PNG规范第二版：无损压缩图像格式

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"Portable Network Graphics (PNG) 是一种用于无损、便携且高效压缩栅格图像的可扩展文件格式。PNG 可以替代 GIF，并且在许多情况下可以替代 TIFF。它支持索引颜色、灰度和真彩色图像，还可以选择性地包含一个alpha通道。位深度范围从1到16位。此规范的第二版由W3C推荐，并由ISO/IEC15948:2003(E)标准化。" PNG（Portable Network Graphics）规范的详细说明： PNG 文件格式设计的目的是提供一个不受专利限制的解决方案，以替代当时受到专利限制的 GIF 图像格式，同时还能在某些场景下替代功能更为复杂的 TIFF 格式。PNG 支持以下特性： 1. **无损压缩**：PNG 使用了一种无损压缩算法，这意味着图像在压缩后解压时，不会出现任何质量损失，原始数据可以完全恢复。 2. **颜色模式**：PNG 支持多种颜色模式，包括： - **索引颜色**：使用调色板存储有限数量的颜色，适合颜色深度较小的图像。 - **灰度**：仅包含不同深浅的灰色，适用于黑白或单色图像。 - **真彩色**：可以表示超过1600万种颜色，类似于RGB格式。 3. **alpha通道**：PNG 还允许包含一个透明度通道，使得图像元素可以具有不同程度的透明度，从而实现半透明效果。 4. **位深度**：PNG 图像的位深度可变，从1位到16位不等，这使得它可以适应各种不同的应用场景，包括低彩色需求和高动态范围图像。 5. **块布局**：PNG 文件由一系列称为“块”（Chunks）的数据结构组成，每个块都有自己的类型和数据，这使得PNG能够灵活地扩展和存储额外的信息，如元数据、颜色空间信息和图像数据。 6. **国际标准**：PNG 格式被国际标准化组织（ISO）和国际电工委员会（IEC）联合制定了ISO/IEC15948标准，确保其在全球范围内的广泛接受和兼容性。 7. **W3C推荐**：PNG 规范是W3C（万维网联盟）的一项推荐标准，这意味着它在Web开发中得到了广泛支持和使用。 8. **错误校正**：PNG 文件结构包含了一些机制来检测和处理潜在的错误，增加了文件的可靠性。 PNG 的这些特性使其成为互联网上常用的图像格式之一，特别是在需要高质量、透明效果或者无损压缩的情况下。无论是网页设计、软件界面还是图形设计，PNG 都是一个不可或缺的工具。

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2016/3/19 Portable Network Graphics (PNG) Specification (Second Edition)

https://www.w3.org/TR/PNG/#5Chunk-layout 8/46

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For high-end applications the first method provides the most flexibility and control. The second method enables one particular colour space to be indicated.

The third method enables the exact chromaticities of the RGB data to be specified, along with the gamma correction (the power function relating the

desired display output with the image samples) to be applied (see Annex C: Gamma and chromaticity). It is recommended that explicit gamma information

also be provided when either the first or second method is used, for use by PNG decoders that do not support full ICC profiles or the sRGB colour space.

Such PNG decoders can still make sensible use of gamma information. PNG decoders are strongly encouraged to use this information, plus information

about the display system, in order to present the image to the viewer in a way that reproduces as closely as possible what the image's original author saw

Gamma correction is not applied to the alpha channel, if present. Alpha samples always represent a linear fraction of full opacity.

4.3 Reference image to PNG image transformation

4.3.1 Introduction

A number of transformations are applied to the reference image to create the PNG image to be encoded (see figure 4.3). The transformations are applied in

the following sequence, where square brackets mean the transformation is optional:

[alpha separation]

indexing or ( [RGB merging] [alpha compaction] )

sample depth scaling

When every pixel is either fully transparent or fully opaque, the alpha separation, alpha compaction, and indexing transformations can cause the recovered

reference image to have an alpha sample depth different from the original reference image, or to have no alpha channel. This has no effect on the degree of

opacity of any pixel. The two reference images are considered equivalent, and the transformations are considered lossless. Encoders that nevertheless

wish to preserve the alpha sample depth may elect not to perform transformations that would alter the alpha sample depth.

reference image

alpha separation

RGB merging

alpha compaction

indexing

sample depth scaling

PNG image

Figure 4.3 — Reference image to PNG image transformation

4.3.2 Alpha separation

If all alpha samples in a reference image have the maximum value, then the alpha channel may be omitted, resulting in an equivalent image that can be

encoded more compactly.

4.3.3 Indexing

If the number of distinct pixel values is 256 or less, and the RGB sample depths are not greater than 8, and the alpha channel is absent or exactly 8 bits

deep or every pixel is either fully transparent or fully opaque, then an alternative representation called indexed-colour may be more efficient for encoding.

Each pixel is replaced by an index into a palette. The palette is a list of entries each containing three 8-bit samples (red, green, blue). If an alpha channel

is present, there is also a parallel table of 8-bit alpha samples.

2016/3/19 Portable Network Graphics (PNG) Specification (Second Edition)

https://www.w3.org/TR/PNG/#5Chunk-layout 9/46

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Indexed colour Palette Alpha table

R G B A

Figure 4.4 — Indexed-colour image

A suggested palette or palettes may be constructed even when the PNG image is not indexed-colour in order to assist viewers that are capable of

displaying only a limited number of colours.

For indexed-colour images, encoders can rearrange the palette so that the table entries with the maximum alpha value are grouped at the end. In this case

the table can be encoded in a shortened form that does not include these entries.

4.3.4 RGB merging

If the red, green, and blue channels have the same sample depth, and for each pixel the values of the red, green, and blue samples are equal, then these

three channels may be merged into a single greyscale channel.

4.3.5 Alpha compaction

For non-indexed images, if there exists an RGB (or greyscale) value such that all pixels with that value are fully transparent while all other pixels are fully

opaque, then the alpha channel can be represented more compactly by merely identifying the RGB (or greyscale) value that is transparent.

4.3.6 Sample depth scaling

In the PNG image, not all sample depths are supported (see 6.1: Colour types and values), and all channels shall have the same sample depth. All

channels of the PNG image use the smallest allowable sample depth that is not less than any sample depth in the reference image, and the possible

sample values in the reference image are linearly mapped into the next allowable range for the PNG image. Figure 4.5 shows how samples of depth 3

might be mapped into samples of depth 4.

Figure 4.5 — Scaling sample values

Allowing only a few sample depths reduces the number of cases that decoders have to cope with. Sample depth scaling is reversible with no loss of data,

because the reference image sample depths can be recorded in the PNG datastream. In the absence of recorded sample depths, the reference image

sample depth equals the PNG image sample depth. See 12.5: Sample depth scaling and 13.12: Sample depth rescaling.

2016/3/19 Portable Network Graphics (PNG) Specification (Second Edition)

https://www.w3.org/TR/PNG/#5Chunk-layout 10/46

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Truecolour with alpha

Greyscale with alpha

Truecolour

Greyscale

Indexed-colour

Palette Alpha table

Figure 4.6 — Possible PNG image pixel types

4.4 PNG image

The transformation of the reference image results in one of five types of PNG image (see figure 4.6) :

a. Truecolour with alpha: each pixel consists of four samples: red, green, blue, and alpha.

b. Greyscale with alpha: each pixel consists of two samples: grey and alpha.

c. Truecolour: each pixel consists of three samples: red, green, and blue. The alpha channel may be represented by a single pixel value. Matching

pixels are fully transparent, and all others are fully opaque. If the alpha channel is not represented in this way, all pixels are fully opaque.

d. Greyscale: each pixel consists of a single sample: grey. The alpha channel may be represented by a single pixel value as in the previous case. If

the alpha channel is not represented in this way, all pixels are fully opaque.

e. Indexed-colour: each pixel consists of an index into a palette (and into an associated table of alpha values, if present).

The format of each pixel depends on the PNG image type and the bit depth. For PNG image types other than indexed-colour, the bit depth specifies the

number of bits per sample, not the total number of bits per pixel. For indexed-colour images, the bit depth specifies the number of bits in each palette

index, not the sample depth of the colours in the palette or alpha table. Within the pixel the samples appear in the following order, depending on the PNG

image type.

a. Truecolour with alpha: red, green, blue, alpha.

b. Greyscale with alpha: grey, alpha.

c. Truecolour: red, green, blue.

d. Greyscale: grey.

e. Indexed-colour: palette index.

4.5 Encoding the PNG image

4.5.1 Introduction

A conceptual model of the process of encoding a PNG image is given in figure 4.7. The steps refer to the operations on the array of pixels or indices in the

PNG image. The palette and alpha table are not encoded in this way.

a. Pass extraction: to allow for progressive display, the PNG image pixels can be rearranged to form several smaller images called reduced images or

passes.

b. Scanline serialization: the image is serialized a scanline at a time. Pixels are ordered left to right in a scanline and scanlines are ordered top to

bottom.

c. Filtering: each scanline is transformed into a filtered scanline using one of the defined filter types to prepare the scanline for image compression.

d. Compression: occurs on all the filtered scanlines in the image.

e. Chunking: the compressed image is divided into conveniently sized chunks. An error detection code is added to each chunk.

f. Datastream construction: the chunks are inserted into the datastream.

4.5.2 Pass extraction

Pass extraction (see figure 4.8) splits a PNG image into a sequence of reduced images where the first image defines a coarse view and subsequent

images enhance this coarse view until the last image completes the PNG image. The set of reduced images is also called an interlaced PNG image. Two

interlace methods are defined in this International Standard. The first method is a null method; pixels are stored sequentially from left to right and scanlines