Transcoding Dolby Vision profile 8.4 to HLG can be performed on both Dolby Vision enabled devices as well as on non-Dolby Vision devices. It is recommended to use Dolby Vision decoder on Dolby Vision enabled devices vs hardware or in app HEVC 10bit decoder on non-Dolby Vision devices. This article explains how to verify if a device has the necessary components to perform this operation and provides guidance on how to correctly configure the transcoding process to produce an HLG video output derived from Dolby Vision profile 8.4.

The Dolby Vision profile 8.4 to HLG transcoding on Android mobile involves several steps. First, the Dolby Vision video track is selected using MediaExtractor. Next, the decoding is performed with MediaCodec to convert the Dolby Vision bitstream (or on Non-Dolby Vision devices the HLG base layer) into a format suitable (i.e. raw video buffers) for optional editing. Editing steps, such as effects, text overlay, trimming, and cropping, may be applied to the raw video buffers. Once edited, the HLG raw video buffer needs to be encoded into the desired target format (e.g., HEVC). You can utilize MediaCodec to handle this process. Finally, the transcoded content is packaged with MediaMuxer, producing an MP4 file ready for playback.

Please see the diagrams below for an overview of the Dolby Vision profile 8.4 to HLG transcoding workflows.

Dolby Vision profile 8.4 video track selection

The first step is to determine if and which track contains a Dolby Vision video bitstream. 

At this stage, we can use MediaExtractor to check if the MIME type is equal to MIMETYPE_VIDEO_DOLBY_VISION, the CodecProfileLevel is DolbyVisionProfileDvheSt and the transfer function is COLOR_TRANSFER_HLG, indicating that it is Dolby Vision profile 8.4 video. Below is sample code demonstrating how to do this.

MediaFormat mediaFormat = null;
     
for (int i = 0; i < mediaExtractor.getTrackCount(); i++) {
  mediaFormat = mediaExtractor.getTrackFormat(i);
  if (mediaFormat.getString(MediaFormat.KEY_MIME).equals(MediaFormat.MIMETYPE_VIDEO_DOLBY_VISION) &&
      mediaFormat.getInteger(MediaFormat.KEY_PROFILE) == 
             MediaCodecInfo.CodecProfileLevel.DolbyVisionProfileDvheSt &&
      mediaFormat.getInteger(MediaFormat.KEY_COLOR_TRANSFER) == MediaFormat.COLOR_TRANSFER_HLG ) {
        mediaExtractor.selectTrack(i);
        break;
  }
}

Decoding Dolby Vision profile 8.4 on Dolby Vision enabled devices

Proper Dolby Vision video decoding requires a device with the Dolby Vision decoder.

Determining Dolby Vision Decoder

To determine if the Dolby Vision decoder is available on the device, you can use the findDecoderForFormat method, which is available within the MediaCodecList class.

The workflow involves retrieving information about available decoders and finding one that supports MIMETYPE_VIDEO_DOLBY_VISION.

The hasDolbyVisionDecoder() method below provides a simple way to check if a device supports Dolby Vision decoding.

public boolean hasDolbyVisionDecoder() {
  MediaFormat format = new MediaFormat();
  MediaCodecList mediaCodecList = new MediaCodecList(MediaCodecList.REGULAR_CODECS);

  format.setString(MediaFormat.KEY_MIME, MediaFormat.MIMETYPE_VIDEO_DOLBY_VISION);

  String decoder = mediaCodecList.findDecoderForFormat(format);

  // true if a Dolby Vision decoder is found, false otherwise.
  return decoder != null;
}

You can also determine whether a decoder supporting given video format is implemented on the device. Instead of manually setting MediaFormat.KEY_MIME, retrieve the complete format information directly from the stream using MediaExtractor.getTrackFormat().

A method to find the decoder based on format information might look like the one below.

public String findDecoderForFormat(MediaFormat format) {
  MediaCodecList mediaCodecList = new MediaCodecList(MediaCodecList.REGULAR_CODECS);

  String decoder = mediaCodecList.findDecoderForFormat(format);

  // Return the name of the decoder, or null if none is found
  return decoder;
}

Configuring Dolby Vision Decoder

The decoder shall be configured to use the hybrid-log-gamma (HLG) transfer function for the decoded image. Depending on the type of Dolby Vision decoder integrated into the device, the appropriate color transfer request should be set either using MediaFormat.KEY_COLOR_TRANSFER_REQUEST or the vendor tag (vendor.dolby.codec.transfer.value). This approach ensures compatibility with the broadest range of Dolby Vision enabled devices. Check the example below to better understand how to do it. 

String VENDOR_DOLBY_CODEC_TRANSFER_PARAMKEY = "vendor.dolby.codec.transfer.value";

if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.S) {
  MediaCodec.ParameterDescriptor descriptor = 
      codec.getParameterDescriptor(VENDOR_DOLBY_CODEC_TRANSFER_PARAMKEY);
  if (descriptor != null) {
    Bundle transferBundle = new Bundle();
    transferBundle.putString(VENDOR_DOLBY_CODEC_TRANSFER_PARAMKEY, "transfer.hlg");
    codec.setParameters(transferBundle);
  } else {
    format.setInteger(MediaFormat.KEY_COLOR_TRANSFER_REQUEST, MediaFormat.COLOR_TRANSFER_HLG);
  }
} else {
  //The device doesn't support Dolby Vision Decoding
  //In this case use HEVC decoder to decode the HLG base layer
}

codec.configure(format, surface, null, 0);

Obtaining image in YUV format

The next element to focus on is the format in which the image is decoded, to ensure it aligns with what you use in your application during image processing stage. The decoder can output either RGB or YUV.

To obtain a decoded image in YUV format, configure the decoder format as shown in the example below.

// set the output format as 10-bit YUV
format.setInteger(MediaFormat.KEY_COLOR_FORMAT, MediaCodecInfo.CodecCapabilities.COLOR_FormatYUVP010);

Decoding Dolby Vision profile 8.4 base layer on non-Dolby Vision enabled devices

If the device does not have a Dolby Vision decoder, then for Dolby Vision profile 8.4, it is possible to only decode the HLG cross compatible base layer. This process can be done using a standard HEVC (Main10) decoder that supports BT2020 HLG decoding.

When hasDolbyVisionDecoder() (described in the "Determining Dolby Vision Decoder" section) returns false, the next step is to determine a decoder that supports HEVC Main10 BT2020 HLG. Below is an example of how to achieve this.

public boolean hasHevcHlgDecoder(){
  MediaFormat format = new MediaFormat();
  MediaCodecList mediaCodecList = new MediaCodecList(MediaCodecList.REGULAR_CODECS);

  //Dolby Vision profile 8.4 base layer
  format.setString(MediaFormat.KEY_MIME, MediaFormat.MIMETYPE_VIDEO_HEVC);
  format.setInteger(MediaFormat.KEY_PROFILE, MediaCodecInfo.CodecProfileLevel.HEVCProfileMain10);
  format.setInteger(MediaFormat.KEY_COLOR_TRANSFER,MediaFormat.COLOR_TRANSFER_HLG);
  format.setInteger(MediaFormat.KEY_COLOR_STANDARD, MediaFormat.COLOR_STANDARD_BT2020)

  String decoder = mediaCodecList.findDecoderForFormat(format);

  // true if an HEVC decoder is found, false otherwise.
  return decoder != null;
}

Processing decoded frames

At this stage, your application should handle the necessary image modifications (e.g., resizing) and then send the modified frames to the encoder. You must remember that editing must be performed in 10-bit HLG BT2020.

An example of editing process can be found in our Dolby Vision Editor project.

Conversion between RGB and YUV

If your existing editing workflow operates in a different format than the one provided by the decoder (or required by the encoder), you can use the functions linked below to convert from RGB to YUV and from YUV to RGB.

RGB to YUV conversion: vec3 RGB2YCC_REC2020(vec3 pixel)

YUV to RGB conversion: vec3 YCC2RGB_REC2020(vec3 pixel)

HLG encoding

The next step is to encode the raw images into a desired format (e.g HEVC). 

The following example illustrates how to produce an HLG 10-bit BT2020 HEVC video bitstream.

MediaCodecList list = new MediaCodecList(MediaCodecList.ALL_CODECS);
MediaFormat format = new MediaFormat();
int level = 0;

format.setString(MediaFormat.KEY_MIME, MediaFormat.MIMETYPE_VIDEO_HEVC);
format.setInteger(MediaFormat.KEY_PROFILE, MediaCodecInfo.CodecProfileLevel.HEVCProfileMain10);
//The values for HEVC tier/level must be appropriately matched to the encoding parameters
format.setInteger(MediaFormat.KEY_LEVEL, MediaCodecInfo.CodecProfileLevel.HEVCMainTierLevel51);

format.setInteger(MediaFormat.KEY_COLOR_TRANSFER, MediaFormat.COLOR_TRANSFER_HLG);
format.setInteger(MediaFormat.KEY_COLOR_STANDARD, MediaFormat.COLOR_STANDARD_BT2020);
format.setInteger(MediaFormat.KEY_COLOR_RANGE, MediaFormat.COLOR_RANGE_LIMITED);

format.setInteger(MediaFormat.KEY_WIDTH, videoResolution.getWidth());
format.setInteger(MediaFormat.KEY_HEIGHT, videoResolution.getHeight());
format.setInteger(MediaFormat.KEY_BIT_RATE, bitrate);
format.setInteger(MediaFormat.KEY_FRAME_RATE, fps);
format.setFloat(MediaFormat.KEY_I_FRAME_INTERVAL, iFrameInterval);
format.setInteger(MediaFormat.KEY_PRIORITY, 0);

(...)

String codecName = list.findEncoderForFormat(format)

if(codecName != null && codecName != "") {
  MediaCodec codec = MediaCodec.createByCodecName(codecName);
  codec.configure(format, null, null, MediaCodec.CONFIGURE_FLAG_ENCODE)
}

HLG Mp4 muxing

To complete the transcoding process, the final step is to create an MP4 file. For this purpouse, you can simply use the MediaMuxer class.

Dolby Vision Editor sample app

Refer to our Dolby Vision Editor sample application for technical guidelines.