18
edits
APNG Specification: Difference between revisions
m
fix typos
(clarified that hidden frame still counted in num_frames) |
Herbyderby (talk | contribs) m (fix typos) |
||
| Line 40: | Line 40: | ||
To be recognized as an APNG, an `acTL` chunk must appear in the stream before any `IDAT` chunks. The `acTL` structure is described in the next section. | To be recognized as an APNG, an `acTL` chunk must appear in the stream before any `IDAT` chunks. The `acTL` structure is described in the next section. | ||
The default image may be included as the first frame of the animation by the presence of a single `fcTL` chunk before `IDAT`. Otherwise, the default image is not part of the animation, and the output buffer must be completely initialized to fully transparent black rectangle, with width and height dimensions from the `IHDR` chunk, at the beginning of each play. | The default image may be included as the first frame of the animation by the presence of a single `fcTL` chunk before `IDAT`. Otherwise, the default image is not part of the animation, and the output buffer must be completely initialized to a fully transparent black rectangle, with width and height dimensions from the `IHDR` chunk, at the beginning of each play. | ||
Subsequent frames are encoded in `fdAT` chunks containing almost the same structure of content as `IDAT` chunks. Information for each frame about placement and rendering is stored in `fcTL` chunks. The full layout of `fdAT` and `fcTL` chunks is described below. | Subsequent frames are encoded in `fdAT` chunks containing almost the same structure of content as `IDAT` chunks. Information for each frame about placement and rendering is stored in `fcTL` chunks. The full layout of `fdAT` and `fcTL` chunks is described below. | ||
| Line 50: | Line 50: | ||
== Chunk Sequence Numbers == | == Chunk Sequence Numbers == | ||
The `fcTL` and `fdAT` chunks have a 4 byte sequence number. Both chunk types share the sequence. The purpose of this number is to detect (and optionally correct) sequence errors in an Animated PNG, since the PNG specification does not impose | The `fcTL` and `fdAT` chunks have a 4 byte sequence number. Both chunk types share the sequence. The purpose of this number is to detect (and optionally correct) sequence errors in an Animated PNG, since the PNG specification does not impose ordering restrictions on ancillary chunks. | ||
The first `fcTL` chunk must contain sequence number 0, and the sequence numbers in the remaining `fcTL` and `fdAT` chunks must be in order, with no gaps or duplicates. | The first `fcTL` chunk must contain sequence number 0, and the sequence numbers in the remaining `fcTL` and `fdAT` chunks must be in order, with no gaps or duplicates. | ||