Dana Bell

When speaking with non-designers about graphics, it is often easy to forget how little they know about graphics. The most that most people know about graphics is the resolution. High resolution is good; low resolution is bad.

Unfortunately, it is not that simple. Without taking the time to include illustrations, some of this may not be so obvious, but it's something I'd like to try to start explaining. Consider this a first draft.

dpi and Resolution
When a graphic artist thinks about resolution, he is very often thinking about the dpi of a bitmap image. And while the higher the dpi, the better, measuring in dpi alone can be misleading. A 600 pixel x 800 pixel screen image at 72 dpi and a 600 pixel x 800 pixel screen image at 300 dpi, is the same image. On computer, the issue is not the dpi. 600 x 800 is 600 x 800. The total pixel size of an image is the determining factor. The computer screen only displays in one resolution.

The difference is in reproduction. The dpi (dots per inch) translates to image size and resolution when printed on a inkjet or laser printer. If you print this image at 72 dpi, it will be larger, but lower resolution. If you print the image at 300 dpi, it will be smaller but higher resolution.

I'm talking pure dots here. You could enlarge a 300 x 400 image to 600 x 800, but in effect the quality of the image is still 300 x 400, since all those extra dots are created out of thin air and don't actually create any new details in the image. That's why it is important to keep the original image in its created size until you know how you are going to use it. Anytime you reduce an image, you are throwing away dots, dots that you can't get back by re-enlarging.

lpi and Printing
In addition to the dpi reproduction of an image, there is lpi. That is the printing reproduction resolution. When a graphic is printed, the print process does it's own conversion to a dot pattern, where lpi stands for the numbers of lines of dots per inch. The printing lpi ranges from about 120 - 200 or more for commercial offset. In order to have the best quality conversion to printing lpi, the finished graphic should be at least 1.5 times the print resolution. Thus, a 300 dpi image would be used for 200 lpi printing.

Bitmap or Vector
All of the concern about bitmap images is due to the fact that bitmap images are simply color dots (squares, actually) placed proportionally on a grid to represent the photo. Since there is so such thing as a half-dot, the image will appear jagged where dots attempt to align on an edge. If everything you do is for the web, that may be all you need to be concerned with since computer monitors are dot-based devices.
If you're printing, however, you'll want to use vector graphics for anything but photos if possible. In short, vector graphics are digitally drawn objects, normally printed on a postscript device.

Various programs create vector, bitmap or combined images. Selecting the correct program, or file format is critical to getting the quality image that you want.

Fonts
By default, modern font files are all vector based, so in order to maintain vector quality, they must be in their native format. Type formatted in a page layout program preserves the vector characteristics of the font. Similarly, type created in most drawing programs will usually be vector. Cases where this type can be converted to bitmap is in graphics programs such as Photoshop. Note however, if you attempt to use a custom color, the image must be broken down into component colors of CMYK, and represented by dots.

Drawings
Drawings made is programs such as Illustrator are also vector based. That is not to say you cannot create bitmap characteristics in an illustrator. Some of Illustrator's effects are bitmap.

Photos
Realistic photos are primarily bitmap images, and there is very little you can do to make them into vector artwork. Things like clipping paths can be preserved in photos, but they are separate items that can be created or imported from other vector programs.

Effects
Shading and gradients are two things that are often combined with other images to enhance them. While they are bitmap effects, you may have to take special steps to make sure that the vector images that they are a part of are not also rasterized or converted to bitmap images.

Flattening
Most graphics programs now use layers for the different elements in an image or graphic. When you use the program to flatten the image, in most cases, the flattened file is a bitmap version of all layers combined, even though some of them may be vector objects. Backup and think twice before flattening. Once the file is written to disk and the program is closed you may not be able to get the vector objects back.

File formats
Often people may be misled by demanding their artwork be in various formats. Used to be that EPS files were the thing to ask for, since they are vector images. The problem is, again, not that simple. EPS can contain both vector and bitmap images. Nowadays, the same presumption is being made about PDF files. PDF files, however, can also contain both vector and bitmap images. The same is true of AI. Since many items contains both vector (type and drawings) and bitmap (photos), these formats are often the most logical choices. It all depends on how you create the files, and the options you use when saving or exporting. Of course, with the combination of various files and formats, transparency is another issue, but that is another dragon to slay.
P.S. Present day JPEG images never contain vector artwork. It is a bitmap format designed specifically for photographers and photographs.

Creating quality infinitely reproducible art is a process with a lot of elements to consider. The above contains just some of the things you need to be concerned with for the highest quality graphics. If it were that easy, anyone could do it. Once you know how it all works, you'll be on the way to a less worry-free life as a graphic designer.

Tip Summary

• Use high resolution original images (larger image sizes)
• In press reproduction, the image dpi should be at least 1.5 times the lpi being used in printing.
• Use Vector artwork for drawings and text
• Maintain vector artwork when adding bitmap effects and creating files for output.