PNG (Portable Network Graphics) Specification
W3C Recommendation draft-02-Nov-1998
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The Portable Network Graphics (PNG) format provides 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 (Compuserve's Graphics Interchange Format), the design provides
some useful new features not available in GIF, with minimal cost to
GIF features retained in PNG include:
- Indexed-color images of up to 256 colors.
- Streamability: files can be read and written 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 followed by gradual improvement of detail.
- Transparency: portions of the image can be marked as transparent,
creating the effect of a non-rectangular image.
- 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 information, which supports automatic display of images
with correct brightness/contrast regardless of the machines used to
originate and display the image.
- Reliable, straightforward detection of file corruption.
- Faster initial presentation in progressive display mode.
PNG is designed to be:
- Simple and portable: developers should be able to implement PNG
- Legally unencumbered: to the best knowledge of the PNG authors, no
algorithms under legal challenge are used. (Some considerable effort
has been spent to verify this.)
- Well compressed: both indexed-color 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 must 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 gives the definition of the
file format and recommendations for encoder and decoder behavior. An
appendix gives the rationale for many design decisions. Although the
rationale is not part of the formal specification, reading it can help
implementors understand the design. Cross-references in the main text
point to relevant parts of the rationale. Additional appendixes, also
not part of the formal specification, provide tutorials on gamma and
color theory as well as other supporting material.
The words "must", "required", "should", "recommended", "may", and
"optional" in this document are to be interpreted as described in
[RFC-2119], which is consistent
with their plain English meanings. The word "can" carries the same
force as "may".
See Rationale: Why a new file
format?, Why these features?,
Why not these features?, Why not use format X?.
PNG is pronounced "ping".
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