heif extraction
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We discuss here how to extract items from HEIF file collections. For track extraction, use the regular tools from GPAC.
A HEIF image collection will contain several items packed in a single meta box in ISOBMFF. These items will usually share the same coding type, although this is not a requirement.
We only discuss here HEIF version 1 files, for which each item is an intra picture. We assume the image collection is made of HEVC items.
You can use MP4Box to manually extract each item, see MP4Box -h meta:
MP4Box -dump-item 1:path=dump.hvc source.heic
This will dump item 2 into dump.hvc.
This however requires the item ID, hence an inspection of the file prior to extracting the item.
To avoid this, you can use gpac as follows:
gpac -i source.heic -o dump_$ItemID$.hvc
This will dump each item in dump_$ItemID$.hvc, with $ItemID$ being replaced by the item ID.
Transcoding is not possible using MP4Box. Using gpac, it is a fairly simple process:
gpac -i source.heic -o dump_$ItemID$.jpg
This will transcode each item to JPEG in dump_$ItemID$.jpg, with $ItemID$ being replaced by the item ID. Changing the output extension to png will transcode to PNG.
Each item will be declared as a media PID. This implies that there will be one decoder instance and one encoder instance created for each media PID.
One way to optimize the previous drawback of high resource usage is by declaring all items as a single track using -itt option of the MP4 demultiplexer:
gpac -i source.heic:itt -o dump_$ItemID$.jpg
gpac -i source.heic:itt -o dump_$num$.jpg
In this case, a single HEVC PID will be declared, hence a single HEVC decoder and a single JPEG encoder.
Note: Using $num$ templating allows producing filenames independent from the item ID.
The same approach can be used to quickly view all items declared in a file:
gpac -i source.heic:itt vout