“Raster” vs. “Vector” Art
These two words sound rather complex, but understanding their meaning and choosing the correct option can ensure success in inserting both drawings and photos into your InDesign art files.
“Raster” just means bitmapped: a grid of pixels (on your computer screen) or printer spots of ink or toner (on your laser printer). Pixels can be on or off (black or white). They can also contain various levels of gray (intermediate stages between black and white). Or they can contain various levels of color. Although books can be written on what to do with bitmapped art (and how to bring it into your computer through scanning techniques or “paint” programs), essentially a raster is a grid of squares of ink.
You can recognize bitmaps by file extension types, such as TIFF and JPEG. Unfortunately, while they do retain their appearance when reduced in size, they become visibly pixellated as they are enlarged. That is, you can see the squares in the bitmapped grid (whether color or black and white). It's not that they weren't there before. They were just below the threshold of visibility, and enlarging the bitmap made them visible (and ugly, or at least distracting).
The rule of thumb for raster art is to scan it (if you're scanning a photo) or create it (if you're using a paint program) at 300dpi at 100 percent of the size you're planning to use it. The 300dpi indicates the resolution, and the 100 percent size means you're not going to place the art in InDesign and then enlarge it.
What about vector art?
Vector art is made up of lines and curves, not pixels or printer spots. These lines and curves are defined mathematically in the PostScript page description language. Unlike raster art, vector art can be enlarged or reduced without losing the crispness of its lines. And since it is not made up of dots (or spots), it never becomes pixellated no matter how large you make it.
You can recognize vector art from its “EPS” (or encapsulated PostScript) suffix (such as “name of file”.eps). If you create it yourself in a program such as Adobe Illustrator, the suffix after the period may be “.ai” before you re-save the file as an EPS.
In either case, although it's important to remember the distinction between the two types of files, it's even more important to remember not to enlarge raster files.
Tracing Raster Files and Rasterizing Vector Files
Many illustration programs now have automatic “tracing” capabilities. If, for instance, you have a hard-copy of a logo you want to bring into the computer, you have a few options:
1. You can scan the logo at a high resolution and large magnification and then reduce its size, which will increase its resolution (i.e., make it less fuzzy when printed).
2. Or you can scan the logo and then trace the bitmapped file in Adobe Illustrator (or another drawing application). Doing this by hand using the Illustrator pen tool will probably be labor intensive. Hence, most drawing programs (again, these are vector, not raster, applications) have some form of autotracing ability. The software just detects the edges of the bitmapped image and draws shapes to recreate the raster art as vector art. Unfortunately, this is seldom completely accurate, and more often than not you have to spend a considerable amount of time tweaking all the lines and curves in Illustrator. But it's often less labor intensive than tracing the bitmapped art yourself. At least it's a start.
You can go the other way, too. You can take vector art and rasterize it (turn it into a bitmap). Why would you want to do this? Let's say you are creating a composite image in Photoshop, and you want to combine a photo with some type and a logo (that you only have as an EPS file). When you open the EPS file of the logo in Photoshop, Photoshop will “rasterize” the logo based on certain specifications that you can adjust. You can then save the composite image of type, photo, and logo as a TIFF file before placing it in an InDesign document.
Sound complicated? You quickly get used to it. And you only have to do this rasterization process when you're combining elements in Photoshop to create a final bitmapped image.
(Remember, that even the type in a Photoshop file eventually gets rasterized. And to be completely candid, when your printer uses your InDesign file to make plates, he rasterizes the file to burn the plates. So all elements of the art file eventually get rasterized.)
Line Art Scans
Here's a shortcut to avoid both autotracing and hand-tracing art you have scanned. It's based on the principle that if you scan something at a sufficient resolution, the bitmapped raster dots will be too small to see.
If you're scanning a logo as single-color line art, for instance, you can scan it at 1200dpi at its final printed size (100 percent of size), and it will be considered acceptable digital line art.
This can save you a huge amount of time and stress.
The eye is quite forgiving. Not only does it ignore raster dots if they're small enough. It also ignores subtle variations in color. For instance, the sky can be a lot of different shades of blue without looking wrong. It's only when it becomes extreme (i.e., gets to the purple range, for instance) that you know something is amiss.
In contrast, such colors as orange (for the fruit) or green (for grass) can be a little off, and you'll find them unacceptable. These are called memory colors. There is very little latitude in making these absolutely right in a Photoshop image file.
White is essentially the same. If it has anything but a completely neutral cast, the human eye will see the difference.
How is this information useful? What it really does is give you an idea of what you can sacrifice to get an acceptable photo in Photoshop. All printing work (and prepress work) involves compromise. It's just wise to identify the “memory colors” first and then make sure their hues are faithful to the original and that there are no color casts.
Particularly when you're designing a book or brochure—when you're just playing around with typefaces and point sizes, before you've absolutely settled on the final design—it's remarkably easy to not use style sheets.
Style sheets are the collection of type attributes you define and then apply to highlighted text. Once you've defined the style sheet parameters, you just point the mouse, select the type, and everything conforms to all the specifications in the style sheet (typeface, point size, leading, tracking, kerning, set width, space between paragraphs, etc.). Even better, if you decide later to change all headlines, for instance, or set the type in a different font or point size altogether, you can just tweak the style sheets, and all type in your document will conform to the redefined styles.
However, creating style sheets requires more of a “production” mindset than a “design” mindset. (It relies on the creative, spatial side of the brain rather than the linear, logical side.) I know. I've made the mistake myself. It's incredibly easy to use “local formatting” instead of style sheets as you play with the design.
Avoid this. Or, once you have finalized the design of a few pages, then create a series of styles for the type based on the rough layout you have just completed.
Then and only then is it safe to proceed with your book or brochure production.
[Steven Waxman is a printing consultant. He teaches corporations how to save money buying printing, brokers printing services, and teaches prepress techniques. Steven has been in the printing industry for thirty-three years working as a writer, editor, print buyer, photographer, graphic designer, art director, and production manager.]