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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 fully transparent black rectangle, with width and height dimensions from the `IHDR` chunk, at the beginning of each iteration.
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.
