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Animated Portable Network Graphics

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Animated Portable Network Graphics
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Filename extension .png +, .apng
Internet media type image/png +
Magic number Graphics format
Developed by Mozilla Corporation +
File formats category - v  e   edit

The Animated Portable Network Graphics (APNG) file format is an extension to the Portable Network Graphics (PNG) specification. It allows for animated PNG files that work similarly to animated GIF files, while supporting 24-bit images and 8-bit transparency not available for GIFs. It also retains backward compatibility with non-animated PNG files.

The first frame of an APNG file is stored as a normal PNG stream, and so most old PNG decoders will be able to display the first frame of an APNG file. The frame speed data and extra animation frames are stored in extra chunks (as provided for by the original PNG specification).

APNG competes with Multiple-image Network Graphics (MNG), a powerful format for bitmapped animations created by the same team as PNG. APNG's advantage is the smaller library size and compatibility with older PNG implementations.

History Edit

The APNG specification was created in 2004 by Stuart Parmenter and Vladimir Vukicevic of the Mozilla Corporation. APNG support was added to the ubiquitous libpng by a Seneca College student during the Google Summer of Code in 2006. Mozilla Firefox eventually added support for APNG in Firefox 3 trunk builds on March 23, 2007.[1]

The PNG group officially rejected APNG as an official extension on April 20, 2007.[2] There have been several subsequent proposals for a simple animated graphics format based on PNG using several different approaches. Among the maintainers of the PNG and MNG formats, APNG was not received well for several reasons. MNG provides all the features APNG provides, but is not supported at all by PNG-only decoders. APNG uses a technically feasible solution for storing any frames except the first, but the majority of the PNG group thinks this conflicts with the purpose of the PNG format – which is to store a single image. APNG would be compatible to this vision with alterations to its signature and intended MIME type, but these would break the desired backwards compatibility.

In 2008, WorldDMB adopted APNG as a backward compatible extension to enable animation as part of the MOT SlideShow user application for Digital Radio. "APNG 1.0 Specification - Animated Portable Network Graphics" is included as normative Annex A in the ETSI standard TS 101 499 V2.2.1 (2008-07).

Mozilla's role in extending the PNG format to APNG echoes Netscape's much earlier role in popularizing animated GIFs.

Technical details Edit

A PNG file consists of the PNG Signature (8 special bytes), followed by a series of so called chunks. A chunk consists of four parts: Length (4 bytes), Chunk type (4 bytes), Chunk data (length bytes) and CRC (Cyclic Redundancy Code / Checksum, 4 bytes).

Structure of a single PNG chunk (bytes)
Offset Size Description
- 4 Length
- 4 Chunk data
- length Chunk data
- 4 CRC

There are about 20 different chunk types, but for a minimal PNG, only 3 are required: The IHDR (image header) chunk, one or more IDAT (image data) chunks and the IEND (image end) chunk.

Structure of a very simple PNG file
PNG signature
(8 bytes)
IHDR
Image header
IDAT
Image data
IEND
Image end

The APNG specification introduces three new chunks: The animation control chunk (acTL), the frame control chunk (fcTL) and the frame data chunk (fdAT). The animation control chunk is a kind of "marker" chunk, telling the parser that this is an animated png. It contains information about how many frames the animation consists of and how many times the animation should play before coming to rest. The frame control chunk contains several bits of information, the most important of which is the display time of the following frame. The frame data chunks have the same structure as the IDAT chunks, except preceded by a sequence number.

A program wanting to assemble several individual PNG files to an animated PNG could proceed as follows:

  1. Take all chunks of the first PNG file as a building basis.
  2. Insert an animation control chunk (acTL) after the image header chunk (IHDR).
  3. If the first PNG is to be part of the animation, insert a frame control chunk (fcTL) before the image data chunk (IDAT).
  4. For each of the remaining frames, add a frame control chunk (fcTL) and a frame data chunk (fdAT) before the image end chunk (IEND). The content for the frame data chunks (fdAT) is taken from the image data chunks (IDAT) of their respective source images.

The PNG specification was designed with future extensions in mind. An application reading a PNG file is supposed to simply ignore any chunks which it does not understand. This is the reason why APNG is "backwards compatible". Existing applications just recognize the first frame and ignore the additional animation chunks.

AlternativesEdit

The MNG file format is a more powerful alternative to APNG, although is a more complex format and has less web browser support.

The GIF file format has better application and browser support than APNG, but it is limited to 256 colors per frame and provides only up to 1 bit alpha transparency.

Animation is also possible using SVG.

An alternative method for animations in web pages is to use conventional static images and animate them using JavaScript, Adobe Flash, Java or other plugin based technologies.

References Edit

External links Edit

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