Kohler's Corner - Files, Files and More Files, Part Two

A couple of issues ago, I explained the various files used today on computers and for printing. I discussed bitmap, vector and combination files and how they worked. So where do printers use these files? Can we use a bitmap file for printing? Digital? Web? Presentation program? Where do they all fit?

Bitmap files are made up of individual pixels, vector files are mathematical based and combination files are made up of both bitmap and mathematical data. Bitmap files can be saved in several types of formats. Files can be saved in a "lossey" or "lossless" format. "Lossey" format means the file has loss data from the original. To shrink the size of the file, the printer must "throw out" or lose some of the data hence the term "lossey." If the file is kept intact – no data lost – the file size might be very large. An example would be if we took an image from a digital camera that was 10 megabytes and tried to put that image on the web - download time would be extremely long. Printers shrink it or "lose" some of the data to reduce the file size. The file in its original form might have been a bitmap TIF (tagged image file format) file but to shrink it we will convert it to a JPG file by losing some of the data. The file might be 800 kilobytes which is more manageable on the web or in a presentation program.

As we shrink and toss data out, we are also degrading the overall quality of the image. While the lower quality will work for the web or in presentation programs, the lower quality will not work for high quality printing. If you remember from my last article, computer or video screens display only 72 pixels per inch whereas printing needs far more pixels to generate high quality continuous images. To produce a high quality continuous tone in print, have to have at least images with 300 pixels per inch. If we were to take a file of low quality and try to print it, it would be very pixilated. The smaller the file size, the larger the pixels and the lower the quality.

In the industry today the most common file types are:

Lossy: .jpg, .png, .bmp, .psd

Lossless: .tif, .eps, .ai

Combination: .wmf (Microsoft), psd (photoshop)

There are, of course, other types of file formats but the ones listed above are the most common.

Working in a web, video, presentation or similar format, the use of a JPG or PNG file is the best as it will be the smallest and easiest to deal with. For printing high quality images, work with lossless files such as TIF or EPS, which keep all the data intact.

Digital printing does not reach the high quality of offset printing, but it is far better than a computer screen. If you are just printing to a home or small ink-jet or laser printer, JPG files will work just fine as there resolution output is low to begin with. If, on the other hand, you are going to print on a high volume digital production press such as a Xerox iGen3 or an HP Indigo, ask the press operator what format they would like the file in.

File types can get very confusing to say the least. If you want more information or advice, feel free to contact me at: jim.kohler@ipaper.com