UNIT - V

DHARANI KUMAR.S/AP/MECH

• CAD Standards are a set of guidelines for the way Computer-aided

design (CAD) drawings should appear, to improve productivity and

interchange of CAD documents between different offices and CAD

programs, especially in architecture and engineering.

DHARANI KUMAR.S/AP/MECH

DHARANI KUMAR.S/AP/MECH

Standards for computer graphics-Need forgraphic standards

• Need for portability of the geometric model among different hardware

platforms.

• Exchange drawing database among software packages.

• Need for exchanging graphic data between different computer system.

• Need for the requirement of graphic data exchange formats and their

details such as IGES ,DXF,STEP.

SOME OF GRAPHIC STANDARDS

DHARANI KUMAR.S/AP/MECH

FOUR TYPES OF MODELNG DATA USED IN PRODUTDESCRIPTION

• Shape data contains information of both geometrical and topographical

information along with surface features.

• Non shape data contains shade images and model global data.

• Design data

• Manufacturing data

Requirements for the Exchange• Shape data: both geometric and topological information, part or form features.

Fonts, color, annotation are considered part of the geometric information.

• Non-shape data: graphics data such as shaded images, and model global data as

measuring units of the database and the resolution of storing the database numerical

values.

• Design data: information that designers generate from geometric models for

analysis purposes. Mass property and finite element mesh data belong to this type of

data.

• Manufacturing data: information as tooling, NC tool paths, tolerancing, process

planning, tool design, and bill of materials (BOM).

CLASSIFICATION OF CAD STANDARDS

• Graphics and computing Standards-GKS, OpenGL

• Data Exchange Standards-IGES,DXF,STEP,CALS

• Communication Standards

DHARANI KUMAR.S/AP/MECH

Graphics standard

• Focus of this standard is that the application program should be device

independent and should interface to any input device handler and to any

graphics display through a device driver.

• The graphics system is divide into two parts ; Kernel system which is

hardware –independent and the device handler /driver ,which is naturally

hardware dependent.

• Kernel system act as buffer between the application program and the specific

hardware to ensure the independent and portability of the program .

Graphics standard

Graphicsstandards ingraphicsprogramming

Graphics Kernel system

• GKS is basically a set of procedure which can be called by user programs

to carry out certain generalized functions such as arc,circle,ellipse etc.

• It is a standardized system of graphical functions for processing graphical

data to create and process 2D images.

• GSK implementations have been made by many hardware manufactures in

many languages.

LAYER MODEL OF GKS

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Features of GKS

• It is an independent device ,it can work with all types of input and output

devices.

• All text and annotation can be prepared and stored in natural languages.

• Graphic functions are defined for both 2D and 3D

• GKS defines an international coordinate system called normalized device

coordinate system .

Classifications of GKS

1. control functions

2. Output attributes

3. Output primitives

4. Segment functions

5. Transformations

6. Input functions

7. Metafile functions

8. Inquiry functions

Output primitives & Output attributes

• Circle is a primitives

• Attributes may be color, line width and line types

Examples

• POLYLINE

• POLYMARKER

• TEXT

• FILL AREA

GKS INQUIRY FUNCTIONS

• Functions are used to find the current status of any variable in GKS.

• It is mainly used to isolate the predefined values of variables if

anything is required.

Example : transformation and clippings

GKS INPUT FUNCTIONS

• A set of functionality is defined in the category is known as input

functions.

• The devices which are connected in a graphic system are to act an

input element such as tablets, mouse, keyboard and joystick.

Metafile functions

• Files to store, retrieve and display the graphical data to handle device

independent formats.

PHIGS

Data Exchange Standards

IGES (Initial Graphics ExchangeSpecification)

• First developed by National Institute of Standards and Technology (NIST)

in 1980.

• Then adopted by the American National Standards Institute (ANSI) in the

same year. exchanges primarily shape (both geometric and topological) and

non-shape data, which is referred as CAD-to-CAD exchange.

IGES (Initial Graphics ExchangeSpecification)

• codes a superset of common entities of all CAD/CAM systems to

facilitate the translation between various systems.

Development of IGES

IGES (InitialGraphics ExchangeSpecification) –Geometry Entities

IGES (Initial Graphics ExchangeSpecification) –Annotation Entities

IGES (Initial Graphics ExchangeSpecification) –Structure Entities

IGES (Initial Graphics ExchangeSpecification) –File structure

Flag section• Used only with the compressed ASCII and binary format .

• IGES data in a file can be represented in two formats :ASCII and binary

• ASCII - American Standard Code for Information Interchange.

• ASCII format comprises two types : fixed 80-character record length and

compressed format .

• Compressed format contains compressed file by eliminating spaces from the

records.

Start section

• Provides a human –readable description of the file, such as

the sending system that generated the original data, the

pre-processor ,and the product being described.

Global section

• Included information telling the pre-processor and

information needed by the post –processor to interpret the

files.

• Some item specified example : the data and time of the file

generation .

Directory Entry section• Is a list of all the entities together with certain of their attributes.

• All product definition data are expressed as a list of predefined entities (geometry

and annotation entitles ) .

• Entry each entity occupies two 80 –character records that are divide into a total of

twenty 8-character fields .

• First and the eleventh field contains entity type number .

• Second field contains pointer to the Parameter data entry for the entity in the

parameter data section.

• Remain entity will contain (line fonts ,layer number, transformation matrix ).

Parameter data section• Section contains the actual data defining each entity listed in the directory entry

section .

• Example: straight line entity is defined by the 6 coordinates its two end points .

• Each entity always has two records in the directory entry section and number of

records need for each entity in the parameter data section.

• Parameter data is placed in free format of columns 1 through 64.

• Column 65 left blank.

• Columns 66 to 72 on all parameter data records contain the entity pointer

specified in the first record of the entity in the data entry section.

Terminate data section

• Section contains a single record which specifies the number of

records in each of the four preceding sections for checking purpose .

DRAWING INTERCHANGE FORMAT (DXF)

• DXF into file formats that could be read and used by other CAD/CAM/CAE

sfiles were originally developed to give user flexibility in managing data and

translating AutoCAD drawings ystems.

• DXF standard of interchanging CAD drawing files for almost all

CAD/CAM/CAE systems .

• In fact, almost every newly introduced CAD/CAM/CAE sytem tends to provide

translators to and form the DXF file.

DXF file is an ASCII test file and consist of five sections

• Header- describes the AutoCAD drawing environment that existed when the DXF file was

created.

• Table-contains information about line types, layers, text styles, and views that have been

defined in the drawing.

• Block-contains a list of graphic entities that are defined as a group.

• Entity –immediately follows the block section, and serves as the main part of the DXF

file, with all entities of the drawing described in it.

• Terminate-indicates the end of the file.

Limitations of IGES and DXF

• IGES and DXF files were developed to exchange product definition

data instead of product data. By product data we mean the data

relevant to entire life cycle of a product (eg: design ,manufacturing,

quality assurance ,testing and support).

PDES (Product Data Exchange Standard)(then Product Data Exchange Using STEP)

• STEP (Standard for the Exchange of Product model data) is a new

International Standard (ISO 10303) for representing and exchanging

product model information.

• Support any industrial application such as mechanical, electric, plant

design, and architecture and engineering construction.

• To include all four types of data which is relevant to the entire life-

cycle of a product: design, analysis, manufacturing, quality assurance,

testing, support, etc.

• PDES is a much more comprehensive and complex standard than IGES

or any other predecessors

• The user interface is not as simple as “put IGES” and “get IGES.”

MECHANICALPRODUCTS

ELECTRICALPRODUCTS

AECPRODUCTS

APPLICATION N

GENERIC ENTITIESAPPLICATION –SPECIFIC

ENTITIES

FILE FORMAT AND DATA STRUCTURE

STEP ARCHITECTURE

Application

layer

Logical

layer

Physical

layer

Three layers of STEP

• Application layer –consist of information of various application areas.

• Logical layer-provide a consistent , computer-independent description of

the data constructs that contain information to be exchanged.

• Physical layer –deals with the data structures and data format for exchange

file itself.

• EXPRESS is a formal data specification language used to specify the

representation of product information.

Standard for the Exchange of Product modeldata• The goal was to define a standard to cover all aspects of a product (i.e.

geometry, topology, tolerances, materials, etc.), during its lifetime.

• Fundamental difference in IGES and STEP .

• The central unit of data exchange in the IGES model is the entity.

• The central unit of data exchange in the STEP model is the application,

which contains various types of entities.

• Data is exchanged between systems it is done in terms of application units.

Continuous acquisition and life –cycle support (CALS).

• It was developed by US department of defense.

• It prescribes the formats for storage and exchange of technical data.

• Focuses mainly technical publications

Important CALS standards

1.Standard generalized markup language (SGML) is an important standard

.developed ibn 1960s IBM . It has the document description language.

2.Computer graphics metafile (CGM) is next important standard .developed in 1986

it is used for the vector file format for illustrations and drawings.

Continuous acquisition and life –cycle support(CALS)

• CALS is an attempt to integrate text, graphics and image data into standard

document architecture.

• All technical publications for the federal government must conform to the

CALS standard.

• CALS has also come into wide use in the commercial computer industry

such as in CAD/CAM applications and in the aerospace industry which

owes a large part of its business to government and military contracts.

Continuous acquisition and life –cycle support (CALS)

• Two types of CALS raster formats as defined by MIL-STD-28002A.

• TYPE I –raster data files contain single monochrome image

compressed using CCITT encoding algorithm .

• TYPE II – image file contain one or two monochrome images which

are also stored using CCITT encoding algorithm.

Continuous acquisition and life –cycle support (CALS)

Type I file format consist of the following

I. Header

II. Image data

Type II file format looks similar to the format given below.

Header

Document profile

Presentation styles

Document layout

Root layout

Layout object page 1

Tile index

Image data

Layout object page 1

Tile index

Image data

Layout object page 1

Tile index

Image data

Continuous acquisition and life –cycle support (CALS)

I. File details

II. Header record data block

III. Image record identifiers

IV. SourceDocId

V. DestDocId

VI. Text File ID

I. FiguerID

II. SourceGraph

III. Docclass

IV. Raser type

V. Orientation

VI. Pelcount

VII.Density

Open GL –image exchange using Open GL

Open GL –image exchange using Open GL

What's wrong with using IGES files?

• Technically there's nothing wrong with using the IGES format, however, IGES files tend to

be surface models, and often get translated with gaps between the surfaces, missing faces

and in some instances surfaces with faces in the wrong orientation. While there is is an

IGES standard for solids (IGES-MSBO), it is not often used.

• When a user creates an IGES file, they are translating their native CAD file into the IGES

format. When this file is shared, it is imported into the next CAD package and is translated

again. File translations, could potentially bring in inaccuracies in the geometry.

• The benefit of using the Native CAD file, is that it only requires 1 translation, which is

theoretically more accurate.

Why is STEP better than IGES?

• STEP is newer technology, where IGES which is older 1990's technology,

that has not been updated in over 20 years. STEP files tend to be solid

models, where IGES files are more often surface models, which could

have gaps and missing faces.

• STEP is a mature file format, it has been available for over 20 years with

continual development and wide adoption between CAD vendors.

• STEP is widely used, and is even used in the construction industry, the

IFC file format is a version of STEP.