Files, files, and more files

There are so many electronic file types these days it is easy to become confused. So many file acronyms (PCX, TIF, PICT, WMF, CGM. GIF. JPEG) can lead to more confusion. How do you know which file to use for the web, presentations, digital or when going to an offset press? Not to complicate matters even more, we have differing computer technologies (Mac versus Windows and now Linux) running various computer programs ranging from word proces­sors to page layout programs. Fortunately for us, programs that are written for the Mac (for example) will, in most cases, work on the PC or Linux today and visa-versa. This was far more difficult in years past when DOS ruled. With all the various file types, let us in this article concentrate on graphic file types.

These graphic formats are nothing more than the way a computer application writes your data to a disk, how software interprets it, and how it will be displayed. Some file formats are definitely right for a particular task, some just clearly wrong. With all the various computer programs and systems available, choosing the right file format can get confusing. Let's explore the different types of formats. There are three basic graphic file types:

· Bitmap files

· Vector files

· Metafile files

A bitmap file stores pictures as matrices with rows and columns of squares known as pixels. Each pixel can have a particular gray or color value, which is known as gray depth, color depth or bit depth. Bitmap files are usually created by image‑editing programs such as Adobe Photoshop®, Corel Painter, Microsoft Paint, scanner software, etc. The most common bitmap files are TIF(F) (tag image file format), BMP (Windows bitmap), PICT, (MacPaint) and PCX (Windows Paint). The tag image format would be file suffixed as a tiff on a Mac but is the same type of tag image file format on the Windows machine and suffix as a tif. Because of the pixels we tend to talk about the file in terms such as “pixels per inch” or PPI. The higher the ppi number, the higher the image resolution. You will normally see the ppi displayed as 72 dpi, 200 dpi, 300 dpi, etc.

 
A vector file is different from the bitmap file, as it does not store pictures as pixels. Vector files contain sets of mathematical instructions for drawing objects, which are usually geometric shapes such as lines, ellipses, polygons, rectangles and arches. These sets of drawing instructions tell the computer to "start this line at this point and draw to that point over there" or "this rectangle is made up of these line segments." CAD (computer aided design) programs usually create vector files, for example. Typical vector files would be the DXF (dynamic exchange format). Another program that would use vector files would be Adobe Illustrator that saves its files in its native ai or converts it to an eps (encapsulated postscript). Generally, image files of this type are used for printing (offset or otherwise).

A metafile can contain either vector and bitmap graphics or both. The Macintosh PICT, Adobe Illustrator (AI), EPS (encapsulated PostScript), CGM (computer graphics metafile), Photoshop (eps) and WMF (Windows metafile or enhanced metafile) are all examples of metafile formats.

The purpose of these formats is to allow the movement from one computer application to another and/or for printout or display. All computer programs will support their own native format but the saved file may or may not be supported in another computer application program. Many computer programs have the ability to read and write several types of file formats making use of these various formats "easy". Adobe InDesign®, for example, can import native Adobe Illustrator® (.ai) files but Microsoft Word cannot. Those programs that can import file formats created by other programs usually conduct some sort of computer language translation. Many times after translating (importing), the translated version can be slightly different from the original version - sometimes with little difference, sometimes with large differences. Another reason files are different is dependant on the display device that will be used.

Now that we have some background on all the file types available, which ones should we be using in the programs that we use? Bitmap? Vector? Metafile? When and where? When do I use a TIF file? When do I use a JPEG file? This is where compression comes in. Image files can be very big, even 20 or 30 megs. You sure wouldn’t want to use big files like that for the web or presentations. To shrink files we have to compress or throw some data away to make the file smaller. As you toss out information, the file size gets smaller but the image loses resolution or image quality. When printing offset, however, you need all the data so the image quality is good. For displaying on the web, which only displays at 72 dpi, you don’t need all the data. Now you can see the trade-offs.

Files that compress will be the JPEG, PNG and GIF and are normally used in web pages, presentations, anywhere you need to keep the file small without loss of image quality. Wait! But doesn’t getting rid of data in the file reduce image quality? Computer or television screens display at 72 dots/inch (dpi), which is low resolution so the low resolution JPEG files display just fine even with the loss of data. If you tried to, however, use the same type of a file for offset printing it would appear very poor due to the low resolution. A high amount of jagged edges would be seen and the image would appear somewhat blurred. To print for offset press, a resolution of around 300 dpi is needed to provide high resolution. TIFF and EPS files keep all the information and can provide high resolution but the file size will remain very high.

Unlike JPEG files, TIFF files can compress without the loss of data. Files that compress can be either “lossy” or “lossless”. JEPG, GIF and PNG are all lossy – which means ridding of.some of the image and extraneous data. TIF(F) files can compress but without the loss of image data. TIF(F) files eliminate extraneous non-use data only.

In summary, if you are going to the web, or using images in PowerPoint, Word, etc. JPEG is the best format. GIF should be used only for the web. The GIF format is best for line type work whereas JPEG is best for photographic images due to how many colors the file can handle. A GIF file can only handle 8 bits of color meaning 256 colors; that is why only GIF files should be used for line type art. JPEG files can handle millions of colors. For digital or offset printing, even though most digital printers/press are only 600 dpi devices (unlike offset printing at 2400 dpi or higher), TIFF or EPS files should be used. With the lower resolution; however, you can use JPEG files for images. TIF and EPS files also handle line art data well on these digital postscript devices where as JPEG files do not.

This short article only scratched the surface and I’ll continue to explore file formats in another issue of LINK. You may also contact me at jim.kohler@ipaper.com.