Before sending artwork for platemaking, and even before you begin the design process for letterpress, it’s important to understand how different file formats work. The decision whether or not to create a vector image in Adobe Illustrator or a raster image in Adobe Photoshop, for instance, can dramatically affect how your printing will look in the end.
Types of Images
A raster image is created from a matrix of dots. The sharpness of the image depends on the resolution of the image, or the density of dots per square inch (DPI). At high resolutions, a raster image can look seamless and sharp to the human eye. Scanned artwork is always a raster image initially, and scanning is the most common way to create a raster image for printing.
A vector image is described by mathematical equations rather than dots. Vector images consist of points, Bezier curves, lines, and fills. They are often drawn from scratch using the drawing tools inside a vector image editing application. Vector images are ideal for producing images and type with crisp edges, since they don’t create the gray areas and fuzzy edges that raster images do. Since making plates starts with a negative, it’s crucial to have digital artwork that can be reduced to black and white–no grays–to in turn produce crisp letterpress plates with smooth images that are never pixelated.
In some situations, raster images can be successfully converted to vector images and vice versa.
Raster vs. Vector: When to Use and Why
The first consideration on choosing between creating a raster or a vector image is considering what graphic design application you have at your disposal. Adobe Photoshop is the most common raster image editing application and Adobe Illustrator is the most common vector editing application. If you have a choice, you will then consider how your artwork should look on the printed page. Both bitmap and vector have their strengths and weaknesses when it comes to letterpress printing.
Why Use a Raster Image?
Calligraphers and designers who work primarily with line art, calligraphy, or hand-drawn artwork will find that raster images work best. All scans are initially made as raster files and contain all the imperfections and character of the original artwork. See 18.104.22.168 Scanning and image touchup for more information on how to best scan images for printing. Generally, a raster image should have a resolution of at least 600 DPI at the output resolution (the effective resolution after it has been scaled up or down).
Raster images are not ideal if you need to enlarge the artwork much. A 600 dpi scan, when enlarged to three times its scanned size, will have an output resolution of 200dpi (600/3=200). This resolution is coarse enough to look very jagged in the finished printing. If you start with a 2400 dpi scan, though, your output resolution would be 800dpi in the example (2400/3=800), and fine enough to print well. So, scanning at a high resolution at the outset will give you more flexibility to resize the image.
Why Use a Vector Image?
Vector files are better for scaling and manipulating without losing detail or resolution. Because their paths can be translated, scaled, and dilated without extreme distortion, vector artwork retains crisp edges and forms without sacrificing the artwork’s sharpness. An vector image that measures 1 inch by 1 inch can be scaled up or down infinitely before printing without a loss of quality.
Fonts are technically vectors, thus allowing them to scale up or down without losing quality. You should attempt to keep fonts vectors, and if your design has typography, only use file formats that support both vector and raster. A vector graphics program such as Adobe Illustrator lends itself to fine typography much better than a raster graphics application such as Adobe Photoshop.
File Format for Images
The image type (whether an image is raster or vector) is different than the file format (whether you save the image as PDF, TIFF, EPS, etc). Many file formats support both raster and vector formats, but in many cases a file format can determine the type of image. Here is a short list of the common file types so that you can know when the file type determines the image type:
Conversion Between Image Types
Convert images between vector and raster image types carefully. This conversion is a bit uncommon and only recommended for in certain situations.
Raster to Vector
Why would you turn a raster image into a vector image? This might be a good idea if you want the image to look very graphic or if you plan to scale and distort the image a fair amount. In Adobe Illustrator, you can select the placed artwork and Object>Live Trace to trace a raster image and create a rough vector image. Stark, graphic artwork with high contrast works best for this conversion, and the resulting image will usually require a fair amount of manipulation to look its best.
If you turn intricate or handmade images into vectors, you will most likely sacrifice the gradations and imperfections of the artwork. Thin lines or subtle details might disappear entirely. For example, you may want to convert a high resolution calligraphy sample with very delicate flourishes or letter embellishments. If you then open the artwork in Adobe Illustrator and use the Live Trace function in Illustrator to vectorize their work, you may find that the thinner details and delicate lines disappear or are distorted.
Vector to Raster
It’s very uncommon to convert a vector image into a raster image for printing. There are not too many good reasons to do this, except if you are deliberately trying to create a jagged image or need to apply a Photoshop filter. In Adobe Illustrator, you can select the object and then Object>Rasterize to rasterize your vector image. Alternately, you can rasterize an entire Illustrator file by opening it directly in Photoshop. In both cases, you will need to specify what resolution to use. The resolution should be 600dpi at the final output size.