ${notification.device.name}');
});

// notification.event is a MiniAVDeviceChangeEvent:
//   .added, .removed, .defaultChanged

Multiple listeners on the same module are independent; disposing one does not affect others.

Context-Lost Notifications

For contexts that are actively capturing, you can subscribe to a notification when the underlying device becomes unavailable (e.g. a webcam is unplugged, an audio endpoint is removed, or a captured window is closed).

final context = await MiniCamera.createContext();
await context.configure(deviceId, format);

final cancelLost = context.addLostListener((reason) {
  // Fired from the capture thread — schedule UI work on the main isolate
  print('Device lost (code $reason), stopping capture...');
});

await context.startCapture((buffer, _) {
  MiniAV.releaseBuffer(buffer);
});

// ...later
cancelLost(); // unsubscribe
await context.stopCapture();
await context.destroy();

addLostListener is available on MiniCameraContext, MiniAudioInputContext, MiniLoopbackContext, and MiniScreenContext.

Important: The lost callback is fired from a native capture thread. Do not call context.destroy() synchronously inside the callback — schedule it on the main isolate instead.

High-Performance Buffers

• Zero-Copy Design: Direct access to native buffers where possible
• GPU Integration: Ready for use with compute shaders and WebGPU
• Explicit Release: Manual buffer management prevents resource leaks
• Multiple Formats: Support for RGB, YUV, and compressed formats

Cross-Platform APIs

// Camera capture
final cameras = await MiniCamera.enumerateDevices();
final context = await MiniCamera.createContext();
final cancelCamera = MiniCamera.addDeviceChangeListener((n) { /* ... */ });

// Screen capture with audio
final displays = await MiniScreen.enumerateDisplays();
final screenContext = await MiniScreen.createContext();
final cancelDisplay = MiniScreen.addDisplayChangeListener((n) { /* ... */ });
final cancelWindow = MiniScreen.addWindowChangeListener((n) { /* ... */ });

// Audio input
final audioDevices = await MiniAudioInput.enumerateDevices();
final audioContext = await MiniAudioInput.createContext();
final cancelAudio = MiniAudioInput.addDeviceChangeListener((n) { /* ... */ });

// System audio loopback (where supported)
final loopbackDevices = await MiniLoopback.enumerateDevices();
final loopbackContext = await MiniLoopback.createContext();
final cancelLoopback = MiniLoopback.addDeviceChangeListener((n) { /* ... */ });

// Input capture (keyboard, mouse, gamepad)
final gamepads = await MiniInput.enumerateGamepads();
final inputContext = await MiniInput.createContext();
final cancelGamepad = MiniInput.addGamepadChangeListener((n) { /* ... */ });

Input Capture

MiniAV includes a unified input capture module for keyboard, mouse, and gamepad events.

Basic Input Capture

import 'package:miniav/miniav.dart';

Future<void> captureInput() async {
  // Create and configure an input context
  final context = await MiniInput.createContext();
  await context.configure(MiniAVInputConfig(
    inputTypes: MiniAVInputType.keyboard.value |
        MiniAVInputType.mouse.value |
        MiniAVInputType.gamepad.value,
    mouseThrottleHz: 120, // Limit mouse events to 120 Hz
    gamepadPollHz: 60,    // Poll gamepads at 60 Hz
  ));

  // Start capture with per-type callbacks
  await context.startCapture(
    onKeyboard: (event, userData) {
      final action = event.action == MiniAVKeyAction.down ? 'DOWN' : 'UP';
      print('Key $action: keyCode=${event.keyCode} scanCode=${event.scanCode}');
    },
    onMouse: (event, userData) {
      print('Mouse ${event.action.name}: (${event.x}, ${event.y})');
    },
    onGamepad: (event, userData) {
      print('Gamepad ${event.gamepadIndex}: '
          'buttons=0x${event.buttons.toRadixString(16)} '
          'LStick=(${event.leftStickX}, ${event.leftStickY}) '
          'triggers=(${event.leftTrigger}, ${event.rightTrigger})');
    },
  );

  // Capture for 10 seconds
  await Future.delayed(Duration(seconds: 10));

  // Cleanup
  await context.stopCapture();
  await context.destroy();
}

Gamepad Enumeration

final gamepads = await MiniInput.enumerateGamepads();
for (final pad in gamepads) {
  print('Gamepad: ${pad.name} (${pad.deviceId})');
}

Input Types

Input types are configured as a bitmask, so you can capture any combination:

// Keyboard only
MiniAVInputConfig(inputTypes: MiniAVInputType.keyboard.value);

// Mouse + gamepad
MiniAVInputConfig(
  inputTypes: MiniAVInputType.mouse.value | MiniAVInputType.gamepad.value,
);

Permissions

macOS

Add to macos/Runner/Info.plist:

<key>NSCameraUsageDescription</key>
<string>This app uses the camera for video capture</string>
<key>NSMicrophoneUsageDescription</key>
<string>This app uses the microphone for audio recording</string>

For screen recording, manually enable in System Preferences > Security & Privacy > Privacy > Screen Recording.

Windows

Camera and microphone access controlled via Windows 10+ Privacy Settings. Screen capture generally requires no special permissions for desktop applications. Input capture (keyboard/mouse hooks, XInput gamepads) works without additional permissions.

Linux

User must be in video and audio groups:

sudo usermod -a -G audio,video $USER

Install required development packages:

# Ubuntu/Debian
sudo apt install libasound2-dev libpulse-dev libpipewire-0.3-dev libv4l-dev

# Fedora
sudo dnf install alsa-lib-devel pulseaudio-libs-devel pipewire-devel libv4l-devel

Web

Requires HTTPS for camera, microphone, and screen capture APIs. All capture requires user gesture and permission.

Architecture

MiniAV follows a modular architecture:

• miniav_platform_interface: Abstract interface definitions
• miniav_ffi: Native implementation using FFI and C library
• miniav_web: Web implementation using browser APIs
• miniav_c: Core C library with platform-specific backends

Buffer Management

MiniAV uses explicit buffer release for optimal performance:

await context.startCapture((buffer, userData) {
  // Process buffer data
  final videoData = buffer.data as MiniAVVideoBuffer;
  
  // Access raw pixel planes
  final plane0 = videoData.planes[0]; // Y plane for YUV, or RGB data
  final plane1 = videoData.planes[1]; // U plane for YUV
  final plane2 = videoData.planes[2]; // V plane for YUV
  
  // CRITICAL: Always release when done
  MiniAV.releaseBuffer(buffer);
});

GPU Integration

Buffers can contain GPU handles for zero-copy workflows:

if (buffer.contentType == MiniAVBufferContentType.gpuD3D11Handle) {
  // Direct GPU texture handle (Windows)
  final gpuHandle = videoData.planes[0];
  // Pass to minigpu or other GPU library
}

GPU Interop with minigpu

MiniAV is designed to feed directly into minigpu for zero-copy GPU processing — camera frames and screen captures can be imported as GPU textures and processed with custom WGSL compute shaders.

Buffer contract

Every MiniAVVideoBuffer delivered in a capture callback carries the fields that minigpu's importVideoFrame needs:

Field	Role
contentType	cpu = copy into GPU texture; gpuD3D11Handle = zero-copy shared texture (Windows)
pixelFormat	rgba32 or nv12 are supported by minigpu on all platforms
width / height	Frame dimensions in pixels
planes[n]	Raw Uint8List pixel data (CPU path) — one plane for RGBA, two for NV12
strideBytes[n]	Row stride in bytes per plane
nativeHandles[n]	Platform GPU handle (D3D11 texture pointer, DMA-BUF fd, etc.) for GPU path
nativeFence	Sync fence for GPU-path handoff (D3D11 fence, sync_fd, or Metal shared event)

Camera → GPU texture → RGBA readback (CPU path)

import 'dart:ffi';
import 'dart:typed_data';
import 'package:ffi/ffi.dart';
import 'package:miniav/miniav.dart';
import 'package:minigpu/minigpu.dart';

Future<void> cameraToGpu() async {
  final gpu = Minigpu();
  await gpu.init();

  MiniAV.setLogLevel(MiniAVLogLevel.warn);
  final devices = await MiniCamera.enumerateDevices();
  if (devices.isEmpty) return;

  final fmt = await MiniCamera.getDefaultFormat(devices.first.deviceId);
  final ctx = await MiniCamera.createContext();
  await ctx.configure(devices.first.deviceId, fmt);

  await ctx.startCapture((buffer, _) async {
    if (buffer.type != MiniAVBufferType.video) {
      MiniAV.releaseBuffer(buffer);
      return;
    }
    final vb = buffer.data as MiniAVVideoBuffer;

    // Copy the CPU plane onto the native heap for the duration of the GPU import.
    final plane = vb.planes[0]!;
    final ptr = malloc<Uint8>(plane.length);
    ptr.asTypedList(plane.length).setAll(0, plane);

    final tex = gpu.importVideoFrame(ExternalVideoBuffer(
      contentType: ExternalContentType.cpu,
      pixelFormat: ExternalPixelFormat.rgba32, // match fmt.pixelFormat
      width: vb.width,
      height: vb.height,
      planes: [
        ExternalPlane(
          dataPtr: ptr.address,
          width: vb.width,
          height: vb.height,
          strideBytes: vb.strideBytes[0],
        ),
      ],
    ));

    if (tex != null) {
      final out = tex.toRGBA();
      // ... dispatch a compute shader or read back ...
      out.destroy();
      tex.destroy();
    }

    malloc.free(ptr);
    MiniAV.releaseBuffer(buffer);
  });

  await Future.delayed(const Duration(seconds: 5));
  await ctx.stopCapture();
  await ctx.destroy();
}

Camera → custom compute shader (colour inversion example)

const kInvertShader = '''
@group(0) @binding(0) var in_tex : texture_2d<f32>;
@group(0) @binding(1) var<storage, read_write> out_buf : array<u32>;
struct Params { width: u32, height: u32 }
@group(0) @binding(2) var<storage, read_write> params : Params;
@compute @workgroup_size(8, 8, 1)
fn main(@builtin(global_invocation_id) gid: vec3<u32>) {
  if (gid.x >= params.width || gid.y >= params.height) { return; }
  let px = textureLoad(in_tex, vec2<u32>(gid.x, gid.y), 0);
  let r = u32((1.0 - px.r) * 255.0);
  let g = u32((1.0 - px.g) * 255.0);
  let b = u32((1.0 - px.b) * 255.0);
  out_buf[gid.y * params.width + gid.x] =
      r | (g << 8u) | (b << 16u) | (255u << 24u);
}
''';

// In capture callback (after building tex as above):
final W = vb.width, H = vb.height;
final outBuf    = gpu.createBuffer(W * H * 4, BufferDataType.uint8);
final paramsBuf = gpu.createBuffer(8, BufferDataType.uint32);
await paramsBuf.write(Uint32List.fromList([W, H]), 2, dataType: BufferDataType.uint32);

final cs = gpu.createComputeShader();
cs.loadKernelString(kInvertShader);
tex.setOnShader(cs, 0);         // imported texture → binding 0
cs.setBufferAtSlot(1, outBuf);  // output  → binding 1
cs.setBufferAtSlot(2, paramsBuf); // params → binding 2
await cs.dispatch((W + 7) ~/ 8, (H + 7) ~/ 8, 1);

final result = Uint8List(W * H * 4);
await outBuf.read(result, W * H * 4, dataType: BufferDataType.uint8);

cs.destroy();  outBuf.destroy();  paramsBuf.destroy();
tex.destroy(); malloc.free(ptr);

NV12 camera frame → GPU BT.709 conversion

Request NV12 output from the camera for more efficient GPU upload, then let minigpu's built-in toRGBA() apply BT.709 full-range conversion:

// Configure for NV12 output
final fmt = MiniAVVideoInfo(
  width: 1280, height: 720,
  pixelFormat: MiniAVPixelFormat.nv12,
  frameRateNumerator: 30, frameRateDenominator: 1,
  outputPreference: MiniAVOutputPreference.cpu,
);
await ctx.configure(deviceId, fmt);

// In the capture callback:
final yPlane  = vb.planes[0]!;  // width × height bytes
final uvPlane = vb.planes[1]!;  // width × (height/2) bytes

final yPtr  = malloc<Uint8>(yPlane.length)..asTypedList(yPlane.length).setAll(0, yPlane);
final uvPtr = malloc<Uint8>(uvPlane.length)..asTypedList(uvPlane.length).setAll(0, uvPlane);

final tex = gpu.importVideoFrame(ExternalVideoBuffer(
  contentType: ExternalContentType.cpu,
  pixelFormat: ExternalPixelFormat.nv12,
  width: vb.width, height: vb.height,
  planes: [
    ExternalPlane(dataPtr: yPtr.address,  width: vb.width,      height: vb.height,      strideBytes: vb.strideBytes[0]),
    ExternalPlane(dataPtr: uvPtr.address, width: vb.width ~/ 2, height: vb.height ~/ 2, strideBytes: vb.strideBytes[1]),
  ],
));

final rgba = tex!.toRGBA(); // BT.709 NV12 → RGBA on GPU
// ... use rgba buffer ...
rgba.destroy(); tex.destroy();
malloc.free(yPtr); malloc.free(uvPtr);

Advanced Usage

Multiple Stream Synchronization

// Start multiple streams
await cameraContext.startCapture(onCameraFrame);
await audioContext.startCapture(onAudioFrame);

void synchronizeStreams(MiniAVBuffer cameraBuffer, MiniAVBuffer audioBuffer) {
  final timeDiff = cameraBuffer.timestampUs - audioBuffer.timestampUs;
  if (timeDiff.abs() < 16667) { // Within ~16ms for 60fps
    // Process synchronized frame
  }
}

Custom Format Selection

final formats = await MiniCamera.getSupportedFormats(deviceId);
final preferredFormat = formats.firstWhere(
  (f) => f.width >= 1920 && f.pixelFormat == MiniAVPixelFormat.nv12,
  orElse: () => formats.first,
);
await context.configure(deviceId, preferredFormat);

Error Handling

try {
  await context.startCapture(callback);
} on MiniAVException catch (e) {
  switch (e.code) {
    case MiniAVResultCode.errorNotSupported:
      // Handle unsupported operation
      break;
    case MiniAVResultCode.errorInvalidArg:
      // Handle invalid parameters
      break;
    default:
      print('Capture error: ${e.message}');
  }
}

Performance Tips

1. Release Buffers Promptly: Delayed release can cause frame drops
2. Use Appropriate Formats: Choose formats matching your processing needs
3. Minimize Copies: Prefer direct buffer access over copying data
4. GPU Preference: Set outputPreference: gpu for zero-copy workflows
5. Background Processing: Move heavy processing off the capture callback thread

Dependencies

Native Dependencies

• Windows: Media Foundation, DirectX 11, WASAPI, XInput
• macOS: AVFoundation, Core Graphics, Core Audio
• Linux: PipeWire, PulseAudio, ALSA, V4L2

Build Dependencies

• CMake 3.15+
• Platform-appropriate C++ compiler
• pkg-config (Linux)

Troubleshooting

Common Issues

No devices found: Check permissions and platform-specific requirements

Frame drops or freezes: Ensure timely buffer release and avoid blocking operations in callbacks

Build failures: Verify CMake version and platform dependencies are installed

Permission denied: Add user to required groups (Linux) or enable privacy settings

Debug Logging

MiniAV.setLogLevel(MiniAVLogLevel.debug);
MiniAV.setLogCallback((level, message) {
  print('[$level] $message');
});