"PNG-v1.2 英文原版规范定义：便携式网络图像无损压缩格式"

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PNG-v1.2 英文原版规范定义了便携式网络图像无损压缩格式（Portable Network Graphics）的详细规范，内部包含了文件头信息和zlib压缩算法。该规范是PNG Development Group发布的版本，初始发布于1999年7月。此书的作者是G. Randers-Pehrson等人，详细的作者列表可以在Credits（第19章）中查看。该文档的地位是PNG 1.0规范的修订版，PNG 1.0规范已经作为RFC-2083和W3C推荐标准发布。这一修订版由PNG Development Group发布，但尚未得到任何标准机构的批准。PNG规范处于ISO/IEC JTC 1 SC 24的监督下，并正在标准化轨道上。 PNG-v1.2规范涵盖了与这一无损压缩格式相关的各个方面，包括文件结构、颜色空间、图像数据、IDAT块格式、物理像素尺寸等。其中还介绍了用于向后兼容的新标记、剖析和存储透明像素等内容，以及响应浏览器和用户代理的建议。这一规范的发布旨在为开发者和工程师提供一个权威的参考，帮助他们设计、开发和实现符合PNG格式的图像处理和应用。该规范也为标准化机构和技术社区提供了一个框架，使得PNG格式能够得到更广泛的推广和应用。 PNG-v1.2规范的内容非常详细，并对PNG格式的各个方面进行了深入的描述和解释。无损压缩算法的实现细节、文件头信息的结构、颜色空间的定义等内容都有详细的说明，使得开发者在实际应用中能够更好地理解和使用PNG格式。该规范还包括了PNG格式的发展历程和版本演变，让读者可以清晰地了解到PNG格式的发展脉络和技术更新。同时，该规范对于以后的版本升级和扩展也提供了一定的参考和指导，使得PNG格式能够不断地得到完善和改进。总的来说，PNG-v1.2规范是一个权威且全面的文档，对于了解和应用PNG格式具有重要的参考价值。它不仅为开发者提供了技术细节和实现指南，也为标准化机构和技术社区提供了一个框架，使PNG格式能够得到更广泛的推广和应用。通过不断地完善和更新，PNG格式将继续成为网络图像处理和应用领域的重要标准和技术。

4. CHUNK SPECIFICATIONS

3.4 CRC algorithm

Chunk CRCs are calculated using standard CRC methods with pre and post conditioning, as deﬁned by ISO

3309 [ISO-3309] or ITU-T V.42 [ITU-T-V42]. The CRC polynomial employed is

+x+1

The 32-bit CRC register is initialized to all 1’s, and then the data from each byte is processed from the least

signiﬁcant bit (1) to the most signiﬁcant bit (128). After all the data bytes are processed, the CRC register

is inverted (its ones complement is taken). This value is transmitted (stored in the ﬁle) MSB ﬁrst. For the

purpose of separating into bytes and ordering, the least signiﬁcant bit of the 32-bit CRC is deﬁned to be the

coefﬁcient of the x

term.

Practical calculation of the CRC always employs a precalculated table to greatly accelerate the computation.

See Sample CRC Code (Chapter 15).

4 Chunk Speciﬁcations

This chapter deﬁnes the standard types of PNG chunks.

4.1 Critical chunks

All implementations must understand and successfully render the standard critical chunks. A valid PNG

image must contain an IHDR chunk, one or more IDAT chunks, and an IEND chunk.

4.1.1 IHDR Image header

The IHDR chunk must appear FIRST. It contains:

Width: 4 bytes

Height: 4 bytes

Bit depth: 1 byte

Color type: 1 byte

Compression method: 1 byte

Filter method: 1 byte

Interlace method: 1 byte

Width and heightgivethe imagedimensions in pixels. They are 4-byteintegers. Zero is an invalidvalue. The

maximum for each is 2

− 1 in order to accommodate languages that have difﬁculty with unsigned 4-byte

values.

Bit depth is a single-byte integer giving the number of bits per sample or per palette index (not per pixel).

Valid values are 1, 2, 4, 8, and 16, although not all values are allowed for all color types.

PNG (PORTABLE NETWORK GRAPHICS) SPECIFICATION

Color type is a single-byte integerthat describes the interpretationof the image data. Color type codes repre-

sent sums of the following values: 1 (palette used), 2 (color used), and 4 (alpha channel used). Valid values

are0,2,3,4,and6.

Bit depth restrictions for each color type are imposed to simplify implementations and to prohibit combina-

tions that do not compress well. Decoders must support all valid combinations of bit depth and color type.

The allowed combinations are:

Color Allowed Interpretation

Type Bit Depths

0 1,2,4,8,16 Each pixel is a grayscale sample.

2 8,16 Each pixel is an R,G,B triple.

3 1,2,4,8 Each pixel is a palette index;

a PLTE chunk must appear.

4 8,16 Each pixel is a grayscale sample,

followed by an alpha sample.

6 8,16 Each pixel is an R,G,B triple,

followed by an alpha sample.

The sample depth is the same as the bit depth except in the case of color type 3, in which the sample depth

is always 8 bits.

Compression method is a single-byte integer that indicates the method used to compress the image data. At

present, only compression method 0 (deﬂate/inﬂate compression with a sliding window of at most 32768

bytes) is deﬁned. All standard PNG images must be compressed with this scheme. The compression method

ﬁeld is provided for possible future expansion or proprietary variants. Decoders must check this byte and

report an error if it holds an unrecognized code. See Deﬂate/Inﬂate Compression (Chapter 5) for details.

Filter method is a single-byte integer that indicates the preprocessing method applied to the image data be-

fore compression. At present, only ﬁlter method 0 (adaptive ﬁltering with ﬁve basic ﬁlter types) is deﬁned.

As with the compression method ﬁeld, decoders must check this byte and report an error if it holds an unrec-

ognized code. See Filter Algorithms (Chapter 6) for details.

Interlace method is a single-byte integer that indicates the transmissionorder of the image data. Two values

are currently deﬁned: 0 (no interlace) or 1 (Adam7 interlace). See Interlaced data order (Section 2.6) for

details.

4.1.2 PLTE Palette

The PLTE chunk contains from 1 to 256 palette entries, each a three-byte series of the form:

4. CHUNK SPECIFICATIONS

Red: 1 byte (0 = black, 255 = red)

Green: 1 byte (0 = black, 255 = green)

Blue: 1 byte (0 = black, 255 = blue)

The number of entries is determined from the chunk length. A chunk length not divisible by 3 is an error.

This chunk must appear for color type 3, and can appear for color types 2 and 6; it must not appear for color

types 0 and 4. If this chunk does appear, it must precede the ﬁrst IDAT chunk. There must not be more than

one PLTE chunk.

For color type 3 (indexed color), the PLTE chunk is required. The ﬁrst entry in PLTE is referenced by pixel

value 0, the second by pixel value 1, etc. The number of palette entries must not exceed the range that can

be represented in the image bit depth (for example, 2

=16for a bit depth of 4). It is permissible to have

fewer entries than the bit depth would allow. In that case, any out-of-range pixel value found in the image

data is an error.

Forcolortypes2 and 6(truecolorand truecolorwithalpha), the PLTE chunk is optional. Ifpresent,it provides

a suggested set of from 1 to 256 colors to which the truecolor image can be quantized if the viewer cannot

display truecolor directly. If neither PLTE nor sPLT is present, such a viewer will need to select colors on its

own, but it is often preferable for this to be done once by the encoder. (See Recommendations for Encoders:

Suggested palettes, Section 9.5.)

Note that the palette uses 8 bits (1 byte) per sample regardless of the image bit depth speciﬁcation. In par-

ticular, the palette is 8 bits deep even when it is a suggested quantization of a 16-bit truecolor image.

There is no requirement that the palette entries all be used by the image, nor that they all be different.

4.1.3 IDAT Image data

The IDAT chunk contains the actual image data. To create this data:

1. Begin with image scanlinesrepresentedas describedin Image layout(Section 2.3); the layout andtotal

size of this raw data are determined by the ﬁelds of IHDR.

2. Filter the image data according to the ﬁltering method speciﬁed by the IHDR chunk. (Note that with

ﬁlter method 0, the only one currently deﬁned, this implies prepending a ﬁlter-type byte to each scan-

line.)

3. Compress the ﬁltered data using the compression method speciﬁed by the IHDR chunk.

The IDAT chunk contains the output datastream of the compression algorithm.

To read the image data, reverse this process.

There can be multiple IDAT chunks; if so, they must appear consecutivelywith no other intervening chunks.

The compressed datastream is thenthe concatenation of thecontentsof all the IDAT chunks. The encoder can

divide the compressed datastream into IDAT chunks however it wishes. (Multiple IDAT chunks are allowed

4. CHUNK SPECIFICATIONS

Each entry indicates that pixels of the corresponding palette index must be treated as having the speciﬁed

alphavalue. Alpha valueshave the same interpretationas in an 8-bit full alpha channel: 0 is fully transparent,

255 is fully opaque, regardless of image bit depth. The tRNS chunk must not contain more alpha values than

there are palette entries, but tRNS can contain fewer values than there are palette entries. In this case, the

alpha value for all remaining palette entries is assumed to be 255. In the common case in which only palette

index 0 need be made transparent, only a one-byte tRNS chunk is needed.

For color type 0 (grayscale), the tRNS chunk contains a single gray level value, stored in the format:

Gray: 2 bytes, range 0..2

bitdepth

− 1

(If the image bit depth is less than 16, the least signiﬁcant bits are used and the others are 0.) Pixels of the

speciﬁed gray level are to be treated as transparent (equivalent to alpha value 0); all other pixels are to be

treated as fully opaque (alpha value 2

bitdepth

− 1).

For color type 2 (truecolor), the tRNS chunk contains a single RGB color value, stored in the format:

Red: 2 bytes, range 0..2

bitdepth

− 1

Green: 2 bytes, range 0..2

bitdepth

− 1

Blue: 2 bytes, range 0..2

bitdepth

− 1

(If the image bit depth is less than 16, the least signiﬁcant bits are used and the others are 0.) Pixels of the

speciﬁed color value are to be treated as transparent (equivalent to alpha value 0); all other pixels are to be

treated as fully opaque (alpha value 2

bitdepth

− 1).

tRNS is prohibited for color types 4 and 6, since a full alpha channel is already present in those cases.

Note: whendealingwith16-bitgrayscale ortruecolordata,itisimportantto compare bothbytesofthe sample

values to determine whether a pixel is transparent. Although decoders may drop the low-order byte of the

samples for display, this must not occur until after the data has been tested for transparency. For example, if

the grayscale level 0x0001is speciﬁed to be transparent, it wouldbe incorrect to compare only the high-order

byte and decide that 0x0002 is also transparent.

When present, the tRNS chunk must precede the ﬁrst IDAT chunk, and must follow the PLTE chunk, if any.

4.2.2 Color space information

These chunks relate the image samples to the desired display intensity.

4.2.2.1 gAMA Image gamma

The gAMA chunk speciﬁes the relationship between the image samples and the desired display output in-

tensity as a power function:

sample = light

out

gamma

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