PNG (Portable Network Graphics) Specification, Tenth Draft
Revision date: 5 May, 1995
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The PNG format is intended to provide a portable, legally
unencumbered, well-compressed, well-specified standard for
lossless bitmapped image files.
Although the initial motivation for developing PNG was to replace GIF,
the design provides some useful new features not available in
GIF, with minimal cost to developers.
GIF features retained in PNG include:
- Palette-mapped images of up to 256 colors.
- Streamability: files can be read and written strictly
serially, thus allowing the file format to be used as a communications
protocol for on-the-fly generation and display of images.
- Progressive display: a suitably prepared image file can be
displayed as it is received over a communications link,
yielding a low-resolution image very quickly
with gradual improvement of detail thereafter.
- Transparency: portions of the image can be marked as transparent,
allowing the effect of a nonrectangular image area to be achieved.
- Ancillary information: textual comments and other data can be
stored within the image file.
- Complete hardware and platform independence.
- Effective, 100% lossless compression.
Important new features of PNG, not available in GIF, include:
- Truecolor images of up to 48 bits per pixel.
- Grayscale images of up to 16 bits per pixel.
- Full alpha channel (general transparency masks).
- Image gamma indication, allowing automatic brightness/contrast adjustment.
- Reliable, straightforward detection of file corruption.
- Faster initial presentation in progressive display mode.
PNG is intended to be:
- Simple and portable: PNG should be widely implementable with
reasonably small effort for developers.
- Legally unencumbered: to the best of the knowledge of the PNG
authors, no algorithms under legal challenge are used.
- Well compressed: both palette-mapped and truecolor images are
compressed as effectively as in any other widely used lossless format,
and in most cases more effectively.
- Interchangeable: any standard-conforming PNG decoder will read
all conforming PNG files.
- Flexible: the format allows for future extensions and
private add-ons, without compromising interchangeability of basic PNG.
- Robust: the design supports full file integrity checking as well
as simple, quick detection of common transmission errors.
The main part of this specification simply gives the definition of
the file format. An appendix gives the rationale for many design
decisions. Although the rationale is not part of the formal
specification, reading it may help implementors to understand the design.
Cross-references in the main text point to relevant parts of the
rationale.
See Rationale:
Why a new file format?,
Why these features?,
Why not these features?,
Why not use format XYZ?.
Pronunciation
PNG is pronounced "ping".
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