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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 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.
== 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 orderng 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.
