[Feature Request] Support encoding progressive tiled image

[macdavis]

Please add support for encoding --progressive --grid AxB. This would be particular useful for large dimension images.

[y-guyon]

Thank you for your interest in libavif.

Could you share more details about your use case?
For example: is it for streaming, do you need a preview of the whole image or does each tile need to be progressive, etc.
That would help us check if progressive layers are the best trade-off option.

In comparison, if you are already using container-level grids (as opposed to codec-level tiles) with enough rows and columns, accessing a particular area of the image should already be fast enough without a need for progressive layers, which come at a cost.

[macdavis]

A preview of the whole image will do, each tile doesn't have to be progressive.

I hope for a better viewing experience on iOS, not sure if the issue is on Apple's side. On iOS 26.2, I noticed fast zooming in and scrolling is slightly smoother with tiled images, either at container or codec-level (Please correct me if I am wrong, I avoid codec-level tiles as AI summarised codec-level tiles will cause visual degradation at edges.). However, it will take 7-8 seconds for the system to fully load 48 MP image, which then shows sharp details. With progressive AVIF, this loading reduced to 3-4 seconds. That's why I think, if both can work together, maybe it will load faster and also preserve smoother scrolling.

[y-guyon]

From the use case you described I would suggest a HEIF thumbnail image in the same file, with lower dimensions and/or quality than the primary item. thmb is unfortunately unsupported in libavif but feel free to open a separate feature request for that.