Grid image arranges tiles in wrong order if IDs in the iref are not sequential

[farindk]

libavif apparently assumes that the tile IDs of a grid image are sequential. If that is not the case, the tiles are composed into the output image in an incorrect arrangement.

Here is an example image that is decoded by libavif like this:

[y-guyon]

libavif apparently assumes that the tile IDs of a grid image are sequential. If that is not the case, the tiles are composed into the output image in an incorrect arrangement.

The tile IDs should not matter at all. HEIF spec ISO/IEC 23008-12 says:

The input images are inserted in row-major order, top-row first, left to right, in the order of
SingleItemTypeReferenceBox of type 'dimg' for this derived image item within the ItemReferenceBox. 

However libavif does not respect that pattern as of today (libavif is used in Chrome for example):

[farindk]

Yes, that's what I meant but expressed incorrectly. It should respect the order in the iref, which it currently doesn't.

[y-guyon]

Not sure what GIMP 2.10.36 uses under the hood but it opens the out-of-order dimg association file correctly.

[farindk]

Not sure what GIMP 2.10.36 uses under the hood but it opens the out-of-order dimg association file correctly.

It uses libheif. Same as I used to create the file.

[y-guyon]

(Thank you for taking the time to report this issue by the way.)

[wantehchang]

To test the fix for this issue, we also need an image grid in which multiple grid tiles reference the same image item. (In the extreme case, all grid tiles reference the same image item.)

@farindk Dirk: do yo happen to have such an image available?

[y-guyon]

To test the fix for this issue, we also need an image grid in which multiple grid tiles reference the same image item. (In the extreme case, all grid tiles reference the same image item.)

I manually edited
https://github.com/AOMediaCodec/libavif/blob/676a6cb82c0261cf5474c086f8e94f0f65ad4645/tests/data/sofa_grid1x5_420.avif
to repeat the same tile twice in dimg:

The image does not work in Chrome (libavif).
However it does not work in GIMP (libheif) either:

Opening '/home/yguyon/Downloads/sofa_grid1x5_420_dimg_repeat.avif' failed:
Loading HEIF image failed: Invalid input: Unspecified: 'iref' has double references

Which is compliant with the ISOBMFF specification ISO/IEC 14496-12:

8.11.12.1 Definition
The items linked to are then represented by an array of to_item_IDs;
within a given array, a given value shall occur at most once.

A workaround would be an intermediate iden derived image item (which is also unsupported by libavif).

Anyway I believe this is a different issue than #2311.

[wantehchang]

Yannis: That's good news. I was mislead by the incorrect comment in libavif. Thank you for finding proof that the image grid I requested is not compliant with ISOBMFF.