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
8. 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
11. 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.
12. 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
13. 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: