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AV1 OBU Stream Structure Analysis Guide

Overview

This guide documents the AV1 video streams captured from Axis camera (10.176.12.35) with togglable overlays feature for analyzing the OBU (Open Bitstream Unit) structure.

Captured Files

1. outputvivek.ivf (7.90 MB)

  • Description: Standard AV1 stream without togglable overlays
  • Parameters: videocodec=av1&streamprofile=vivek
  • Duration: 60 seconds
  • Resolution: 3840x2160 (4K)
  • Frame Rate: 25 fps
  • Bitrate: ~1105 kbits/s
  • Use Case: Baseline AV1 stream for comparison

2. outputvivek_layers_all.ivf (14.72 MB)

  • Description: AV1 stream with togglable overlays enabled (all overlays visible)
  • Parameters: videocodec=av1&streamprofile=vivek&videolayers=1&overlays=all
  • Duration: 60 seconds
  • Resolution: 3840x2160 (4K)
  • Frame Rate: 25 fps
  • Bitrate: ~2057 kbits/s
  • Use Case: Analyze OBU structure with overlay layer included
  • Key Features:
    • Contains both base layer and overlay layer
    • Includes OBU_META_DATA with UUID: aaaaaaaa-aaaa-aaaa-aaaa-70661eab1e02
    • Each GOP starts with KEY FRAME + INTER FRAME structure
    • Subsequent TUs contain two INTER FRAMEs marked as "not shown"

3. outputvivek_layers_off.ivf (9.96 MB)

  • Description: AV1 stream with togglable overlays enabled (overlays hidden)
  • Parameters: videocodec=av1&streamprofile=vivek&videolayers=1&overlays=off
  • Duration: 60 seconds
  • Resolution: 3840x2160 (4K)
  • Frame Rate: 25 fps
  • Bitrate: ~1392 kbits/s
  • Use Case: Analyze base layer only with togglable overlay metadata

OBU Stream Structure (Togglable Overlays)

Metadata OBU Identification

The togglable overlays feature uses OBU_META_DATA (OBU Type 5) with:

  • Metadata Type: 25 (Unregistered user private data)
  • UUID: aaaaaaaa-aaaa-aaaa-aaaa-70661eab1e02
  • Location: Beginning of GOP within the Temporal Unit containing the keyframe

Observed Structure in av1_layers_all.ivf:

  • OBU Header: Type 5 (Metadata)
  • Metadata Type: 25
  • Payload:
    • Byte 0: 0x80 (128) - Prefix/Flag
    • Bytes 1-16: 0xAA 0xAA ... 0x70 0x66 0x1E 0xAB 0x1E 0x02
      • This matches the UUID: aaaaaaaa-aaaa-aaaa-aaaa-70661eab1e02
    • Bytes 17+: Overlay configuration data

GOP Structure

Each GOP in a togglable overlays stream follows this pattern (observed in trace):

First Temporal Unit (TU 0)

  1. OBU_SEQUENCE_HEADER
  2. OBU_META_DATA (Type 25, containing the UUID)
  3. OBU_FRAME (Base Layer)
    • Frame Type: KEY FRAME (0)
    • show_frame: 1 (Displayed)
  4. OBU_FRAME (Overlay Layer)
    • Frame Type: INTER FRAME (1)
    • show_frame: 0 (Hidden)
    • showable_frame: 1

Subsequent Temporal Units (e.g., TU 1)

  1. OBU_FRAME (Base Layer)
    • Frame Type: INTER FRAME (1)
    • show_frame: 0 (Hidden)
    • showable_frame: 1
  2. OBU_FRAME (Overlay Layer)
    • Frame Type: INTER FRAME (1)
    • show_frame: 0 (Hidden)
    • showable_frame: 1
  3. OBU_FRAME (Frame Header / Show Existing)
    • show_existing_frame: 1
    • frame_to_show_map_idx: Determines which frame to display (Base or Overlay)

Comparison with Standard AV1 Stream

For reference, the standard AV1 stream (av1_standard.ivf) has a simpler structure:

Standard GOP Structure (TU 0):

  1. OBU_SEQUENCE_HEADER
  2. OBU_FRAME (Base Layer)
    • Frame Type: KEY FRAME (0)
    • show_frame: 1 (Displayed)
    • Note: No OBU_METADATA and no hidden overlay frames.

Standard Subsequent TUs:

  1. OBU_FRAME (Base Layer)
    • Frame Type: INTER FRAME (1)
    • show_frame: 1 (Displayed)

How to Modify the Stream

To Show Base Layer Only

  1. Identify KEY FRAME via OBU_SEQUENCE_HEADER
  2. Identify OBU_META_DATA with togglable overlay UUID
  3. Drop the OBU_META_DATA in first TU of GOP
  4. Drop the first OBU_FRAME in each TU of GOP
  5. Drop all subsequent OBU_FRAMEs in same GOP (or maintain counter and drop when count > 1)

To Show Overlay Layer Only

  1. Identify KEY FRAME via OBU_SEQUENCE_HEADER
  2. Identify OBU_META_DATA with togglable overlay UUID
  3. Drop the OBU_META_DATA in first TU of GOP
  4. Keep only the overlay layer frames

Analysis Tools

Using FFmpeg to Inspect OBUs

# Show detailed frame information
ffmpeg -i outputvivek_layers_all.ivf -vf showinfo -f null -

# Extract raw OBU data
ffmpeg -i outputvivek_layers_all.ivf -c copy -bsf:v trace_headers -f null - 2>&1 | grep -i obu

Using aomdec (AV1 reference decoder)

# Decode and show frame details
aomdec --summary outputvivek_layers_all.ivf -o /dev/null

Using av1-grain (AV1 analysis tool)

# Analyze OBU structure
av1-grain outputvivek_layers_all.ivf

Key Observations

Bitrate Comparison

  • Standard AV1: ~1105 kbits/s
  • Layers OFF: ~1392 kbits/s (+26%)
  • Layers ALL: ~2057 kbits/s (+86%)

The increased bitrate with videolayers=1 confirms that both layers are being encoded and transmitted, even when overlays=off is set (the metadata is still present).

File Size Comparison

  • Standard: 7.90 MB
  • Layers OFF: 9.96 MB (+26%)
  • Layers ALL: 14.72 MB (+86%)

VAPIX Commands Used

Capture Standard AV1

ffmpeg -analyzeduration 10M -probesize 10M \
  -i "http://root:Axis%40125Apass@10.176.12.35/axis-cgi/media.cgi?videocodec=av1&streamprofile=vivek" \
  -t 60 -c copy -f ivf outputvivek.ivf

Capture with Togglable Overlays (All)

ffmpeg -analyzeduration 10M -probesize 10M \
  -i "http://root:Axis%40125Apass@10.176.12.35/axis-cgi/media.cgi?videocodec=av1&streamprofile=vivek&videolayers=1&overlays=all" \
  -t 60 -c copy -f ivf outputvivek_layers_all.ivf

Capture with Togglable Overlays (Off)

ffmpeg -analyzeduration 10M -probesize 10M \
  -i "http://root:Axis%40125Apass@10.176.12.35/axis-cgi/media.cgi?videocodec=av1&streamprofile=vivek&videolayers=1&overlays=off" \
  -t 60 -c copy -f ivf outputvivek_layers_off.ivf

References

Next Steps for Analysis

  1. Use a hex editor to examine the raw OBU structure
  2. Look for the UUID aaaaaaaa-aaaa-aaaa-aaaa-70661eab1e02 in the metadata
  3. Count OBU_FRAME occurrences per Temporal Unit
  4. Verify the "show_frame" flags in frame headers
  5. Compare GOP structures between standard and togglable overlay streams