HEVC/Alpha

• https://developer.apple.com/videos/play/wwdc2019/506/
  • https://devstreaming-cdn.apple.com/videos/wwdc/2019/506lqy7sprpfyo800/506/506_hevc_video_with_alpha.pdf

Both Premultiplied Alpha and Straight Alpha are supported

Single video track with ‘hvc1’ codec type
• Two layers
- Base layer: main profile; 4:2:0 video-range; nuh_layer_id 0
- Alpha layer: main profile; 4:2:0 full-range, neutral chroma channel; nuh_layer_id 1
• Base and alpha layers share one VPS
- VPS extension required — indicates presence of alpha channel
• Base and alpha layers have distinct PPS and SPS with different IDs
• Base and alpha layers must have identical frame types and dependency structure
• SEI alpha_channel_info message in hvcC in video format description
- Indicates premultiplied alpha or straight alpha

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• 動画エフェクト負荷を大きく軽減した HEVCPlayerView 詳解
  • https://engineering.dena.com/blog/2022/07/hevcplayerview/