Relax property association order on readers

[y-guyon]

Section 6.5.1 of HEIF says:

The semantics of the descriptive properties specified in 6.5 are specified for the image before the transformations, if any, are applied. Readers shall allow and ignore descriptive properties following the first transformative or unrecognized property, whichever is earlier, in the sequence associating properties with an item.

This leads to some issues:

• Compliant but potentially backward-incompatible files depending on the reader, for example a HEIFv2 prop before a HEIFv1 prop, see Ignore descr props after transf props AOMediaCodec/libavif#2571 (comment)
• Existing non-compliant files, see Wrong property association order in kimono files link-u/avif-sample-images#8
• There is conflicting wording between an essential property which is an "item property that readers are required to process", and "readers shall allow and ignore [potentially these same essential] descriptive properties".

Saying that descriptive properties describe items before any transformation should be enough in my opinion.

HEIF amendment suggestion

The semantics of the descriptive properties specified in 6.5 are specified for the image before the
transformations, if any, are applied.
- Readers shall
+ NOTE: In previous versions of this specification, readers had to 
allow and ignore descriptive properties following the first transformative or unrecognized property,
whichever is earlier, in the sequence associating properties with an item.

+ Writers should arrange the descriptive properties prior to any transformative property in the
+ sequence associating properties with an item.
Writers should arrange the descriptive properties specified in 6.5 prior to any other properties in the
sequence associating properties with an item.

[mhannuksela]

Hi @y-guyon,

I agree that the sentence in 6.5.1, which appeared already in HEIF v1, is problematic. I think it might have tried to say that even though descriptive properties are specified in 6.5 of HEIF v1 to appear before any transformative properties, future versions of HEIF or other specifications may specify a descriptive property propA that follows a transformative image property and describes the output of that transformation, in which case HEIF v1 readers shall not misinterpret the descriptive property propA to describe the reconstructed image (before transformations). The changes you are proposing are going to the right direction IMO. Additionally, I'd propose to add an explanation to the NOTE that you proposed:

NOTE: It is uncertain if readers would be able to correctly interpret descriptive properties that follow the first transformative property or the first unrecognized essential property, whichever is earlier, in the sequence associating properties with an item, because those descriptive properties possibly describe the output image after the transformation(s). In previous versions of this document, readers had to allow and ignore descriptive properties following the first transformative or unrecognized property, whichever is earlier, in the sequence associating properties with an item.

[cconcolato]

We agree to include this text into the Amd2 of HEIF.