png格式详解

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Portable Network GraphicsPNG

A PNG image with an 8-bit transparency channel (top). The same image is overlaid onto a checkered background (bottom), typically used ingraphics software to indicate transparency.Filename extension.pngInternet media typeimage/pngType codePNGfPNGUniform Type Identifierpublic.pngDeveloped byPNG Development Group (donated to W3C)Initial releaseOctober 1, 1996Type of formatlossless bitmap image formatand video codecExtended toAPNG, JNG and MNGStandard(s)ISO/IEC 15948[1], IETF RFC 2083Open format?YesPortable Network Graphics (PNG, pronounced /ˈpɪŋ/[2] ping) is a bitmapped image format andvideo codec that employs lossless data compression. PNG was created to improve upon and replace GIF (Graphics Interchange Format) as an image-file format not requiring a patentlicense. The PNG acronym is optionally recursive, unofficially standing for PNG's Not GIF.[2]

PNG supports palette-based images (with palettes of 24-bit RGB

PNG files nearly always use file extension "PNG" or "png" and are assigned MIME

Contents

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∙\t1 History and development

∙\t2 PNG Working Group

∙\t3 Technical details

o\t3.1 File header

o\t3.2 "Chunks" within the file

▪\t3.2.1 Critical chunks

▪\t3.2.2 Ancillary chunks

o\t3.3 Color depth

o\t3.4 Transparency of image

o\t3.5 Compression

▪\t3.5.1 Filtering

o\t3.6 Interlacing

o\t3.7 Animation

∙\t4 Comparison to other file formats

o\t4.1 Comparison to Graphics Interchange Format (GIF)

o\t4.2 Comparison to JPEG

o\t4.3 Comparison to JPEG-LS

o\t4.4 Comparison to TIFF

∙\t5 Software support

o\t5.1 Bitmap graphics editor support for PNG

o\t5.2 Web browser support for PNG

o\t5.3 Operating system support for PNG icons

∙\t6 File size and optimization software

o\t6.1 Compared to GIF

o\t6.2 File size factors

o\t6.3 Image editing software

o\t6.4 Optimizing tools

▪\t6.4.1 Ancillary chunk removal

▪\t6.4.2 Filter optimization

▪\t6.4.3 DEFLATE optimization

▪\t6.4.4 Wrapper tools

o\t6.5 Icon optimization

∙\t7 See also

∙\t8 References

∙\t9 Further reading

∙\t10 External links

o\t10.1 libpng.org

o\t10.2 W3C

o\t10.3 Others[edit]History and development

See also: Graphics Interchange Format#Unisys and LZW patent enforcement

The motivation for creating the PNG format was in early 1995, after it became known that the Lempel–Ziv–Welch

A January 1995 precursory discussion thread, on the usenet newsgroup

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The original PNG specification was authored by an ad-hoc group of computer graphics experts and enthusiasts. Discussions and decisions about the format were done exclusively via email. The original authors listed on RFC 2083

Editor: Thomas Boutell

Authors' names in alphabetical order: Mark Adler

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A PNG file starts with an 8-byte; the decimal values are 137 80 78 71 13 10 26 10. Each of the header bytes is there for a specific reason:[6]

After the header comes a series of chunks, each of which conveys certain information about the image. Chunks declare themselves as criticalor 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

A chunk consists of four parts: length (4 bytes), chunk type/name (4 bytes), chunk data (length bytes) and CRC

The case of the first letter indicates if 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 if the chunk is "public" (either in the specification or the registry of special purpose public chunks) or "private" (not standardised). 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 unrecognised chunk.

The case of the fourth letter indicates if a 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.

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A decoder must be able to interpret these to read and render a PNG file.

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The PLTE chunk is essential for color type 3 (indexed color). It is optional for color types 2 and 6 (truecolor and truecolor with alpha) and it must not appear for color types 0 and 4 (grayscale and grayscale with alpha).

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Other image attributes that can be stored in PNG files include gamma

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The lowercase first letter in these chunks indicates that they are not needed for the PNG specification. The lowercase last letter in some chunks indicates that they are safe to copy, even if the application concerned does not understand them.

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PNG color options[10]Bits per pixelBits per channelColor optionChannels124816Indexed11248Grayscale1124816Grayscale & alpha21632Truecolor32448Truecolor & alpha432PNG images can either use palette-indexed color or be made up of one or more channels (numerical values directly representing quantities about the pixels). When there is more than one channel in an image all channels have the same number of bits allocated per pixel (known as the bit depth of the channel). Although the PNG specification always talks about the bit depth of channels, most software and users generally talk about the total number of bits per pixel (sometimes also referred to as bit depth or color depth). Since multiple channels can affect a single pixel, the number of bits per pixel is often higher than the number of bits per channel, as shown in the illustration at right.

The number of channels will depend on whether the image is grayscale or color and whether it has an alpha channel

The color type is specified in the color type field, which is a bit field

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With indexed color images, the palette is always stored in RGB at a depth of 8 bits per channel (24 bits per palette entry). Additionally, an optional array of 8-bit alpha values of the palette entries may be included. The palette must not have more entries than the image bit depth allows for, but it may have fewer (for example, if an image only uses 90 colors then it does not need palette entries for all 256 colors).

Indexed color PNGs are allowed to have 1, 2, 4 or 8 bits per pixel by the standard; grayscale images with no alpha channel allow for 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.

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PNG offers a variety of transparency options. With truecolor and grayscale images either a single pixel value can be declared as transparent or an alpha channel

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 they must to be compliant with the specification) but only output at 8 bits per channel (the norm for all but the highest end systems).

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PNG uses a 2-stage compression process:

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PNG uses a non-patented lossless data compression

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PNG's filter method 0 can use the data in pixels A, B, and C to predict the value for X.

Before DEFLATE is applied, the data is precompressed, via a prediction method: a single filter methodis used for the entire image, while for each image line, a filter type is chosen that transforms the data so that it is hopefully more easily compressed.[11]

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

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 (A), above (B), above and to the left (C) 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:[12]

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

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.

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An illustration of Adam7 interlacing over a 16×16 image.

PNG offers an optional 2-dimensional, 7-pass interlacing

However, the 7-pass scheme tends to reduce the data's compressibility more than simpler schemes.

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PNG itself does not support animation at all. MNG

The complexity of MNG led to the proposal of APNG

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▪ When an alpha channel is added, up to bits per pixel (before compression) are possible.

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Composite image comparing JPEG and PNG: notice artifacts in JPEG versus solid PNG background.

JPEG

Also, JPEG suffers from generation loss

JPEG has historically been the format of choice for exporting images containing gradients, as it could handle the color depth much better than the GIF format. However, any compression by the JPEG would cause the gradient to become blurry, but a 24-bit PNG export of a gradient image often comes out identical to the source image, and at a small file size. As such, the PNG format is the optimal choice for exporting small, repeating gradients for web usage.

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JPEG-LS

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Tagged Image File Format

The most common general-purpose, lossless compression algorithm used with TIFF is Lempel–Ziv–Welch

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Main article: Comparison of raster graphics editors

Popular graphics programs which support the PNG format include Adobe Photoshop, Corel's Photo-Paint and Paint Shop Pro, the GIMP,GraphicConverter, Helicon Filter, Inkscape, IrfanView, Konvertor, Universal Converter, Pixel image editor, Paint.NET and Xara. Some programs bundled with popular operating systems which support PNG include Microsoft's Paint and Apple's 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 meta data 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 meta data for use on web pages, etc.[23]

Some image processing programs have PNG compression problems, mainly related to lack of full implementation of the PNG compressor library.[citation needed

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Adobe's Fireworks is sometimes placed in this category, but its difficulties are less severe than the other entries. The confusion stems from a misunderstanding of the mechanics of its Save format: though PNGs, the intermediate images produced by that option include large, private chunks containing complete layer and vector information, which allows further, lossless editing. Properly saved with the Exportoption, 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.

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Main article: Web browser image format support

PNG support first appeared in Internet Explorer

Despite calls by the Free Software Foundationcitation needed]

GIF

▪\tNo support on old browsers (such as Internet Explorer

▪\tNo animation, still images only (unlike GIF, though Mozilla's unofficial APNG

PNG compatible browsers include: Apple Safari

However, Internet Explorer

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PNG icons have been supported in most distributions of Linux

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PNG file size can vary significantly depending on how it is encoded and compressed; this is discussed and a number of tips are given inPNG: The Definitive Guide.[22]

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Compared to GIF files, a PNG file with the same information (256 colors, no ancillary chunks/metadata), compressed by a good compressor will often be smaller than GIF, though lack of agreed data sets make any controlled comparisons subjective.[22] 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.[22]

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PNG files vary in size due to a number of factors:

color depth

Color depth can range from 1 to bits per pixel.

ancillary chunks

PNG supports much 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.[22]

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.[clarification needed

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 one will choose different trade-offs: 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.[22]

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Image editing software varies in its treatment of PNGs.

Because GIF is de facto limited to 256 colors

Some versions of Adobe Photoshop

Adobe Photoshop

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Various tools are available for optimizing PNG files; they do this by:

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As some tools are PNG-specific, while others only optimize DEFLATE, in general one must use a combination of 2 tools in sequence for optimal compression: one which optimizes filters (and removes ancillary chunks), and one which optimizes DEFLATE. Most commonly, OptiPNG is used for the first (non-DEFLATE) step, and either of AdvanceCOMP or PNGOUT is used for the DEFLATE step.

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For removing ancillary chunks, pngcrush

pngcrush -rem gAMA -rem cHRM -rem iCCP -rem sRGB InputFile.png OutputFile.png

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For optimizing filters, OptiPNG and pngcrush

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AdvanceCOMP

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Wrapper tools that simplify this workflow include: ImageOptim

The littleutils

The current version of IrfanView

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Since icons

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▪\tComparison of graphics file formats

▪\tComputer graphics

▪\tComparison of layout engines (graphics)

▪\tImage editing

▪\tImage file formats

▪\tlibpng

▪\tRelated graphics file formats

▪\tAPNG

▪\tJPEG Network Graphics

▪\tMultiple-image Network Graphics

▪\tSimilar file formats

▪\tGraphics Interchange Format

▪\tX PixMap

▪\t.svg

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1.\t^ a b c "ISO/IEC 15948:2004 - Information technology -- Computer graphics and image processing -- Portable Network Graphics (PNG): Functional specification". Retrieved 2011-02-19.

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▪\tRoelofs, Greg (April 1997). "Linux Gazette: History of the Portable Network Graphics (PNG) Format". Linux Journal (Specialized Systems Consultants, Inc.) 1997 (36es). ISSN 1075-3583.

▪\tRoelofs, Greg (2003). PNG: The Definitive Guide (2nd ed.). O'Reilly Media. ISBN 15659224.

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▪\tPNG Home Site

▪\tlibpng Home Page

▪\tThe Story of PNG by Greg Roelofs

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▪\tPNG Specification (Latest Edition)

▪\tTest inline PNG images

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▪\tAn introduction to the PNG image format — Including test images, file editing tips, and reviews of PNG image tools for Windows.

▪\tRFC 2083

▪\tPNG transparency test

▪\t"The Lonely Planet" — PNG-based animation for web browsers

▪\tMore information about PNG color correction

▪\tThe GD-library to generate dynamic PNG-files with PHP

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