Remove fields in AV2CodecConfigurationBox

[y-guyon]

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Built from commit 75d8fb2 · Updated: Mon, 10 Aug 2026 14:02:39 GMT

[skotagiri]

My recommendation would be to add the following fields. This will allow us to extend this box by adding new fields after the configOBUs[] and make the parsing more deterministic.

bit(24) flags;
unsigned int(32) config_obus_length;

[y-guyon]

bit(24) flags;

• Regarding the name, I do not mind using flags instead of reserved, but I do not see the benefit of it.
• For the length, I would argue 8 bits is enough. It is rare for a box to need more than 8 flags, and even so more can be added in another field at the end of the box.

unsigned int(32) config_obus_length;

Good point. I was hoping everything would be handled by the contents of config_obus but it has to be flexible just in case.
To avoid adding 4 bytes to all AV2-ISOBMFF files forever "just in case", I can think of these alternatives:

1. Use a flag bit to signal trailing fields:

   aligned(8) class AV2CodecConfigurationBox extends Box('av2C')
     unsigned int(8)  flags;
     if (flags & 1) {
       unsigned int(32) config_obus_length;
     }
     unsigned int(8)  config_obus[config_obus_length];
     if (flags & 1) {
       unsigned int(8)  reserved[];
     }
   }

2. Use a flag bit to signal heading fields:

   aligned(8) class AV2CodecConfigurationBox extends Box('av2C')
     unsigned int(8)  flags;
     if (flags & 1) {
       unsigned int(32) reserved_length;
       unsigned int(8)  reserved[reserved_length];
     }
     unsigned int(8)  config_obus[];
   }

3. I assume 256 OBUs to be enough (no need for flags because fields can be added at the end of the box in the future):

   aligned(8) class AV2CodecConfigurationBox extends Box('av2C')
     unsigned int(8) config_obus_count_minus1;
     for (i = 0; i <= config_obus_count_minus1; ++i) {
       unsigned int(8)  config_obu[];  // leb128() num_bytes_in_obu + a single OBU
     }
   }

4. Variable-length length field (no need for flags because fields can be added at the end of the box in the future):

   aligned(8) class AV2CodecConfigurationBox extends Box('av2C')
     leb128() config_obus_length;  // Or similar behavior; ISOBMFF has no vl type defined so far
     unsigned int(8)  config_obus[config_obus_length];
   }

5. Never append anything to AV2CodecConfigurationBox, but introduce a new box in AV2SampleEntry later if necessary:

   class AV2SampleEntry extends VisualSampleEntry('av02') {
     AV2CodecConfigurationBox config;
     AV2MultistreamCodecConfigurationBox multistream_config;
   }
   aligned(8) class AV2CodecConfigurationBox extends Box('av2C')
   {
     unsigned int(8)  reserved1 = 0;
     unsigned int(8)  config_obus[];  // Till end of box
   }
   aligned(8) class AV2MultistreamCodecConfigurationBox extends Box('av2M')
   {
     // whatever
   }

Any other approach in mind?