ACIS SAT文件格式详解：文本与二进制解析

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"ACIS（Advanced Computer Aided Interference System）是一种强大的三维几何建模内核，它能够将建模信息存储在外部文件中，这些文件被称为保存文件，通常有两种类型：SAT（Standard ACIS Text）和SAB（Standard ACIS Binary）。SAT文件是可读的ASCII文本格式，而SAB文件则是二进制格式，两者包含的模型数据信息是相同的，只是表示形式不同。" ACIS的SAT文件格式是其数据交换的核心部分，允许与非基于ACIS的应用程序进行交互。这种开放的文件格式使得外部应用能够访问ACIS的几何模型数据，增强了软件之间的兼容性和数据交换能力。 SATSaveFileFormat章节主要讨论了保存和恢复操作。要理解ACIS文件格式，需要关注以下几个关键点： 1. **文件结构**：SAT文件由一系列有序的数据块组成，每个块包含了特定类型的几何信息或元数据。文件的结构通常是固定的，包括文件头、实体定义、属性信息等。 2. **数据封装**：ACIS使用特定的语法和标记来封装几何数据，例如，实体的边界表示法（B-rep）和拓扑数据。这些数据通过ASCII文本编码，使得它们可以被文本编辑器查看和理解。 3. **数据类型**：SAT文件中记录了多种数据类型，包括基本几何形状（如点、线、面）、复杂的实体（如曲面和体）、以及相关的拓扑信息。这些数据类型定义了模型的几何结构和相互关系。 4. **子类型和引用**：在ACIS中，几何对象可以有多个子类型，每个子类型可能具有不同的属性或行为。此外，文件中的数据通常包含对其他数据的引用，以表示几何对象间的连接，如面的边和顶点。 5. **SAT和SAB的区别**：SAT文件以文本形式存储，易于阅读和调试，但文件大小较大。SAB文件则使用二进制格式，文件体积小，读取速度快，但不便于直接查看。在不需要文本可读性的情况下，通常会优先选择SAB格式以提高效率。 6. **扩展名**：SAT文件通常以`.sat`为扩展名，而SAB文件的扩展名是`.sab`。由于两者存储的数据实质相同，所以当不需要区分格式时，统称为SAT文件。理解ACIS的SAT文件格式对于开发和维护与ACIS兼容的应用程序至关重要，这涉及到读取、解析和重建模型的能力。无论是用于数据导入、导出，还是进行几何运算和分析，熟悉这种格式都能提高软件的性能和用户友好性。

SAT Format

7.0

The save file sometimes strings save identifiers together using a dash (“--”). This often reflects

the ACIS class derivation. For example, the save identifiers “

plane--surface”haveaclass

derivation consisting of

ENTITY, SURFACE,andPLANE. The save identifiers

“

colour--tsl--attrib” have the class derivation ENTITY, ATTRIB, ATTRIB_TSL,and

ATTRIB_COL.

Unique save identifier strings are required for private attributes that a developer may create.

This is achieved by deriving a private base class from

ATTRIB and giving it a unique name.

Developers may use any class name for their private attributes, yet the full identifiers are

unique in the save file.

If the save identifiers are not completely recognized by ACIS, a data structure from just the

recognized classes is constructed and restored. The remaining data at the end of the record is

remembered so that it is not lost by a later save. For unrecognized classes whose derivation is

two or more levels removed from

ENTITY,suchasclassesderivedfromCURVE, SURFACE,

ATTRIB, the minimum is to create a recognizable data structure so that references to the

data structure are correct. For example, if a record from a derived class of

ATTRIB is not

recognized, an

ATTRIB record is created in such a way that the chain of attributes remains

connected for the entity owning the unrecognized attribute.

Character Sets Supported

Topic: *SAT Save and Restore

Beginning with ACIS Release 7.0, the SAT file will delimit strings using an @ sign. This

allows SAT files to contain and support any character set, including Japanese or graphics

characters. For backward compatibility, SAT files with strings that contain SAT delimiters will

be modified when being saved in an earlier version. The SAT delimiters in the string will be

replaced with underscores. This will insure that the file can be read in earlier v ersions of

ACIS.

Entity Encapsulation in a Record

Topic: *SAT Save and Restore

The data for an entity is encapsulated in the order of its derivation from the basic ENTITY,

from left to right. This is in the opposite order of the derivation of the identifier. The data for

ENTITY is written first, followed by the d ata for the class directly derived from ENTITY,

continuing down to the leaf class. For readability in the SAT file, each data field is separated

by white space: a space, carriage return, or new line.

Because it is known that the encapsulation (and derivation) starts from

ENTITY,theENTITY

save identifier is not written. In figure 2-1, this is shown in the middle by the empty double

quotation marks, (“”).

SAT Format

7.0

identifier data

type ntype2type1type n-- type2-- type1

“” data #

...

ENTITY

Figure 2 -1. Format Entity Record

Starting from inside and going out, the ENTITY data is preceded by identifiers and is f ollowed

by the data for the identifiers. The identifiers correspond to classes derived directly from

ENTITY. This type of encapsulation continues for all class derivations until the leaf class is

reached.

In figure 2-1 for a SAT file, the class identifiers are separated from one another with dashes,

while their data fields are separated with white space. The white space is typically a space, but

may also be a new line. The last element of the record is the terminator character (#).

Terminator

Topic: *SAT Save and Restore

The pound sign (#) terminates the entity data in the SAT file. Tag 17 is the terminator in the

SAB file. This allows unknown entities and attributes to be read and the start of the next entity

to be located.

Subtypes and References

Topic: *SAT Save and Restore

Subtypes are frequently used in a save file in many of the entity records. Subtypes specify in

more detail the ch aracteristics of a geometry type. Interpolated curves (

intcurve) and spline

surfaces (

spl_sur) are two of the major geometry types that make extensive use of subtypes.

Subtype definitions contain the bulk of the geometry information of a model and generally

span numerous lines within a record of a SAT file. The subtype definition can be preceded and

followed by other data pertinent to that record, and subtypes can be nested.

When a subtype definition is created as part of a record in the save file, it is numbered in an

index table starting with 0. Because entity elements within a model often share geometry, the

subtype index numbers can be referenced by other entity element records. When a particular

record uses a subtype object which has been defined by another record, the latter record

simply references the index number of the subtype rather than writing the entire set of data out

again. These references are written out as “

ref n”. The ref indicates that this particular item

has already been written out to the save file. The n stands for the subtype reference number,

counting from the beginning of the file.

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