Portable Network Graphics (PNG)
A basic introductory slides about Portable Network Graphics (PNG), how this is helping in web and what are the mechanism behind its storing technique.
Published on: Mar 4, 2016
Transcripts - Portable Network Graphics (PNG)
– GIF features retained in PNG include
• Indexed-color images of up to 256 colors.
• Progressive display
• 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
Interlaced data order
PNG file signature
Header (137 80 78 71 13 10 26 10)
– Chunk Data
– CRC (Cyclic Redundancy Check)
Chunk naming conventions
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
IDAT Image data
IEND Image trailer
bKGD Background color
gAMA Image gamma
tIME Image last-modiﬁcation time
iCCP Embedded ICC (nIternational Color Consortium)
sRGB Standard RGB color space
pre-compression: filtering (prediction)
The Adam7 algorithm.
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)
– GIF intrinsically supports animated images. PNG supports
animation only via unofficial extensions (see the section on
– PNG images are widely supported (for instance, with modern
web browsers and office software), but not as widely
supported as GIF images.
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
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
– 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
– 6.0 sometimes completely loses ability to display PNGs, but there are
– 6.0 and below has broken alpha-channel transparency support (will
display the default background color instead). However there are