APNG Specification: Difference between revisions

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APNG Specification (view source)

Revision as of 15:39, 5 April 2007

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An APNG stream is a normal PNG stream as defined in the [PNG Specification][pngspec], with three additional chunk types describing the animation and providing additional frame data.  
An APNG stream is a normal PNG stream as defined in the [PNG Specification][pngspec], with three additional chunk types describing the animation and providing additional frame data.  


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 below.


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.
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, which have the same structure as `IDAT` chunks, except preceded by a sequence number. Information for each frame about placement and rendering is stored in `fcTL` chunks.  The full layout of `fdAT` and `fcTL` chunks is described below.


The boundaries of the entire animation are specified by the width and height parameters of the PNG `IHDR` chunk, regardless of whether the default image is part of the animation. The default image should be appropriately padded with fully transparent pixels if extra space will be needed for later frames.
The boundaries of the entire animation are specified by the width and height parameters of the PNG `IHDR` chunk, regardless of whether the default image is part of the animation. The default image should be appropriately padded with fully transparent pixels if extra space will be needed for later frames.
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     `y_offset` + `height` <= `IHDR` height
     `y_offset` + `height` <= `IHDR` height


The `delay_num` and `delay_den` parameters together specify a fraction indicating the delay after displaying the current frame, in seconds.  If the denominator is 0, it is to be treated as if it were 100 (that is, `delay_num` then specifies 1/100ths of a second).  If the the value of the numerator is 0 the decoder should render the next frame as quickly as possible, though viewers may impose a reasonable lower bound on the delay.
The `delay_num` and `delay_den` parameters together specify a fraction indicating the time to display the current frame, in seconds.  If the denominator is 0, it is to be treated as if it were 100 (that is, `delay_num` then specifies 1/100ths of a second).  If the the value of the numerator is 0 the decoder should render the next frame as quickly as possible, though viewers may impose a reasonable lower bound.


`dispose_op` specifies how the output buffer should be changed at the end of the delay (before rendering the next frame).
`dispose_op` specifies how the output buffer should be changed at the end of the delay (before rendering the next frame).
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The `fdAT` chunk has the same purpose as an `IDAT` chunk. It has the same structure as an `IDAT` chunk, except preceded by a sequence number.
The `fdAT` chunk has the same purpose as an `IDAT` chunk. It has the same structure as an `IDAT` chunk, except preceded by a sequence number.


Each frame must contain at least one `fdAT` chunk. The compressed datastream is then the concatenation of the contents of the data fields of all the `fdAT` chunks within a frame. When decompressed, the datastream is a complete PNG image, including the filter byte at the beginning of each scanline. It utilizes the same bit depth, color type, compression method, filter method, interlace method, and palette (if any) as the default image.
At least one `fdAT` chunk is required for each frame. The compressed datastream is then the concatenation of the contents of the data fields of all the `fdAT` chunks within a frame. When decompressed, the datastream is the complete pixel data of a PNG image, including the filter byte at the beginning of each scanline, similar to the uncompressed data of all the `IDAT` chunks. It utilizes the same bit depth, color type, compression method, filter method, interlace method, and palette (if any) as the default image.


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