Cheat Sheet

Your guide to decoding graphics techno talk
By Molly Petrilla

When you hear ppi and dpi, if your first reaction is OMG, this article is for you. Whether you’re new to the industry or a veteran who’s just been faking it, this handy guide will help you to talk shop like a true graphics pro — no pricy Graphics for Dummies books required.

I keep hearing people use the terms “ppi” and “dpi” interchangeably. What do these mean, is there a difference between them, and should I care?
Some folks, including some computer hardware manufacturers, use ppi and dpi interchangeably. Although they are similar, there is a difference.

Dpi stands for dots per inch, and it refers to the printing process. More specifically, it reveals the number of ink dots a printer will place on the page when it prints an image.

Ppi refers to the number of pixels per inch in a digital image file. An image’s ppi (also referred to as resolution or pixel density) affects how large the image will print out and how good its quality will be.

To determine the size an image will print to, simply divide its number of pixels by its ppi. For example, a 1000 x 1200 pixel image printed at 200 ppi would print out as 5 x 6 inches (or, 1000/200 x 1200/200).

Ppi and dpi numbers do not directly correspond to each other because a printer may use several dots of ink to reproduce one pixel. However, they do have one thing in common: “The higher the ppi or dpi number is, the more dots or pixels are compressed in an inch, and the better the image’s quality is,” says Dan Riley, president of Dysgraphic by Design in Louisville, Ky. “Higher numbers mean cleaner, crisper images.”

Although all printers are different and not all pixels are created equal, a good rule of thumb is to figure 3 dots per pixel. So a 1200 dpi print would be roughly equivalent to a 400 ppi image.

Whether you're talking dpi or ppi, if you don't have enough, your image will be lousy. Just like all good things in life, there's no such thing as too much. You can always make an image smaller.

How many ppi should an image be in order to print in wide format?
By far the biggest problem wide-format graphic suppliers have is with source files that do not have enough resolution to produce a large sharp image.

An image that looks sharp on a Web site is a blurred mess if you blow it up 10 feet wide. Also, the closer an image is viewed, the sharper it will need to be. Far less resolution is needed for a billboard than for a trade show graphic.

According to Radius Display Products Graphics Manager Larry Heffington, your image file will need enough resolution to yield a final output resolution of 100 to 150 ppi for trade show use. Keep in mind that this resolution refers to the finished size of the image. A 10-foot wide image at 100 ppi has 12,000 pixels (and many more dots) across that 10’ width. This is important because this means you will need to supply your printer with a file that has 12,000 pixels to arrive at the optimum output resolution. If you scan a 10” wide photo you will need to scan it at 1,200 ppi to accomplish that.

What's the difference between a hi-res and low-res image? See above. So unless you're a washed up, wrinkly brute who can't look at the image in the mirror, a high-resolution image is always the best way to go.

What’s the deal with vector and bitmap art? Is one better than the other?
Vector graphics, commonly known as “line art,” are made up of individual dots connected by lines, while bitmap graphics are comprised of pixels.

Vector files are usually created in Adobe Illustrator, although a number of other packages also output vector files. Adobe Photoshop outputs bitmap files, but a wide variety of packages create other bitmap files with suffixes such as “jpeg”, “bmp”, and “tiff”. Vector files are used for line art such as logos and for text, whereas the bitmap formats are used for photographic images.

Vectors are usually preferable, since a business-card-sized vector graphic can be reproduced as a billboard without losing any of its image quality or resolution. The same can’t be said of bitmap graphics. Bitmaps are difficult to manipulate; due to their fixed resolutions, they cannot be resized without diminishing an image’s quality. Bitmap files also tend to be much larger than vector graphics, taking up more hard drive space.

However, you have no option but to utilize a bitmap file when the subject is a photographic type image. Just be sure to pay attention to the required resolution for these images as discussed above.

It can be difficult to appreciate the difference between vector and bitmap graphics. Vector graphics (top) can be reproduced at any size while retaining their clarity and are easier to manipulate.

I thought I knew what PMS was, but apparently it’s something different in graphics-speak. What does it mean and why is it important?
In graphics, PMS stands for Pantone Matching System — the standardized color guide that assigns each color (thousands in all) a code made up of numbers and/or letters. Originally developed for the graphics industry by Pantone Inc. to solve problems of color matching, PMS is now the standard in a host of other industries as well, including fashion, plastics, architecture and paint. By asking your client to specify a PMS color for their logo you will insure that the right color is supplied on the final output.

When color counts, as it does when creating logos, it's important to know the ins and outs of the color printing process.

What’s dye sublimation, and what does it have to do with printing?
Dye sublimation is a process used for printing large projects on polyester fabric. As Heffington explains, it’s similar to using your inkjet printer at home, except the output is a 100-inch piece of fabric rather than an 8.5 x 11 piece of paper. The process begins with a special sublimation ink applied to paper, which is then put into a transfer machine on top of a piece of fabric. The machine’s pressure and heat (usually around 400 degrees) cause the ink to turn into a gas and dye the fabric’s fibers. The result is a photographic image on fabric that can be folded and even washed.

Is there a difference between printing with a four-color press and printing with spot colors?
With a traditional four-color press, printing is done with the four CMYK colors: cyan, magenta, yellow and key (black). Each color is laid down one layer at a time, and other colors are formed by various combinations of CMYK. The process is used by most newspapers and magazines. As evidenced by the occasional blurry pages we’ve all seen, there is room for error when the layers aren’t perfectly aligned. Modern-day inkjet printers (including dye-sublimation printers) use the CMYK color set to print as well — with far fewer alignment problems.

It can also be harder for four-color presses to reproduce colors as precisely as some graphics people would like them to. In those cases, they turn to the PMS color chart and select specific colors for spot color printing.

This method works best for projects that will use only a few colors, since each spot color requires its own lithographic film and individual formulation and mixing of the required spot colors. The spot color printing process is mostly found in traditional screen printing operations.