PNG - Wikipedia

PNG - Wikipedia
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(Redirected from
Portable Network Graphics
Family of lossless-compression image file formats
This article is about the file format. For the country, see
Papua New Guinea
. For other uses, see
PNG (disambiguation)
Portable Network Graphics
A PNG image of four differently colored
dice
with an 8-bit transparency channel, overlaid onto a checkered background, typically used in
graphics software
to indicate transparency
Filename extension
.png
Internet media type
image/png
Type code
PNGf
PNG
(including a single trailing space)
Uniform Type Identifier (UTI)
public.png
UTI conformation
public.image
Magic number
89 50 4e 47 0d 0a 1a 0a
(8 bytes
hexadecimal
PNG Development Group (donated to
W3C
Initial release
1 October 1996
; 29 years ago
1996-10-01
Latest release
3.0
24 June 2025
; 9 months ago
2025-06-24
Type of format
Lossless
bitmap
image format
Extended to
APNG
JNG
, and
MNG
Standard
ISO
IEC
15948
IETF
RFC 2083
Open format
Yes
Zlib/libpng license
Website
w3
.org
/TR
/png
(specification)
libpng
.org
/pub
/png
(home site)
Portable Network Graphics
PNG
, officially pronounced
PING
colloquially pronounced
iː
dʒ
iː
PEE
-en-
JEE
) is a
raster-graphics
file
format
that supports
lossless data compression
PNG was developed as an improved, non-patented replacement for
Graphics Interchange Format
(GIF).
PNG supports palette-based images (with palettes of 24-bit
RGB
or 32-bit
RGBA
colors),
grayscale
images (with or without an
alpha channel
for transparency), and full-color non-palette-based RGB or RGBA images. The PNG working group designed the format for transferring images on the
Internet
, not for professional-quality print graphics; therefore, non-RGB
color spaces
such as
CMYK
are not supported. A PNG file contains a single image in an extensible structure of
chunks
, encoding the basic
pixels
and other information such as textual comments and
integrity checks
documented in
RFC
2083.
PNG files have the ".png"
file extension
and the "image/png"
MIME
media type.
PNG was published as an
informational
RFC 2083 in March 1997 and as the standard
ISO/IEC 15948:2004
in 2004.
History and development
edit
See also:
Graphics Interchange Format § Unisys and LZW patent enforcement
The motivation for creating the PNG format was the announcement on 28 December 1994 that implementations of the
Graphics Interchange Format
(GIF) format would have to pay royalties to
Unisys
due to their
patent
of the
Lempel–Ziv–Welch
(LZW)
data compression
algorithm used in GIF.
This led to a flurry of criticism from
Usenet
users. One of them was Thomas Boutell, who on 4 January 1995 posted a precursory discussion thread on the
Usenet newsgroup
"comp.graphics" in which he devised a plan for a free alternative to GIF. Other users in that thread put forth many propositions that would later be part of the final file format. Oliver Fromme, author of the popular
JPEG
viewer
QPEG
, proposed the PING name, eventually becoming PNG, a
recursive acronym
meaning
PING is not GIF
10
11
and also the .png
extension
. Other suggestions later implemented included the
deflate compression algorithm
and
24-bit color
support, the lack of the latter in GIF also motivating the team to create their file format. The group would become known as the PNG Development Group, and as the discussion rapidly expanded, it later used a mailing list associated with a
CompuServe
forum.
12
The full specification of PNG was released under the approval of
World Wide Web Consortium
(W3C) on 1 October 1996, and later as
RFC
2083 on 15 January 1997. The specification was revised on 31 December 1998 as version 1.1, which addressed technical problems for
gamma
and
color correction
. Version 1.2, released on 11 August 1999, added the iTXt chunk as the specification's only change, and a reformatted version of 1.2 was released as a second edition of the W3C standard on 10 November 2003,
13
and as an International Standard (
ISO/IEC 15948:2004
) on 3 March 2004.
14
Although GIF allows for
animation
, it was initially decided that PNG should be a single-image format.
15
In 2001, the developers of PNG published the
Multiple-image Network Graphics
(MNG) format, with support for animation. MNG achieved moderate application support, but not enough among mainstream web browsers and no usage among web site designers or publishers. In 2008, certain
Mozilla
developers published the
Animated Portable Network Graphics
(APNG) format with similar goals. APNG is a format that is natively supported by
Gecko
- and
Presto
-based web browsers and is also commonly used for thumbnails on Sony's
PlayStation Portable
system (using the normal PNG file extension). In 2017, Chromium based browsers adopted APNG support. In January 2020,
Microsoft Edge
became
Chromium
based, thus inheriting support for APNG. With this all major browsers now support APNG.
The PNG Working Group has been chartered by the W3C, since September 14, 2021, to maintain and develop for the PNG specification. The third edition of PNG specification, which adds the proper support of APNG,
high dynamic range
(HDR) and
Exif
data, was published as the first public working draft on October 25, 2022,
16
and ultimately as a W3C Recommendation on June 24, 2025.
17
18
PNG Working Group
edit
The original PNG specification was authored by an ad hoc group of
computer graphics
experts and enthusiasts. Discussions and decisions about the format were conducted by email. The original authors listed on RFC 2083 are:
19
Editor: Thomas Boutell
Contributing Editor:
Tom Lane
Authors (in alphabetical order by last name):
Mark Adler
, Thomas Boutell,
Christian Brunschen
Adam M. Costello
Lee Daniel Crocker
Andreas Dilger
Oliver Fromme
Jean-loup Gailly
Chris Herborth
Aleks Jakulin
Neal Kettler
Tom Lane
Alexander Lehmann
Chris Lilley
, Dave Martindale,
Owen Mortensen
Keith S. Pickens
Robert P. Poole
Glenn Randers-Pehrson
Greg Roelofs
, Willem van Schaik,
Guy Schalnat
Paul Schmidt
Tim Wegner
Jeremy Wohl
File format
edit
The PNG image
viewed with a
hex editor
application for
Ubuntu
File header
edit
A PNG file starts with an eight-
byte
signature
20
(refer to hex editor image on the right):
Values (
hex
Purpose
89
Has the high bit set to detect transmission systems that do not support
8-bit data
and to reduce the chance that a text file is mistakenly interpreted as a PNG, or vice versa
50 4E 47
In
ASCII
, the letters
PNG
, allowing a person to identify the format easily if it is viewed in a text editor
0D 0A
DOS
-style
line ending
(CRLF) to detect DOS–Unix line ending conversion of the data
1A
A byte that stops display of the file under DOS when the command
type
has been used—the
end-of-file
character
0A
A Unix-style line ending (LF) to detect Unix–DOS line ending conversion
"Chunks" within the file
edit
After the header, comes a series of
chunks
21
each of which conveys certain information about the image. Chunks declare themselves as
critical
or
ancillary
, and a program encountering an ancillary chunk that it does not understand can safely ignore it. This chunk-based storage layer structure, similar in concept to a
container format
or to
Amiga
IFF
, is designed to allow the PNG format to be extended while maintaining compatibility with older versions—it provides
forward compatibility
, and this same file structure (with different signature and chunks) is used in the associated
MNG
JNG
, and
APNG
formats.
A chunk consists of four parts: length (4 bytes,
22
big-endian
), chunk type/name (4 bytes
23
), chunk data (length bytes) and
CRC
(cyclic redundancy code/checksum; 4 bytes
22
). The CRC is a network-byte-order
CRC-32
computed over the chunk type and chunk data, but not the length.
Length
Chunk type
Chunk data
CRC
4 bytes
4 bytes
Length
bytes
4 bytes
Chunk types are given a four-letter
case sensitive
ASCII type/name; compare
FourCC
. The case of the different letters in the name (bit 5 of the numeric value of the character) is a
bit field
that provides the
decoder
with some information on the nature of chunks it does not recognize.
The case of the first letter indicates whether the chunk is critical or not. If the first letter is uppercase, the chunk is critical; if not, the chunk is ancillary. Critical chunks contain information that is necessary to read the file. If a decoder encounters a critical chunk it does not recognize, it must abort reading the file or supply the user with an appropriate warning.
The case of the second letter indicates whether the chunk is "public" (either in the specification or the registry of special-purpose public chunks) or "private" (not standardized). Uppercase is public and lowercase is private. This ensures that public and private chunk names can never conflict with each other (although two private chunk names could conflict).
The third letter must be uppercase to conform to the PNG specification. It is reserved for future expansion. Decoders should treat a chunk with a lower case third letter the same as any other unrecognized chunk.
The case of the fourth letter indicates whether the chunk is safe to copy by editors that do not recognize it. If lowercase, the chunk may be safely copied regardless of the extent of modifications to the file. If uppercase, it may only be copied if the modifications have not touched any critical chunks.
Critical chunks
edit
A decoder must be able to interpret critical chunks to read and render a PNG file.
IHDR
must be the first chunk; it is 13 data bytes long and contains (in this order) the image's
width (4 bytes)
height (4 bytes)
bit depth (1 byte, values 1, 2, 4, 8, or 16)
color type (1 byte, values 0, 2, 3, 4, or 6)
compression method (1 byte, value 0)
filter method (1 byte, value 0)
interlace method (1 byte, values 0 "no interlace" or 1 "
Adam7
interlace").
13
As stated in the
World Wide Web Consortium
, bit depth is defined as "the number of bits per sample or per palette index (not per pixel)".
13
PLTE
contains the
palette
: a list of colors.
IDAT
contains the image, which may be split among multiple IDAT chunks. Such splitting slightly increases the file size, but makes it possible to generate a PNG in a streaming manner. The IDAT chunk contains the actual image data, which is the output stream of the compression algorithm.
24
IEND
marks the image end; the data field of the IEND chunk has 0 bytes/is empty.
25
The
PLTE
chunk is essential for color type 3 (indexed color). It is optional for color types two and six (truecolor and truecolor with alpha) and it must not appear for color types 0 and 4 (grayscale and grayscale with alpha).
Ancillary chunks
edit
Other image attributes that can be stored in PNG files include
gamma
values, background color, and textual
metadata
information. PNG also supports
color management
through the inclusion of
ICC color profiles
26
bKGD
gives the default background color. It is intended for use when there is no better choice available, such as in standalone image viewers (but not web browsers; see below for more details).
cHRM
gives the
chromaticity
coordinates of the display
primaries
and
white point
cICP
specifies the color space, transfer function and matrix coefficients as defined in ITU-T
H.273
27
It is intended for use with
HDR imagery
without requiring a color profile.
28
dSIG
is for storing digital signatures.
29
eXIf
stores
Exif
metadata.
30
31
gAMA
specifies
gamma
. The gAMA chunk contains only 4 bytes, and its value represents the gamma value multiplied by 100,000; for example, the gamma value 1/3.4 calculates to 29411.7647059 ((1/3.4)*(100,000)) and is converted to an integer (29412) for storage.
32
hIST
can store the histogram, or total amount of each color in the image.
iCCP
is an
ICC color profile
iTXt
contains a keyword and
UTF-8
text, with encodings for possible compression and translations marked with
language tag
. The
Extensible Metadata Platform
(XMP) uses this chunk with a keyword 'XML:com.adobe.xmp'
pHYs
holds the intended pixel size (or pixel aspect ratio); the pHYs contains "Pixels per unit, X axis" (4 bytes), "Pixels per unit, Y axis" (4 bytes), and "Unit specifier" (1 byte) for a total of 9 bytes.
33
sBIT
(significant bits) indicates the color-accuracy of the source data; this chunk contains a total of between 1 and 5 bytes, depending on the color type.
34
35
36
sPLT
suggests a palette to use if the full range of colors is unavailable.
sRGB
indicates that the standard
sRGB color space
is used; the sRGB chunk contains only 1 byte, which is used for "rendering intent" (4 values—0, 1, 2, and 3—are defined for rendering intent).
37
sTER
stereo-image indicator chunk for
stereoscopic
images.
38
tEXt
can store text that can be represented in
ISO/IEC 8859-1
, with one
key-value
pair for each chunk. The "key" must be between one and 79 characters long. Separator is a null character. The "value" can be any length, including zero up to the maximum permissible chunk size minus the length of the keyword and separator. Neither "key" nor "value" can contain null character. Leading or trailing spaces are also disallowed.
tIME
stores the time that the image was last changed.
tRNS
contains transparency information. For indexed images, it stores alpha channel values for one or more palette entries. For truecolor and grayscale images, it stores a single pixel value that is to be regarded as fully transparent.
zTXt
contains compressed text (and a compression method marker) with the same limits as
tEXt
Pixel format
edit
Allowed combinations of color type and bit depth
13
Color type
Channels
Bits per channel
16
Indexed
Grayscale
16
Grayscale and alpha
16
32
Truecolor
24
48
Truecolor and alpha
32
64
Pixels in PNG images are numbers that may be either indices of sample data in the
palette
or the sample data itself. The palette is a separate table contained in the PLTE chunk. Sample data for a single
pixel
consists of a tuple of between one and four numbers. Whether the pixel data represents palette indices or explicit sample values, the numbers are referred to as
channels
and every number in the image is encoded with an identical format.
The permitted formats encode each number as an unsigned integer value using a fixed number of bits, referred to in the PNG specification as the
bit depth
. Notice that this is not the same as
color depth
, which is commonly used to refer to the total number of bits in each pixel, not each channel. The permitted bit depths are summarized in the table along with the total number of bits used for each pixel.
The number of channels depends on whether the image is grayscale or color and whether it has an
alpha channel
. PNG allows the following combinations of channels, called the
color type
0 (000
grayscale
2 (010
red, green and blue: rgb/truecolor
3 (011
indexed: channel containing indices into a palette of colors
4 (100
grayscale and alpha: level of
opacity
for each pixel
6 (110
red, green, blue and alpha
The color type is specified as an 8-bit value however only the low three bits are used and, even then, only the five combinations listed above are permitted. So long as the color type is valid it can be considered as a bit field as summarized in the adjacent table:
PNG color types
Color
type
Name
Binary
Masks
Grayscale
Truecolor
color
Indexed
color, palette
Grayscale and alpha
alpha
Truecolor and alpha
alpha, color
bit value 1: the image data stores palette indices. This is only valid in combination with bit value 2;
bit value 2: the image samples contain three channels of data encoding
trichromatic
colors
, otherwise the image samples contain one channel of data encoding
relative luminance
bit value 4: the image samples also contain an alpha channel expressed as a linear measure of the opacity of the pixel. This is not valid in combination with bit value 1.
With indexed color images, the palette always stores trichromatic colors at a depth of 8 bits per channel (24 bits per palette entry). Additionally, an optional list of 8-bit alpha values for the palette entries may be included; if not included, or if shorter than the palette, the remaining palette entries are assumed to be opaque. The palette must not have more entries than the image bit depth allows for, but it may have fewer (for example, if an image with 8-bit pixels only uses 90 colors then it does not need palette entries for all 256 colors). The palette must contain entries for all the pixel values present in the image.
The standard allows indexed color PNGs to have 1, 2, 4 or 8 bits per pixel; grayscale images with no alpha channel may have 1, 2, 4, 8 or 16 bits per pixel. Everything else uses a bit depth per channel of either 8 or 16. The combinations this allows are given in the table above. The standard requires that decoders can read all supported color formats, but many image editors can only produce a small subset of them.
Transparency of image
edit
PNG offers a variety of transparency options. With true-color and grayscale images either a single pixel value can be declared as transparent or an
alpha channel
can be added (enabling any percentage of partial transparency to be used). For paletted images, alpha values can be added to palette entries. The number of such values stored may be less than the total number of palette entries, in which case the remaining entries are considered fully opaque.
The scanning of pixel values for binary transparency is supposed to be performed before any color reduction to avoid pixels becoming unintentionally transparent. This is most likely to pose an issue for systems that can decode 16-bits-per-channel images (as is required for compliance with the specification) but only output at 8 bits per channel (the norm for all but the highest end systems).
Alpha
storage
can be "associated" ("
premultiplied
") or "unassociated", but PNG standardized
39
on "unassociated" ("non-premultiplied") alpha, which means that imagery is not alpha
encoded
; the emissions represented in RGB are not the emissions at the pixel level. This means that the over operation will multiply the RGB emissions by the alpha, and cannot represent emission and occlusion properly.
Compression
edit
PNG uses a two-stage compression process:
pre-compression: filtering (prediction)
compression:
DEFLATE
PNG uses
DEFLATE
, a non-patented
lossless data compression
algorithm
involving a combination of
LZ77
and
Huffman coding
Permissively licensed
DEFLATE implementations, such as
zlib
, are widely available.
Compared to formats with
lossy compression
, such as JPEG, choosing higher compression settings can slow processing without significantly reducing file size.
Filtering
edit
PNG's filter method 0 can use the data in pixels A, B, and C to predict the value for X.
A PNG with 256 colors, which is only 251 bytes large with pre-filter. The same image as a GIF would be more than thirteen times larger.
Before DEFLATE is applied, the data is transformed via a prediction method: a single
filter method
is used for the entire image, while for each image line, a
filter type
is chosen to transform the data to make it more efficiently compressible.
40
The filter type used for a scanline is prepended to the scanline to enable inline decompression.
There is only one filter method in the current PNG specification (denoted method 0), and thus in practice the only choice is which filter type to apply to each line. For this method, the filter predicts the value of each pixel based on the values of previous neighboring pixels, and subtracts the predicted color of the pixel from the actual value, as in
DPCM
. An image line filtered in this way is often more compressible than the raw image line would be, especially if it is similar to the line above, since the differences from prediction will generally be clustered around 0, rather than spread over all possible image values. This is particularly important in relating separate rows, since DEFLATE has no understanding that an image is a 2D entity, and instead just sees the image data as a stream of bytes.
There are five filter types for filter method 0; each type predicts the value of each byte (of the image data before filtering) based on the corresponding byte of the pixel to the left (
), the pixel above (
), and the pixel above and to the left (
) or some combination thereof, and encodes the
difference
between the predicted value and the actual value. Filters are applied to byte values, not pixels; pixel values may be one or two bytes, or several values per byte, but never cross byte boundaries. The filter types are:
41
Type byte
Filter name
Predicted value
None
Zero (so that the raw byte value passes through unaltered)
Sub
Byte
(to the left)
Up
Byte
(above)
Average
Mean of bytes
and
, rounded down
Paeth
, or
, whichever is closest to
The Paeth filter is based on an algorithm by
Alan W. Paeth
42
Compare to the version of
DPCM
used in
lossless JPEG
, and to the
discrete wavelet transform
using 1 × 2, 2 × 1, or (for the Paeth predictor) 2 × 2 windows and
Haar wavelets
Compression is further improved by choosing filter types adaptively on a line-by-line basis. This improvement, and a heuristic method of implementing it commonly used by PNG-writing software, were created by
Lee Daniel Crocker
, who tested the methods on many images during the creation of the format;
43
the choice of filter is a component of file size optimization, as discussed below.
If interlacing is used, each stage of the interlacing is filtered separately, meaning that the image can be progressively rendered as each stage is received; however, interlacing generally makes compression less effective.
Interlacing
edit
An illustration of Adam7 interlacing over a 16×16 image
PNG offers an optional 2-dimensional, 7-pass
interlacing
scheme—the
Adam7 algorithm
. This is more sophisticated than GIF's 1-dimensional, 4-pass scheme, and allows a clearer low-resolution image to be visible earlier in the transfer, particularly if interpolation algorithms such as
bicubic interpolation
are used.
44
However, the 7-pass scheme tends to reduce the data's compressibility more than simpler schemes.
Animation
edit
An APNG (animated PNG) file (displays as static image in
some web browsers
The core PNG format does not support animation.
MNG
is an extension to PNG that does; it was designed by members of the PNG Group. MNG shares PNG's basic structure and chunks, but it is significantly more complex and has a different file signature, which automatically renders it incompatible with standard PNG decoders. This means that most web browsers and applications either never supported MNG or dropped support for it.
The complexity of MNG led to the proposal of
APNG
by developers at the Mozilla Foundation. It is based on PNG, supports animation and is simpler than MNG. APNG offers fallback to single-image display for PNG decoders that do not support APNG. Today, the APNG format is supported by all major web browsers.
45
APNG is supported in
Firefox
3.0 and up,
Pale Moon
(all versions), and
Safari
8.0 and up.
46
Chromium 59.0 added APNG support,
47
48
followed by Google Chrome.
Opera
supported APNG in versions 10–12.1, but support lapsed in version 15 when it switched to the
Blink
rendering engine; support was re-added in Opera 46 (inherited from Chromium 59).
49
Microsoft Edge
has supported APNG since version 79.0, when it switched to a Chromium-based engine.
The PNG Group decided in April 2007 not to embrace APNG.
50
Several alternatives were under discussion, including ANG, aNIM/mPNG, "PNG in GIF" and its subset "RGBA in GIF".
51
However, currently only APNG has widespread support.
With the release of the third edition of the PNG specification in June 2025, now maintained by the PNG working group,
16
APNG is finally incorporated into the specification as an extension.
52
Examples
edit
Structure of a very simple PNG file
89 50 4E 47 0D 0A 1A 0A
PNG signature
IHDR
Image header
IDAT
Image data
IEND
Image end
Contents of a minimal PNG file representing one red pixel
Hex
As characters
89 50 4E 47 0D 0A 1A 0A
00 00 00 0D 49 48 44 52
00 00 00 01 00 00 00 01 08 02 00 00 00 90 77 53
DE
00 00 00 0C 49 44 41 54 08 D7 63 F8 CF C0 00
00 03 01 01 00 18 DD 8D B0
00 00 00 00 49 45 4E
44 AE 42 60 82
.PNG....
....IHDR
..............wS
....IDAT..c....
.........
....IEN
D.B`.
IHDR Chunk
Offset into chunk
Hex Value
Decimal Value
Text
Meaning
0x0D
13
IHDR chunk has 13 bytes of content
0x49484452
IHDR
Identifies a Header chunk
0x01
Image is 1 pixel wide
12
0x01
Image is 1 pixel high
16
0x08
8 bits per pixel (per channel)
17
0x02
Color type 2 (RGB/truecolor)
18
0x00
Compression method 0 (only accepted value)
19
0x00
Filter method 0 (only accepted value)
20
0x00
Not interlaced
21
0x907753DE
CRC of chunk's type and content (but not length)
IDAT Chunk
Offset into chunk
Hex Value
Meaning
0x0C
IDAT chunk has 12 bytes of content
0x49444154
Identifies a Data chunk
0x08
DEFLATE compression method using a 256-byte window
53
0xD7
ZLIB FCHECK value, no dictionary used, maximum compression algorithm
53
10
0x63F8CFC00000
A compressed DEFLATE block using the static Huffman code that decodes to 0x00 0xFF 0x00 0x00
54
16
0x03010100
The ZLIB check value: the
Adler-32
checksum of the uncompressed data
53
20
0x18DD8DB0
CRC of chunk's type and content (but not length)
Displayed in the fashion of
hex editors
, with on the left side byte values shown in
hex format
, and on the right side their equivalent characters from
ISO-8859-1
with unrecognized and control characters replaced with periods. Offsets 8 and 9 of the IDAT Chunk represent the required 2-byte header of the zlib data format. Offset 16 (of the same chunk) represents the 4-byte footer required by the same format. Additionally the PNG signature and individual chunks are marked with colors. Note they are easy to identify because of their human readable type names (in this example PNG, IHDR, IDAT, and IEND).
Advantages
edit
Reasons to use PNG:
Portability
: Transmission is independent of the software and hardware platform.
Completeness
: it's possible to represent true color, indexed-color, and grayscale images.
Coding and decoding in series
: allows to generate and read data streams in series, that is, the format of the data stream is used for the generation and visualization of images at the moment through serial communication.
Progressive presentation
: to be able to transmit data flows that are initially an approximation of the entire image and progressively they improve as the data flow is received.
Soundness to transmission errors
: detects the transmission errors of the data stream correctly.
Losslessness
: No loss: filtering and compression preserve all information.
Efficiency
: any progressive image presentation, compression and filtering seeks efficient decoding and presentation.
Compression
: images can be compressed efficiently and consistently.
Easiness
: the implementation of the standard is easy.
Interchangeability
: any PNG decoder that follows the standards can read all PNG data streams.
Flexibility
: allows future extensions and private additions without affecting the previous point.
Freedom of legal restrictions
: the algorithms used are free and accessible.
Comparison with other file formats
edit
Main article:
Comparison of graphics file formats
Graphics Interchange Format (GIF)
edit
On small images,
GIF
can achieve greater compression than PNG (see the
section on filesize
, below).
On most images, except for the above case, a GIF file has a larger size than an indexed PNG image.
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.
55
When an alpha channel is added, up to 64 bits per pixel (before compression) are possible.
When converting an image from the PNG format to GIF, the image quality may suffer due to
posterization
if the PNG image has more than 256 colors.
GIF intrinsically supports animated images. PNG supports animation only via extensions (see the
section on animation
, above).
PNG images are less widely supported by older browsers. In particular, IE6 has limited support for PNG.
56
JPEG
edit
Composite image comparing lossy compression in JPEG with lossless compression in PNG: the JPEG artifacts can be easily visible in the background of this kind of image data, where the PNG image has solid color.
The
JPEG
(Joint Photographic Experts Group) format 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, which is typically dominated by soft, low-contrast transitions, and an amount of noise or similar irregular structures. Using PNG instead of a high-quality JPEG for such images would result in a large increase in file size with
negligible
gain in quality. In comparison, when storing images that contain text, line art, or graphics – images with sharp transitions and large areas of solid color – the PNG format can compress image data more than JPEG can. Additionally, PNG is lossless, while JPEG produces visual artifacts around high-contrast areas. (Such artifacts depend on the settings used in the JPG compression; they can be quite noticeable when a low-quality [high-compression] setting is used.) Where an image contains both sharp transitions and photographic parts, a choice must be made between the two effects. JPEG does not support transparency.
JPEG's lossy compression also suffers from
generation loss
, where repeatedly decoding and re-encoding an image to save it again causes a loss of information each time, degrading the image. Because PNG is lossless, it is suitable for storing images to be edited. While PNG is reasonably efficient when compressing photographic images, there are lossless compression formats designed specifically for photographic images, lossless
WebP
and
Adobe DNG
(digital negative) for example. However these formats are either not widely supported, or are proprietary. An image can be stored losslessly and converted to JPEG format only for distribution, so that there is no generation loss.
While the PNG specification does not explicitly include a standard for embedding
Exif
image data from sources such as digital cameras, the preferred method for embedding EXIF data in a PNG is to use the non-critical ancillary chunk label
eXIf
57
Early web browsers did not support PNG images; JPEG and GIF were the main image formats. JPEG was commonly used when exporting images containing gradients for web pages, because of GIF's limited color depth. However, JPEG compression causes a gradient to blur slightly. A PNG format reproduces a gradient as accurately as possible for a given bit depth, while keeping the file size small. PNG became the optimal choice for small gradient images as web browser support for the format improved. No images at all are needed to display gradients in modern browsers, as gradients can be created using
CSS
JPEG-LS
edit
JPEG-LS
is an image format by the
Joint Photographic Experts Group
, though far less widely known and supported than the other lossy JPEG format discussed above. It is directly comparable with PNG,
clarification needed
and has a standard set of test images.
58
On the Waterloo Repertoire ColorSet, a standard set of test images (unrelated to the JPEG-LS conformance test set), JPEG-LS generally performs better than PNG, by 10–15%, but on some images PNG performs substantially better, on the order of 50–75%.
59
Thus, if both of these formats are options and file size is an important criterion, they should both be considered, depending on the image.
JPEG XL
edit
JPEG XL
is another, much improved, lossless or lossy format, that is unfortunately supported much less, developed to replace lossless formats like PNG.
60
JPEG XL is more than 50% smaller than JPEG, and that can happen while it's lossless, therefore making it even smaller than PNG.
61
It also supports
high dynamic range
, wide
colour gamuts
, and large
colour depths
62
JPEG XL is also very efficient at decoding, and provides smooth transitions from the formats it intends to replace, losslessly able to convert from JPEG. It also excels at compressing without compromising on fidelity.
63
TIFF
edit
Tag Image File Format
(TIFF) is a format that incorporates an extremely wide range of options. While this makes TIFF useful as a generic format for interchange between professional image editing applications, it makes adding support for it to applications a much bigger task and so it has little support in applications not concerned with image manipulation (such as web browsers). The high level of extensibility also means that most applications provide only a subset of possible features, potentially creating user confusion and compatibility issues.
The most common general-purpose, lossless compression algorithm used with TIFF is
Lempel–Ziv–Welch
(LZW). This compression technique, also used in GIF, was covered by patents until 2003. TIFF also supports the compression algorithm PNG uses (i.e.
Compression Tag 0008
16
Adobe
-style') with medium usage and support by applications. TIFF also offers special-purpose lossless compression algorithms like
CCITT Group IV
, which can compress
bilevel images
(e.g., faxes or black-and-white text) better than PNG's compression algorithm.
PNG supports non-premultiplied alpha only
39
whereas TIFF also supports "associated" (premultiplied) alpha.
WebP
edit
WebP
is a format invented by
Google
that was intended to replace PNG, JPEG, and GIF.
64
WebP files allow for both lossy and lossless compression, while PNG only allows for lossless compression. WebP also supports animation, something that only
GIF
files could previously accomplish.
65
The main improvements of WebP over PNG, however, are the large reduction in file size and therefore faster loading times when embedded into websites. Google claims that lossless WebP images are 26% smaller than PNG files.
66
WebP has received criticism for being incompatible with various image editing programs and social media websites, unlike PNG.
67
WebP is also not supported across all web browsers, which may require web image hosters to create a fallback image to display to the user, negating the potential storage savings of WebP.
65
AVIF
edit
AVIF
is an image format developed by the
Alliance for Open Media
. AVIF was designed by the foundation to make up for the shortcomings of other image codecs, including PNG,
GIF
, and
WebP
68
AVIF is generally smaller in size than both WebP and PNG.
69
AVIF supports animation while PNG previously did not.
70
However, like WebP, AVIF is supported across fewer applications than PNG.
70
Software support
edit
The official
reference implementation
of the PNG format is the
programming library
libpng
71
It is published as free software under the terms of a
permissive free software license
. Therefore, it is usually found as an important system library in free operating systems.
Bitmap graphics editor support for PNG
edit
Main article:
Comparison of raster graphics editors
The PNG format is widely supported by graphics programs, including
Adobe Photoshop
Corel
's
Photo-Paint
and
Paint Shop Pro
, the
GIMP
GraphicConverter
Helicon Filter
ImageMagick
Inkscape
IrfanView
, Pixel image editor,
Paint.NET
and
Xara Photo & Graphic Designer
and many others (including online graphic design platforms such as
Canva
). Some programs bundled with popular
operating systems
which support PNG include
Microsoft
's
Paint
and
Apple
's
Photos
iPhoto
and
Preview
, with the GIMP also often being bundled with popular
Linux
distributions.
Adobe Fireworks
(formerly by
Macromedia
) uses PNG as its native file format, allowing other image editors and preview utilities to view the flattened image. However, Fireworks by default also stores metadata for layers, animation, vector data, text and effects. Such files should not be distributed directly. Fireworks can instead export the image as an optimized PNG without the extra metadata for use on web pages, etc.
72
Web browser support for PNG
edit
Main article:
Web browser image format support
PNG support first appeared in 1997, in
Internet Explorer
4.0b1 (32-bit only for NT), and in
Netscape
4.04.
73
Despite calls by the
Free Software Foundation
74
and the
World Wide Web Consortium
(W3C),
75
tools such as gif2png,
76
and campaigns such as Burn All GIFs,
77
PNG adoption on websites was fairly slow due to late and buggy support in Internet Explorer, particularly regarding transparency.
78
PNG is the most used image file format on the web since 2018.
79
PNG compatible browsers include: Apple
Safari
Google Chrome
Mozilla Firefox
Opera
Camino
Internet Explorer
Microsoft Edge
and many others. For the complete comparison, see
Comparison of web browsers (Image format support)
Especially versions of Internet Explorer (Windows) below 9.0 (released 2011) had numerous problems which prevented it from correctly rendering PNG images.
80
4.0 crashes on large PNG chunks.
81
4.0 does not include the functionality to view .png files,
82
but there is a registry fix.
80
5.0 and 5.01 have broken OBJECT support.
83
5.01 prints palette images with black (or dark gray) backgrounds under Windows 98, sometimes with radically altered colors.
84
6.0 fails to display PNG images of 4097 or 4098 bytes in size.
85
6.0 cannot open a PNG file that contains one or more zero-length IDAT chunks. This issue was first fixed in security update 947864 (MS08-024). For more information, see this article in the Microsoft Knowledge Base:
947864
MS08-024: Cumulative Security Update for Internet Explorer.
86
6.0 sometimes completely loses ability to display PNGs, but there are various fixes.
87
6.0 and below have broken alpha-channel transparency support (will display the default background color instead).
88
89
90
7.0 and below cannot combine 8-bit alpha transparency AND element opacity (
CSS
– filter: Alpha (opacity=xx)) without filling partially transparent sections with black.
91
8.0 and below have inconsistent/broken gamma support.
80
8.0 and below don't have color-correction support.
80
Operating system support for PNG icons
edit
PNG icons have been supported in most distributions of
Linux
since at least 1999, in desktop environments such as
GNOME
92
In 2006,
Microsoft Windows
support for PNG icons was introduced in
Windows Vista
93
PNG icons are supported in
AmigaOS 4
AROS
macOS
iOS
and
MorphOS
as well. In addition,
Android
makes extensive use of PNGs.
File size and optimization software
edit
PNG file size can vary significantly depending on how it is encoded and compressed; this is discussed and a number of tips are given in
PNG: The Definitive Guide.
59
Compared to GIF
edit
Compared to
GIF
files, a PNG file with the same information (256 colors, no ancillary chunks/metadata), compressed by an effective compressor is normally smaller than a GIF image. Depending on the file and the compressor, PNG may range from somewhat smaller (10%) to significantly smaller (50%) to somewhat larger (5%), but is rarely significantly larger
59
for large images. This is attributed to the performance of PNG's
DEFLATE
compared to GIF's
LZW
, and because the added precompression layer of PNG's predictive filters take account of the 2-dimensional image structure to further compress files; as filtered data encodes differences between pixels, they will tend to cluster closer to 0, rather than being spread across all possible values, and thus be more easily compressed by DEFLATE. However, some versions of
Adobe Photoshop
CorelDRAW
and
MS Paint
provide poor PNG compression, creating the impression that GIF is more efficient.
59
File size factors
edit
PNG files vary in size due to a number of factors:
color depth
Color depth can range from 1 to 64 bits per pixel.
ancillary chunks
PNG supports metadata—this may be useful for editing, but unnecessary for viewing, as on websites.
interlacing
As each pass of the Adam7 algorithm is separately filtered, this can increase file size.
59
filter
As a precompression stage, each line is filtered by a predictive filter, which can change from line to line. As the ultimate DEFLATE step operates on the whole image's filtered data, one cannot optimize this row-by-row; the choice of filter for each row is thus potentially very variable, though heuristics exist.
note 1
compression
With additional computation, DEFLATE compressors can produce smaller files.
There is thus a filesize trade-off between high color depth, maximal metadata (including color space information, together with information that does not affect display), interlacing, and speed of compression, which all yield large files, with lower color depth, fewer or no ancillary chunks, no interlacing, and tuned but computationally intensive filtering and compression. For different purposes, different trade-offs are chosen: a maximal file may be best for archiving and editing, while a stripped down file may be best for use on a website, and similarly fast but poor compression is preferred when repeatedly editing and saving a file, while slow but high compression is preferred when a file is stable: when archiving or posting.
Interlacing is a trade-off: it dramatically speeds up early rendering of large files (improves latency), but may increase file size (decrease throughput) for little gain, particularly for small files.
59
Lossy PNG compression
edit
Although PNG is a lossless format, PNG encoders can preprocess image data in a lossy fashion to improve PNG compression. For example, quantizing a truecolor PNG to 256 colors allows the indexed color type to be used for a likely reduction in file size.
94
Image editing software
edit
Some programs are more efficient than others when saving PNG files, this relates to implementation of the PNG compression used by the program.
Many graphics programs (such as Apple's
Preview
software) save PNGs with large amounts of
metadata
and color-correction data that are generally unnecessary for
Web
viewing. Unoptimized PNG files from
Adobe Fireworks
are also notorious for this since they contain options to make the image editable in supported editors. Also CorelDRAW (at least version 11) sometimes produces PNGs which cannot be opened by Internet Explorer (versions 6–8).
Adobe Photoshop
's performance on PNG files has improved in the CS Suite when using the Save For Web feature (which also allows explicit PNG/8 use).
Adobe's Fireworks saves larger PNG files than many programs by default. This stems from the mechanics of its
format: the images produced by Fireworks' save function include large, private chunks, containing complete layer and vector information. This allows further lossless editing. When saved with the
Export
option, Fireworks' PNGs are competitive with those produced by other image editors, but are no longer editable as anything but flattened bitmaps. Fireworks is unable to save size-optimized vector-editable PNGs.
Other notable examples of poor PNG compressors include:
Microsoft's Paint for Windows XP
Microsoft Picture It! Photo Premium 9
Poor compression increases the PNG file size but does not affect the image quality or compatibility of the file with other programs.
When the color depth of a
truecolor
image is reduced to an 8-bit palette (as in GIF), the resulting image data is typically much smaller. Thus a truecolor PNG is typically larger than a color-reduced GIF, although PNG could store the color-reduced version as a palettized file of comparable size. Conversely, some tools, when saving images as PNGs, automatically save them as truecolor, even if the original data use only 8-bit color, thus bloating the file unnecessarily.
59
Both factors can lead to the misconception that PNG files are larger than equivalent GIF files.
Optimizing tools
edit
Various tools are available for optimizing PNG files; they do this by:
(optionally) removing ancillary chunks,
reducing
color depth
, either:
use a palette (instead of RGB) if the image has 256 or fewer colors,
use a smaller palette, if the image has 2, 4, or 16 colors, or
(optionally) lossily discard some of the data in the original image,
optimizing line-by-line filter choice, and
optimizing DEFLATE compression.
Tool list
edit
pngcrush
is the oldest of the popular PNG optimizers. It allows for multiple trials on filter selection and compression arguments, and finally chooses the smallest one. This working model is used in almost every png optimizer.
advpng and the similar advdef utility in the AdvanceCOMP package recompress the PNG IDAT. Different DEFLATE implementations are applied depending on the selected compression level, trading between speed and file size: zlib at level 1, libdeflate at level 2,
7-zip
's
LZMA
DEFLATE at level 3, and
zopfli
at level 4.
pngout
was made with the author's own deflater (same to the author's zip utility, kzip), while keeping all facilities of color reduction / filtering. However, pngout doesn't allow for using several trials on filters in a single run. It's suggested to use its commercial GUI version, pngoutwin, or used with a
wrapper
to automate the trials or to recompress using its own deflater while keep the filter line by line.
note 2
zopflipng
was also made with its own deflater, zopfli. It has all the optimizing features pngcrush has (including automating trials) while providing a very good, but slow deflater.
A simple comparison of their features is listed below.
Optimizer
Chunk removal
Color reduction
Filtering
Filter reuse
note 3
Multiple trials on filters in a single run
Deflater
note 4
advpng
Yes
No
note 5
No
N/A
note 6
(multiple)
advdef
No
No
Reuses previous filter set
Always
N/A
(multiple)
pngcrush
Yes
Yes
0–4 or adaptive
No
Yes
zlib
pngout
Yes
Yes
0–4 or adaptive
Yes
note 2
No
kzip
zopflipng
Yes
Yes
0–4 or adaptive with 2 different algorithms, or with a brute way
Yes
Yes
zopfli
Before
zopflipng
was available, a good way in practice to perform a png optimization is to use a combination of 2 tools in sequence for optimal compression: one which optimizes filters (and removes ancillary chunks), and one which optimizes DEFLATE. Although pngout offers both, only one type of filter can be specified in a single run, therefore it can be used with a
wrapper tool
or in combination with
pngcrush
note 2
acting as a re-deflater, like
advdef
Ancillary chunk removal
edit
For removing ancillary chunks, most PNG optimization tools have the ability to remove all color correction data from PNG files (gamma, white balance, ICC color profile, standard RGB color profile). This often results in much smaller file sizes. For example, the following command line options achieve this with
pngcrush
pngcrush -rem gAMA -rem cHRM -rem iCCP -rem sRGB
InputFile.png
OutputFile.png
Filter optimization
edit
pngcrush
pngout
, and
zopflipng
all offer options applying one of the filter types 0–4 globally (using the same filter type for all lines) or with a "pseudo filter" (numbered 5), which for each line chooses one of the filter types 0–4 using an adaptive algorithm.
zopflipng
offers 3 different adaptive method, including a brute-force search that attempts to optimize the filtering.
note 7
pngout
and
zopflipng
provide an option to preserve/reuse
note 2
note 8
the line-by-line filter set present in the input image.
pngcrush
and
zopflipng
provide options to try different filter strategies in a single run and choose the best. The freeware command line version of
pngout
doesn't offer this, but the commercial version,
pngoutwin
, does.
note 9
DEFLATE optimization
edit
Zopfli
and the
LZMA SDK
provide
DEFLATE
implementations that can produce higher
compression ratios
than the
zlib
reference implementation at the cost of performance. AdvanceCOMP's
advpng
and
advdef
can use either of these libraries to re-compress PNG files. Additionally,
PNGOUT
contains its own
proprietary
DEFLATE implementation.
advpng
doesn't have an option to apply filters and always uses filter 0 globally (leaving the image data unfiltered); therefore it should not be used where the image benefits significantly from filtering. By contrast,
advdef
from the same package doesn't deal with PNG structure and acts only as a re-deflater, retaining any existing filter settings.
Icon optimization
edit
Since
icons
intended for Windows Vista and later versions may contain PNG subimages, the optimizations can be applied to them as well. At least one
icon editor
, Pixelformer, is able to perform a special optimization pass while saving
ICO
files, thereby reducing their sizes.
Icons for macOS
may also contain PNG subimages, yet there isn't such tool available.
citation needed
See also
edit
Computer graphics
, including:
Image editing
Image file formats
Related
graphics file formats
APNG
Animated PNG
JPEG Network Graphics
(JNG)
Multiple-image Network Graphics
(MNG)
Similar file formats
X PixMap
for portable icons
Scalable Vector Graphics
Digital Negative
WebP
Explanatory notes
edit
The filtering is used to increase the similarity to the data, hence increasing the compression ratio. However, there is theoretically no formula for similarity, nor absolute relationship between the similarity and compressor, thus unless the compression is done, one can't tell one filter set is better than another.
Use
pngout -f6
to reuse previous filter set
The tools offering such feature could act as a pure re-deflater to PNG files.
zlib
, the reference deflate implementation, compression is suboptimal even at the maximum level. See
Zopfli
zip format in 7-zip
and
pngout
Not only does advpng not support color reduction, it also fails on images with a reduced colorspace.
Advpng can only apply filter 0 globally, thus it's neither yes or no, but N/A.
[pngcrush|pngout] -f
or
zopflipng --filters
zopflipng --filters=p
pngoutwin
's setting dialog for optimization offers the user a selection of filter strategies.
References
edit
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cite web
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IDAT
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IEND
Image trailer
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ftp-osl.osuosl.org
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gAMA
Image gamma
W3C 2003
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sBIT
Significant bits
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Roelofs 2003
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"Here's why you have to deal with so many annoying webPs now"
PC Gamer
. Retrieved
22 August
2024
"AVIF: Meet the Next Level Image File Format"
Alliance for Open Media
. 8 November 2023
. Retrieved
26 September
2024
"PNG vs AVIF: The Ultimate Image Format Battle | Coconut©"
www.coconut.co
. Retrieved
26 September
2024
"AVIF vs. WebP: 4 Key Differences and How to Choose"
Cloudinary
. Retrieved
26 September
2024
"libpng"
. Retrieved
13 July
2013
"Fireworks Help / Saving and exporting"
. Adobe Inc
. Retrieved
16 April
2026
"Use of PNG Images to Display Data"
. Oregon Water Science Center. 16 February 2006. Archived from
the original
on 20 August 2008
. Retrieved
21 October
2003
"Why There Are No GIF files on GNU Web Pages"
GNU Operating System
. 16 December 2008.
"PNG Fact Sheet"
World Wide Web Consortium
. 7 October 1996.
"Resource page for gif2png 2.5.11"
catb.org
"Burn All GIFs"
burnallgifs.org
"PNG Transparency in Internet Explorer"
PC Magazine
. 5 October 2004.
"Historical yearly trends in the usage statistics of image file formats for websites"
w3techs.com
"Browsers with PNG Support"
. 14 March 2009.
"Windows Explorer Crashes When I Click on a Fireworks PNG File to View It"
Adobe Systems
. 5 June 2007.
"Unable to view .png images with Internet Explorer 4.0"
Microsoft Knowledge Base
"PNGs That Are Inside of an Object Tag Print as a Negative Image"
Microsoft Knowledge Base
"PNG Images Are Printed Improperly in Internet Explorer 5.01"
Microsoft Knowledge Base
"You cannot view some PNG images in Internet Explorer 6"
Microsoft Knowledge Base
"You cannot use Internet Explorer 6 to open a PNG file that contains one or more zero-length IDAT chunks"
Microsoft Knowledge Base
"PNG Frequently Asked Questions"
"PhD: Portable Network Graphics Lose Transparency in Web Browser"
Microsoft Knowledge Base
"PNG Files Do Not Show Transparency in Internet Explorer"
Microsoft Knowledge Base
Lovitt, Michael (21 December 2002).
"Cross-Browser Variable Opacity with PNG: A Real Solution"
A List Apart
. Archived from
the original
on 18 August 2011
. Retrieved
21 July
2009
"IE7 alpha transparent PNG + opacity"
Channel 9
. Archived from
the original
on 27 August 2011
. Retrieved
23 January
2009
Fulbright, Michael (1999).
"GNOME 1.0 Library Roadmap"
. Archived from
the original
on 30 January 2010
. Retrieved
19 December
2007
"Windows Vista – Icons"
OOne
. 2007. Archived from
the original
on 11 November 2007
. Retrieved
12 November
2007
"PNG can be a lossy format"
. Pngmini.com
. Retrieved
1 February
2014
Further reading
edit
Roelofs, Greg (April 1997).
"Linux Gazette: History of the Portable Network Graphics (PNG) Format"
Linux Journal
1997
(36es). Specialized Systems Consultants, Inc.
ISSN
1075-3583
Roelofs, Greg (2003).
PNG: The Definitive Guide
(2nd ed.). O'Reilly Media.
ISBN
1-56592-542-4
"Portable Network Graphics (PNG) Specification"
(Second ed.).
W3C
. 10 November 2003.
External links
edit
PNG Specification
(Third Edition)
PNG Home Site
libpng Home Page
The Story of PNG
by Greg Roelofs
Test inline PNG images
RFC
2083
More information about PNG color correction
The GD-library to generate dynamic PNG-files with PHP
PNG Adam7 interlacing
Archived
31 August 2024 at the
Wayback Machine
Encoding Web Shells in PNG files
: Encoding human readable data inside an IDAT block.
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