Disclaimer: Work in progress.
There are two ways of signaling content:
The descriptions below match the order of the chunks.
Hard-coded 3 bytes with the value 0x6ffff4.
Using bit-packing, the main image header is 7 to 12-byte long:
| Value | Bits | |
|---|---|---|
| Width | 14 | In [1:16384] |
| Height | 14 | In [1:16384] |
| Orientation | 3 | 0°, 90°, 180°, 270° and/or mirror |
| Is opaque | 1 | If true: no alpha |
| Is animation | 1 | If true: expect ANMF |
| Preview color | 12 | RGB444 |
| Has preview | 1 | If true: preview chunk |
| Has ICC profile | 1 | If true: ICC chunk |
| Has XMP and/or EXIF | 1 | If true: metadata tag |
| RGB bit depth | 1 | 0 means 8-bit RGB, 1 means 10-bit RGB |
| Tile shape | 2 | 256x256, 512x512 etc. |
| Default transfer function | 1 | If false: read custom |
| [Transfer function] | 4 | If not default transfer func |
| [Loop forever] | 1 | For animation only |
| [Custom background color] | 1 | For animation only |
| [[Background color]] | 38 | For animation and custom bg only |
| Padding | Value 0 |
This chunk exists only if signaled in the image header.
The size of the preview chunk is stored as a variable-length integer.
Raw encoded bytes follow.
This chunk exists only if signaled in the image header.
The size of the ICC profile chunk is stored as a variable-length integer.
Raw bytes follow, verbatim.
Frame headers are only present if the image is animated (bit set to 1 in the image header).
Using bit-packing, the frame header begins by storing 1 byte:
| Value | Bits | |
|---|---|---|
| Dispose frame | 6 | Value is 0x33 (replace canvas by bg color) or 0x15 |
| Blend | 1 | Merge new pixels into canvas or replace them |
| Last frame | 1 | Expect another frame after this one if true |
Then using variable-length-integers, the frame header ends by storing 1 to 10 bytes:
| Value | Range |
|---|---|
| Duration in milliseconds | [0:32767] |
| Frame width | [1:image width] |
| Frame height | [1:image height] |
| Frame position X | [0:(image width - frame width)] |
| Frame position Y | [0:(image height - frame height)] |
Thus making each frame header 2 to 11-byte long.
The size of the GlobalParams chunk is stored as a variable-length integer, then the encoded bytes are written. Those are not described here as they are not related to the container. It contains whether the following tiles are lossless, lossy or both, on top of other fields (alpha etc.).
There is one GlobalParams unit per still image or per frame in an animation.
The size of a tile chunk is stored as a variable-length integer.
Encoded pixels follow.
There is at least one tile per still image or per frame in an animation.
Hard-coded 3 bytes with the value 0x61ffe9 (XMP present), 0x62ffe9 (EXIF present) or 0x63ffe9 (both present).
This chunk exists only if signaled in the metadata tag.
The size of the XMP chunk is stored as a variable-length integer.
Raw bytes follow, verbatim.
This chunk exists only if signaled in the metadata tag.
The size of the EXIF chunk is stored as a variable-length integer.
Raw bytes follow, verbatim.
