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Portable Network Graphics (PNG)
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Portable Network Graphics (PNG)


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A basic introductory slides about Portable Network Graphics (PNG), how this is helping in web and what are the mechanism behind its storing technique.

A basic introductory slides about Portable Network Graphics (PNG), how this is helping in web and what are the mechanism behind its storing technique.

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  • 1. Presented by SARMAD MAKHDOOM
  • 2.  Introduction – GIF features retained in PNG include • Indexed-color images of up to 256 colors. • Progressive display • Transparency • Complete hardware and platform independence • Ancillary information – Important new features of PNG, not available in GIF • Truecolor images • Full alpha channel • Image gamma information • Faster initial presentation – PNG is designed to be • Simple and portable • Well compressed
  • 3.  Color values  Image layout  Alpha channel  Filtering  Interlaced data order  Text strings
  • 4.  PNG file signature  Header (137 80 78 71 13 10 26 10)  Chunk layout – Length – ChunkType – Chunk Data – CRC (Cyclic Redundancy Check)  Chunk naming conventions
  • 5.  Chunk Specifications  IHDR Image header Width: 4 bytes Height: 4 bytes Bit depth: 1 byte Color type: 1 byte Compression method: 1 byte Filter method: 1 byte Interlace method: 1 byte
  • 6.  PLTE Palette  IDAT Image data  IEND Image trailer
  • 7.  bKGD Background color  tRNSTransparency  gAMA Image gamma  tIME Image last-modification time  iCCP Embedded ICC (nIternational Color Consortium)  tEXtTextual data  sRGB Standard RGB color space
  • 8.  pre-compression: filtering (prediction)  compression: DEFLATE TYPE NAME 0 None 1 Sub 2 Up 3 Average 4 Paeth
  • 9. The Adam7 algorithm.
  • 10.  Comparison with Graphics Interchange Format (GIF)  On most images, PNG can achieve greater compression than GIF (see the section on filesize, below). – PNG gives a much wider range of transparency options than GIF, including alpha channel transparency. – Whereas GIF is limited to 8-bit indexed color, PNG gives a much wider range of color depths, including 24-bit (8 bits per channel) and 48-bit (16 bits per channel) truecolor, allowing for greater color precision, smoother fades, etc.When an alpha channel is added, up to 64 bits per pixel (before compression) are possible. – GIF intrinsically supports animated images. PNG supports animation only via unofficial extensions (see the section on animation, above). – PNG images are widely supported (for instance, with modern web browsers and office software), but not as widely supported as GIF images.
  • 11.  Comparison with JPEG – JPEG (Joint Photography Experts Group) can produce a smaller file than PNG for photographic (and photo-like) images, since JPEG uses a lossy encoding method specifically designed for photographic image data. Using PNG instead of a high-quality JPEG for such images would result in a large increase in filesize (often 5–10 times) with negligible gain in quality
  • 12.  PNG compatible browsers include: Apple Safari, Google Chrome, Mozilla Firefox, Opera, Camino, Internet Explorer 7 – 4.0 crashes on large PNG chunks. – 4.0 does not include the functionality to view .png files, but there is a registry fix. – 5.0 and 5.01 have broken OBJECT support. – 5.01 prints palette images with black (or dark gray) backgrounds under Windows 98, sometimes with radically altered colors. – 6.0 fails to display PNG images of 4097 or 4098 bytes in size. – 6.0 cannot open a PNG file that contains one or more zero-length IDAT chunks. – 6.0 sometimes completely loses ability to display PNGs, but there are various fixes. – 6.0 and below has broken alpha-channel transparency support (will display the default background color instead). However there are various fixes: