What Is Computer Graphics Metafile



Computer graphics deals with generating images with the aid of computers. Today, computer graphics is a core technology in digital photography, film, video games, cell phone and computer displays, and many specialized applications

Computer Graphics involves technology to access. The Process transforms and presents information in a visual form. The role of computer graphics insensible. In today life, computer graphics has now become a common element in user interfaces, T.V. commercial motion pictures.

Computer Graphics is the creation of pictures with the help of a computer. The end product of the computer graphics is a picture it may be a business graph, drawing, and engineering.

In computer graphics, two or three-dimensional pictures can be created that are used for research. Many hardware devices algorithm has been developing for improving the speed of picture generation with the passes of time. It includes the creation storage of models and image of objects. These models for various fields like engineering, mathematical and so on.

It is the use of computers to create and manipulate pictures on a display device. It comprises of software techniques to create, store, modify, represents pictures.


Two Kinds of Computer Graphics

There are two kinds of computer graphics – raster (composed of pixels) and vector (composed of paths). Raster images are more commonly called bitmap images.

A bitmap image uses a grid of individual pixels where each pixel can be a different color or shade. Bitmaps are composed of pixels.

Vector graphics use mathematical relationships between points and the paths connecting them to describe an image. Vector graphics are composed of paths.

The image to the left below is representative of a bitmap and the image to the right is representative of a vector graphic. They are shown at four times actual size to exaggerate the fact that the edges of a bitmap become jagged as it is scaled up:

Bitmap Image:
Vector Graphic:

With Adobe® Systems’ introduction of the PostScript® page-description language computers could display fonts and images using point-to-point math rather than by pixels alone. The advantage to using a page-description language such as PostScript becomes clear when you scale an image up. The larger you display a bitmap, the more jagged it appears, while a vector image remains smooth at any size. That is why PostScript and TrueType® fonts always appear smooth – they are vector-based.

The jagged appearance of bitmap images can be partially overcome with the use of “anti-aliasing”. Anti-aliasing is the application of subtle transitions in the pixels along the edges of images to minimize the jagged effect (below left). A scalable vector image will always appear smooth (below right):

Anti-Aliased Bitmap Image:
Smooth Vector Image:

Bitmap images require higher resolutions and anti-aliasing for a smooth appearance. Vector-based graphics on the other hand are mathematically described and appear smooth at any size or resolution.

Bitmaps are best used for photographs and images with subtle shading. Graphics best suited for the vector format are page layout, type, line art or illustrations.

Wherever possible use the vector format for all your type, line art and illustrations and only use bitmaps for photos or images with complex or non-uniform shading. If the graphics application recognizes native vector files such as those created by Deneba Canvas™ (a filename with an extension of .CNV-  for versions 6 and 7), Adobe Illustrator® (a filename with an extension of .AI), CorelDRAW® (a filename with an extension of .CDR), or Macromedia® FreeHand® (a filename with an extension of .FH8 –  for version 8), then use them first.

The EPS File Format

If the graphics application you are using cannot read native vector files the next best thing would be to save them as EPS (Encapsulated PostScript) files. These are self-contained PostScript files which contain the same mathematical descriptions as the vector files they are made from. Even bitmaps can be saved in the EPS file format. EPS files are supported by most all graphics applications. It is the most portable format for this reason. It is best to use EPS files for all line art and illustrations because they can be reproduced at any size or resolution and still display exactly as they were drawn. Use them wherever native vector files cannot be used.

A Little of Both In Today’s Applications

Today’s graphic artist has to master both skills – image editing and illustration. In fact, Adobe Photoshop® – an image editor – incorporates vector based paths which can be exported as native vector files. Four of the major illustration programs – Deneba Canvas, Adobe Illustrator, CorelDRAW and Macromedia Freehand – allow bitmaps to be embedded in the vector files they create.

I believe that the ideal page layout program of the future would be primarily a vector-based application which would create, import, display and print native vector objects side by side with bitmaps. As computing power becomes more affordable I believe this will happen.


What Does Metafile Mean?

A metafile is a file that contains specifications for another file. Metafiles are commonly associated with digital graphics, particularly vector images. However metafiles can contain other formats as well, such as bitmaps or other data. Metafiles that contain audio, video and (in some cases) text are often referred to as container files.

The word “metafile” basically translates to “beyond” or “before” file. With that, the metafile is meant to serve some outside function to an existing file. Use of the word “file” to explain electronically stored data can be traced back to an RCA advertisement for electron tubes in 1950. Since then, files have broadly expanded in format, function and capability. The metafile is typically used to outline specifications for a graphic image file, but metafiles have also evolved to perform other functions over the years.


What Does Computer Graphics Metafile (CGM) Mean?

A computer graphics metafile (CGM) is an open and free file format as well as an international standard. It is used in 2-D format for:

  • Vector graphics
  • Raster graphics
  • Text

CGM contains numerous function provisions and entitity representations in its format and it uses object-oriented approaches for image production. A metafile includes information that describes other files.

Computer Graphics Metafile is a free and open international standard file format for 2D vector graphics, raster graphics, and text, and is defined by ISO/IEC 8632.

CGM hosts a graphics data interchange format pertinent to 2-D computer graphics. It stands alone from any specific application, system, device or platform. CGM format contains the data and the instructions for graphical element rebuilding to deliver an image through object-oriented methods. The CGM format supports a varied range of graphic information and geometric primitives where graphic files are detailed in a text source file that can then be gathered into a binary file.

The CGM file format isn’t often used for Web pages since it has been replaced most typically by scalable vector graphics (SVG) and AutoCAD (which has superceded DFX). The WebCGM, however, was developed by the World Wide Web Consortium which supports CGM usage on the Web.

What are CGM files used for?

CGM file is an image saved in a vector graphics format designed to support the needs of many graphics applications. It was originally used for clip art libraries, but newer revisions (based on the ISO/IEC 8632 standard) are now commonly used for CAD drawings.


CGM File Format 

Computer graphics metafiles are basically database for graphical information and provide the means for the capture, storage, and transmission of graphical data. Consequently, there must be a graphical system component for creating the database simultaneously along with the execution of an application in a metafile format. In most cases, this component is the Metafile generator. Alongside, there is a need for another component that can fetch, interpret and render graphical data in a metafile. This need is fulfilled by the presence of a metafile interpreter. The following figure represents the graphical metafile working environment.

The relationship of CGM with other components of a typical graphics system is illustrated in the above figure. It is also evident from the figure that the functionality of metafile is not dependent on the final device output.

Generally, there are two categories of metafile: section capture and picture capture. The primary functionality of picture capture metafile is the capturing of device-independent, multiple picture definitions. While session capture metafiles use the system interface to capture the output dialogue in a graphical system. CGM belongs to the category of static picture capture metafiles. CGM provides a well-organized arrangement of components with a two-level structure.

  1. Metafile descriptor
  2. A pool of logically independent images

Each picture is a collection of picture descriptors and a picture body including a picture definition. metafile descriptor defines descriptive information that equally applies to all pictures of that metafile. This information helps the interpreter to correctly parse a metafile and recognize the resources that are required for the correct rendering of a picture. Though the picture descriptor also encloses the descriptive information yet it can only recognize the picture in which the descriptor resides. In this file format, each picture definition is self-contained and logically sovereign. They are independent of all other picture definitions in a file. Right after the interpretation of the meta-descriptor, pictures can be accessed and interpreted randomly. Change in the state of previous pictures has no effect on their successors. This picture independence is another prominent feature of CGM.CGM consists of coordinates space which are 2D Cartesian coordinates called virtual device coordinates and can be represented through number or precision representing the range and granularity. CGM specifies both the direct selection of colors and index-based selection. In the former, the color specifier consists of an RGB triple while later, the color specifier indicates an index into a color table.

CGM matches the needs of both communication-dependent as well as performance-dependent applications. Centralized and distributed graphics systems can use CGM in an unlimited number of ways. It can be tailored to access graphics devices using a spooling system.

Pros and Cons of Metafiles

Because metafiles are in a sense a combination of bitmap and vector formats, many of the pros and cons associated with these formats also apply to metafiles. Your decision to choose one particular metafile format over another will thus depend on what kind of data (bitmap or vector) makes up the bulk of the file, and on the strengths and weaknesses of that particular type of data. With that said, we can safely generalize as follows:

  • Although many metafile formats are binary, some are character-oriented (ASCII), and these are usually very portable between computer systems.
  • Metafiles containing mixtures of vector and bitmap data can in some cases be smaller than fully-rendered bitmap versions of the same image.
  • Because they can contain high-redundancy ASCII-encoded data, metafiles generally compress well for file transfer.
  • Most metafiles are very complex, because they are usually written by one application for another application. Although their ASCII nature means that theoretically they may be modified with a text editor, modification of a metafile by hand generally requires a skilled eye and special knowledge.
To import a CGM file
1. Click File onestep Import.
2. Locate the folder in which the file is stored.
3. Do one of the following:
(Windows 7 and Windows Vista) Choose CGM – Computer Graphics Metafile (*.cgm) from the list box next to the File name box.
(Windows XP) Choose CGM – Computer Graphics Metafile from the Files of type list box.
4. Click the filename.
5. Click Import.
To export a CGM file
1. Click File onestep Export.
2. Locate the folder in which you want to save the file.
3. Choose CGM – Computer Graphics Metafile from the Save as type list box.
4. Type a filename in the File name list box.
5. Click Export.
6. In the CGM export dialog box, choose a version from the Export version list box.
If the version you selected supports text encoding, enable one of the following encoding options:
Choose an encoding type from the Type list box.

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