Number 299 - April 2008

What Types of Files Do You Have?
by Randall Rich. Sacramento PCUG, January 2000
One of the more confusing tasks in preparing electronics files for publishing comes when sifting through the alphabet soup of graphics file formats. Which is best to use: the smallest file size or the one that looks best on-screen? In fact, there are definite reasons to use one format over another for specific uses, but first let's take a quick overview of the various file types.    All graphics files can be divided into one of two types: either native or interchange. A native file format is unique to the application or program that created the file. This is what you get when you use the command Save in the File menu. This format cannot be read by other applications unless the translation ability has been written into the program or appropriate filters or plug-ins installed. An interchange file format is intended for moving data from one application to another. The more robust your specific application, the greater the variety of interchange file formats it will read.    Interchange file formats can be further separated into three distinct types: bitmap files, vector files, and metafiles.    Bitmap files, also called raster images, store data in rows and columns of pixels, or bits, of a given depth. Thus, as the image size grows, the file size increases exponentially. The same growth rule applies to resolution. Double the resolution and quadruple the file size. Line art, or black and white, bitmaps have a bit depth of one. These are very compact files which can use a variety of dot imaging techniques to emulate grayscale, such as stochastic dithering, diffusion dithering, or pattern dithering. Photographic halftones are an example of screened, single bit bitmaps.    Grayscale bitmaps may have a bit depth up to eight, and thus may take up to eight times the disk space of a one-bit file. Color bitmaps are much larger and come in RGB or CMYK format, with the RGB files 75 percent of the CMYK files size for the same bit depth. Bitmap files are typically generated by scanners and image-editing software, such as Adobe Photoshop or other paint programs. Common bitmap file formats are TIFF (Tagged Image File Format), GIF (Graphics Interchange Format) and PCX (PC Paintbrush).    Vector files use mathematics formulae to describe arcs and shapes so they can be scaled to virtually any size with no loss of image quality. These file types are typically generated by draw programs such as Multimedia Freehand or Adobe Illustrator. Pen-based CAD (Computer-Aided Design) programs, such as Autocad or Turbocad also create vector files. Examples include DXF (Dynamic eXchange Format) and PS (Adobe PostScript) files.	Metafiles have the capability to contain both raster and vector elements. Examples include EPS (Encapsulated PostScript), WMF (Windows MetaFile), CGM (Computer Graphics Metafile), and AI (Adobe Illustrator).    Some of these formats are best used on a given machine, while others lend themselves well to file transfer. Most of the interchange file formats assume that the files are to be used on the system where they were created. These files are device-specific, because they make certain assumptions about the monitor or printer. And we all know what happens when we make assumptions. Bitmap formats will assume the color palette is stable, while vector and metafile formats will assume the font list does not change. These are not valid assumptions to make if you wish to transfer data to another system or, indeed, another platform. WMF, for example, uses GDI (Windows Graphic Device Interface) while PICT (Macintosh PICTure) uses Quickdraw. Both of these languages are exclusive to their platforms and do not transfer well. BMP is intended for on-screen viewing and does not print well, even in Windows, much less on a Macintosh.    Problems in preparing files for publishing sometimes come from the application rather than the file format. You should always use a professional graphics or page layout program designed to read and write a wide variety of formats.    If your file is intended for print, it will most likely be going to a Postscript device. In this case, you want all your scanned and painted images in TIFF format and all your vectors and metafiles in EPS format. Indeed, your final file may be an EPS file.    If your file is intended for on-screen viewing, use GIF for images with areas of single color and sharp transitions, such as vector artwork, and JPEG (Joint Photographic Expert Group) for images with gradual transitions between colors, such as scanned photos.    And please, do not copy and paste. The clipboard, on both Windows and Macintosh platforms, strips information from your graphic in the process of transfer, sometimes altering it through "rounding" differences due to a format change, sometimes eliminating important printing data, and leaving you without an external link to the graphic. Place, don't paste. Save the image in the appropriate format for final print from the application where it was created or edited. Then, place that image in the page layout program. This technique gives you the greatest control over how that image will finally display.    Copyright ) 1999 Sacramento PC Users Group, Inc. All rights reserved.
Number 299 - April 2008