EMF解析：矢量图元结构探索

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本文档是关于Enhanced Metafile Format (EMF)的解析，一种专用于Windows操作系统中的矢量图形文件格式。EMF是由微软公司开发并维护的一种标准，其版本为2010年11月12日更新。该格式主要用于高效地存储和绘制复杂的图形，特别是在需要缩放和打印质量保持一致的情况下，如在打印预览或打印时。在MS-EMF文档中，知识产权声明指出，Microsoft发布这些开放规格文档的目的是为了技术交流和实施，允许开发者复制文档以实现所描述的技术，并在实施过程中分发部分文档，只要是为了正确展示技术实现。开发者可以在自己的文档中包含随文档提供的任何模式、接口定义语言（IDL）或代码示例，无需额外许可。引用文档的行为也受到此许可的覆盖。 EMF文件的核心结构包括对图元（图形元素）的详细描述，这些图元可能包括线条、曲线、矩形、椭圆、文本等基本图形元素，以及更复杂的对象，如图表、图像和矢量图形。解析EMF文件涉及解析这些图元的数据结构，理解它们的位置、颜色、大小、样式和透明度等属性，以便在应用程序中重现原始图形。解析过程通常涉及到以下几个步骤： 1. **头信息读取**：获取文件的标识符、版本信息和版权声明。 2. **图层和颜色表管理**：理解图层组织以及颜色表的使用，这对于处理不同颜色和透明度至关重要。 3. **几何对象描述**：解析描述线条、曲线、填充区域等几何形状的元数据。 4. **文本处理**：提取文本字符串及其相关的字体、大小、颜色等属性。 5. **图形状态**：识别当前绘图模式、混合模式、透明度等设置。 6. **群组和复合对象**：理解嵌套的对象组合，可能涉及路径合并或位图剪切蒙版。 7. **事件序列**：解析命令序列，了解图形对象的创建和变换操作。在实际应用中，程序员和图形处理库通常会使用特定的API（如Windows API中的GDI+或DirectX）来解析和操作EMF文件，以便在各种应用场景中使用，如打印预览、图形编辑软件、文档转换工具等。对于开发者来说，理解和熟练掌握EMF解析方法是实现高效、高质量矢量图形处理的关键。

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1.2 References

1.2.1 Normative References

We conduct frequent surveys of the normative references to assure their continued availability. If

you have any issue with finding a normative reference, please contact dochelp@microsoft.com. We

will assist you in finding the relevant information. Please check the archive site,

http://msdn2.microsoft.com/en-us/library/E4BD6494-06AD-4aed-9823-445E921C9624, as an

additional source.

[ISO/IEC-8859-1] International Organization for Standardization, "Information Technology -- 8-Bit

Single-Byte Coded Graphic Character Sets -- Part 1: Latin Alphabet No. 1", ISO/IEC 8859-1, 1998,

http://www.iso.org/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=28245&ICS1=35&ICS

2=40&ICS3=

Note There is a charge to download the specification.

[JFIF] Hamilton, E., "JPEG File Interchange Format, Version 1.02", September 1992,

http://www.w3.org/Graphics/JPEG/jfif.txt

[MS-WMF] Microsoft Corporation, "Windows Metafile Format", June 2007.

[RFC2083] Boutell, T., "PNG (Portable Network Graphics) Specification Version 1.0", RFC 2083,

March 1997, http://www.ietf.org/rfc/rfc2083.txt

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC

2119, March 1997, http://www.ietf.org/rfc/rfc2119.txt

[UNICODE] The Unicode Consortium, "Unicode Home Page", 2006, http://www.unicode.org/

1.2.2 Informative References

[MS-EMFPLUS] Microsoft Corporation, "Enhanced Metafile Format Plus Extensions", June 2007.

[MS-EMFSPOOL] Microsoft Corporation, "Enhanced Metafile Spool Format", June 2007.

[MS-GLOS] Microsoft Corporation, "Windows Protocols Master Glossary", March 2007.

[MSDN-GDI+] Microsoft Corporation, "GDI+", http://msdn.microsoft.com/en-

us/library/ms533798.aspx

[MSDN-WRLDPGSPC] Microsoft Corporation, "World-Space to Page-Space Transformations",

http://msdn.microsoft.com/en-us/library/ms532657.aspx

[OPENGL] Segal, M. and Akeley, K., "The OpenGL Graphics System: A Specification, Version 2.1",

December 2006, http://www.opengl.org/registry/doc/glspec21.20061201.pdf

1.3 Overview

1.3.1 Metafile Structure

The EMF specifies a collection of data records that contain graphics drawing commands and object

definitions. EMF metafiles provide true device independence; the image defined in an EMF metafile

maintains its dimensions, shape, and proportions on any output device, including printers, plotters,

desktops, and in the client areas of many applications.

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Figure 1: EMF metafile structures

Thus, all EMF metafiles can be considered to have three sections:

EMF Header: The metafile header with extensions that correspond to this type of EMF metafile.

The header record contains information concerning the structure and contents of the metafile,

including an optional description string and pixel format descriptor. See section 2.3.4.2 for details

concerning the EMF header.

EMF Records: An array of EMF records that contains drawing orders, graphics state information,

and graphics object definitions. At least one record has to be present—not counting the EMF

header or EMF end-of-file records—or the metafile is invalid. See sections 2.3.10 and 2.3.5 for

specifications of all EMF record types.

EMF End-of-File: The EMF end-of-file record signals the end of all EMF metafiles. It has to be

the last record in the metafile. If the EMF metafile contains an optional palette, it is in the form

of an array of LogPaletteEntry (section 2.2.18) objects located in the EMF end-of-file record.

Offsets to the palette are present in both the EMF header and EMF end-of-file records.

Within the portions of these files identified as EMF header records, the description and pixel format

substructures are optional, and they can be placed in any order. Their presence, location, and

relative order in a given EMF file are determined by offset values in the EMF header record. If

present, they are part of the EMF header record, and cannot be located at some arbitrary location in

the EMF metafile, such as between EMF records.

EMF records are contiguous; this is required, because the information that is available for traversing

the file from record to record depends on it. That is, from any given EMF record, including the

header record, in order to move to the next sequential record in the file, the length of the record is

used.

1.3.2 Graphics Objects

Graphics objects, which are used in the drawing and painting operations specified in the records of

an EMF metafile, are created by Object Creation record types, specified in section 2.3.7, prior to the

records that specify their use. These objects are designed to be reusable during the course of

processing the EMF metafile.

Throughout this specification, it is assumed that these previously defined, reusable graphics objects

are available when needed for the processing of particular metafile records. This store of available

objects is referred to in the text as the EMF Object Table, which is described in section 3.1.1.1. The

exact characteristics of an object store for EMF objects can be determined by the particular

implementation that parses or writes EMF metafiles.

The types of reusable objects that can be created and managed during EMF metafile playback

include:

Brushes, specified in section 2.2.12

Color spaces, specified in [MS-WMF] sections 2.2.2.11 and 2.2.2.12

Fonts, specified in sections 2.2.13, 2.2.14, 2.2.15, and 2.2.16

Palettes, specified in section 2.2.17

Pens, specified in sections 2.2.19 and 2.2.20

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When one of these objects is created, it is assigned a 32-bit index in the EMF Object Table. An

object can be "activated" by an EMR_SELECTOBJECT record that refers to the index it was assigned.

There are also stock objects that can be selected for use in graphics operations. Stock objects are

also assigned 32-bit indexes, but not at run time—their indexes are predefined. They are

distinguished from the indexes of dynamically created graphics objects by having the most-

significant bit set to 1. The other 31 bits of the index define the particular stock object, according to

the StockObject enumeration, specified in section 2.1.31.

1.3.3 Byte Ordering

Data in metafile records is stored in little-endian format.

Some computer architectures number bytes in a binary word from left to right, which is referred to

as big-endian. The byte numbering used for bitfields in this specification is big-endian. Other

architectures number the bytes in a binary word from right to left, which is referred to as little-

endian. The byte numbering used for enumerations, objects, and records in this specification is

little-endian.

Using the big-endian and little-endian methods, the number 0x12345678 would be stored as shown

in the following table.

Byte order

Byte 0

Byte 1

Byte 2

Byte 3

Big-endian

0x12

0x34

0x56

0x78

Little-endian

0x78

0x56

0x34

0x12

1.4 Relationship to Protocols and Other Structures

EMF is related to the following file formats:

Windows metafile format (WMF) [MS-WMF] uses similar graphics, commands, and objects,

but WMF files are not device-independent.<5>

Enhanced metafile spool format (EMFSPOOL) [MS-EMFSPOOL] is an application of EMF for

print job spooling.

Enhanced metafile format plus extensions (EMF+) [MS-EMFPLUS] specifies object-oriented

structures that can be embedded in EMF metafiles.<6>

1.5 Applicability Statement

Files that adhere to the EMF metafile format can be used as portable, device-independent containers

for images. The graphics supported in EMF metafiles are applicable to document content

representation, including printing and plotting.

1.6 Versioning and Localization

This specification covers versioning issues in the following areas:

Versioning: The EMF structure has been revised twice. The different versions are:

Original: The first version of the EMF structure, supporting records that define drawing

commands and graphics objects.

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