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YUV -> RGB Conversion in Flutter

Full working example of YUV to RGB conversion in Dart with native code (Java)

Why is it needed?

Personally, I encountered a problem while making a real time recognition app, using a model i trained with the FastAI library.
Since the library requires an RGBA image type for prediction, and the Camera plugin produces YUV images, i got stuck for a few days searching for efficient conversion solution.
After reading a lot of half working examples of people stuck in the same situation as I, and with a lot of trial and error, I managed to get a decent solution for my purposes.

Performance of real-time image acquisition in Flutter is bad, at least on Android. It's no surprise that the framework caps the resolution at at 720p. The penalty we must pay to pass an image from the camera to a neural network is high. It is comprised of several steps. First, we

The solution in brief

After trying a few examples, the best solution seemed to be using native code(Java) to convert the image. A MethodChanel is being opened upon page init, allowing a direct connection for transferring the content of the frame forwards and backwards, to and from the conversion function.

Average conversion time:

The conversion speed depends on the phone itself + the quality you chose for the CameraController.
Here are the results for 3 different physical devices tested:

  • Nokia 4.2 original:

    • medium: 720x480 60-90 ms yuv->rgb: 35 ms
    • high: 1280x720 200 ms yuv->rgb: 63 ms

  • Nokia 4.2 with NV21toRGB (after initialization):

    • medium: 720x480 60-90 ms yuv->rgb: 18 ms
    • high: 1280x720 170 ms yuv->rgb: 27 ms
    • veryHigh: 1920x1080 400 ms yuv->rgb: 55 ms

  • Nokia 4.2 with YUB420toRGB (after initialization):

    • medium: 720x480 60-90 ms yuv->rgb: 18 ms bitmap rotation: 39 ms
    • high: 1280x720 170 ms yuv->rgb: 26 ms bitmap rotation: 106 ms
    • veryHigh: 1920x1080 280 ms yuv->rgb: 50 ms; bitmap rotation: 166 ms; Jpeg compression 68 ms

  • Nokia 4.2 with YUB420toRGB (non-static):

    • medium: 720x480 yuv->rgb: 9 ms; bitmap rotation: 24 ms; Jpeg compression 11 ms
    • high: 1280x720 yuv->rgb: 15 ms; bitmap rotation: 74 ms; Jpeg compression 26 ms
    • veryHigh: 1920x1080 yuv->rgb: 31 ms; bitmap rotation: 147 ms; Jpeg compression 58 ms

  • Nokia 4.2 with YUB420toRGB, with rotation:

    • medium: 720x480 yuv->rgb: 11 ms; Jpeg compression 14 ms; prepare 2 ms; call to Flutter 5 ms
    • high: 1280x720 yuv->rgb: 15 ms; Jpeg compression 31 ms; prepare 3 ms; call to Flutter 12 ms
    • veryHigh: 1920x1080 yuv->rgb: 32 ms; Jpeg compression 64 ms; prepare 10 ms; call to Flutter 26 ms

  • Nokia 4.2 using ffi:

  • medium: 720x480 yuv->rgb: 13 ms (with rotation).

  • high: 1280x720 yuv->rgb: 33 ms (with rotation). Much better than Java, but needs 41 ms of preparation. (debug: 166 ms)

  • Nokia 4.2 luminance only with Image:

  • medium: 720x480 Image.fromBytes 70 ms; encodeJpeg 210 ms (after patch)

  • high: 1280x720 Image.fromBytes 110 ms; encodeJpeg 530 ms

  • Nokia 4.2 luminance only via a 8-byte bmp, in dart with ByteData:

  • medium: 720x480 40 ms (without rotation, using iterator)

  • medium: 720x480 2 ms (without rotation, padding) - this adds a black stripe of 48 pixels

  • high: 1280x720 3 ms (without rotation, padding)

  • medium: 720x480 36 ms (without rotation), 49 ms (with rotation)

  • high: 1280x720 60 ms (without rotation), 87 ms (with rotation)

  • Nokia 4.2 uncompressed 32-bit bmp:

  • medium: 720x480 yuv->rgb: 11 ms; to bmp 2 ms; overall in Flutter 42 ms

  • high: 1280x720 yuv->rgb: 15 ms; to bmp 5 ms; overall in Flutter 70 ms

  • Redmi Note 4:

    • Low quality: ~0.03-0.06 Seconds.
    • Medium quality: ~0.1-0.14 Seconds.
    • High quality: ~0.2-0.24 Seconds.

  • Meizu 16:

    • Low quality: ~0.015-0.016 Seconds.
    • Medium quality: ~0.03-0.05 Seconds.
    • High quality: ~0.09-0.1 Seconds.

What does this example includes?

  • Permission handling for the camera(loop until the users accept).
  • Full screen of the live camera preview
  • Channeling camera frames for conversion
  • Optional response stream for the RGBA Jpeg image(Uint8List)

Important notes

  • The minimum SDK version of the application is 21 ( Required by the "Camera" package).
    • Action to preform: Open "android/app/build.gradle" and change minSdkVersion XX to minSdkVersion 21
  • Camera privileges need to be requested at the Manifest level.
    • Action to preform: Open "android/app/src/main/AndroidManifest.xml", add <uses-permission android:name="android.permission.CAMERA"/> to the <manifest> element.
    • Note: Do not insert the element to the <application>). It should be a direct child of the manifest element.

Camera resolution

When initializing the "CameraController" object, an enum called "ResolutionPreset" should be passed to define the camera quality.
Those are the values for each entry as shown in the official flutter site..

  • low → 352x288 on iOS, 240p (320x240) on Android
  • medium → 480p (640x480 on iOS, 720x480 on Android)
  • high → 720p (1280x720)
  • veryHigh → 1080p (1920x1080)
  • ultraHigh → 2160p (3840x2160)
  • max → The highest resolution available.

To get veryHigh resolution or more, disable the cap at high in computeBestPreviewSize().