Vector & Raster Graphics in Offset Printing
One of the simplest graphical concepts to understand before sending files to your printer is that there are two basic types of graphics; vector graphics and raster graphics. Both are perfectly suitable for press, but each is optimal in different situations. In fact, many press ready files will contain a mixture of both of these graphical formats sandwiched into a press ready container such as a PDF.
Raster graphics are image files made up of individual color pixels or dots. An example of a raster graphic would be any image produced by a digital camera. Besides images, other graphics can also be in raster format. For example a logo or illustration may also be in a raster format. Although raster graphics can exist in many file types, the most well known is the jpeg. Raster graphics work best for images and hand produced art such as paintings and drawings. Raster graphics can carry tremendous amounts of color information in a relatively small file, especially if they are stored as jpegs.
When to Use
Raster graphics can be used whenever there are no crisp edges or lines in the files you intend for press. Because raster graphics consists of individual pixels, they do not work well to depict lines that do not exactly follow the boundaries of individual pixels. For example, when rendering a diagonal line with a raster graphic, pixels that are half inside and half outside the line will be rendered as a mix between the line color and the background color resulting in a slightly fuzzy edge. When the print process is ultra high definition (like ours) the difference between raster graphic edges and vector edges can be readily noticed with the naked eye. A few raster graphic lines and shapes won't necessarily ruin your print project (if they are of sufficient resolution), but for the very best results it is better to use raster graphics for photos, textures, and other elements where edge sharpness is not a factor.
Raster File Formats
common raster file formats are .gif, .jpg, .png .bmp and .tiff. In the pre-press environment we often see just .jpg and .tiff. Jpeg graphics have the benefit of a smaller file size and generally print very well on offset. It is common for clients to send us tiff images even when jpg would have done the job. We suggest using jpgs over tiffs to conserve file size and potentially make it easier to transfer your files to your printer. However, when outputting a press file from software such as Illustrator or InDesign, never compress your jpgs below level 8, and use 10 or higher to ensure the best quality.
Rasters and Resolution
Resolution in raster graphics is very important. For example, a 1in. x 1in. image consist of 72 pixels (or dots) along each edge is using a total of 5,184 pixels to render the image. Another 1in. x 1in image consisting of 300 pixels (or dots) along each edge is using a total of 90,000 pixels to render the image. The second image will have a larger file size, but it will also appear much sharper to the naked eye because there is much more information making up the image. We use the term dpi (dots per inch) to convey the amount of information (resolution) of an image. The first example above we call a 72 dpi image, and the second example a 300 dpi image. Most offset printers will need your raster graphics to be at least 240 dpi. This is because the average offset press is not capable of printing more information on paper that what is carried in a 240 dpi image. For our high definition presses we recommend 350 dpi, with 300 being acceptable for most production work. The resolution of your raster graphics can be determined with programs such as Photoshop and InDesign.
Rasters can't be sized up
One final thing to keep in mind about raster graphics is that unlike vector graphics, they cannot be sized up. If you start with a 2x3in. image at 300 dpi and want to print it at 4x6 inches, your resulting print would be at only half the original dpi, or in this case 150 dpi. There is a method called interpolation, where a program injects extra pixels into the image in order to preserve the original dpi. Although interpolating up can help with the unwanted effect of being able to see individual pixels in an image, it does not add any real resolution to your image. Images that are interpolated up end up looking blurry rather than pixel-y.
Vector graphics are not made up of pixels; instead they are produced through a mathematical formula. For example, a simple vector triangle consists of three points in a plane that have a fixed relationship to each other. Because vectors are math based, they can be scaled to any size and retain their precise edges. Vectors needn't be simple, they can also be very complex consisting of hundreds or thousands of points with connecting straight or curved lines, shaded polygons and even color gradients. Vector graphics print beautifully and are best for depicting the precise lines and curves of shapes. Vectors come in many formats, but a common format most people are familiar with is a font file. These files hold the mathematical relationship of points and lines that are necessary for a device to display the letters in any size.
When to use
Vector graphics are created using a professional graphics program such as Adobe Illustrator, InDesign and Photoshop (yes, Photoshop can create vectors too!). They should always be used when crisp edges are desired. For example, text, icons, shapes, lines and logos are better printed from vectors. Vectors are also the format of choice when art needs to be scaled to grand proportions for uses such as banner printing or vehicle graphics. Vectors are also being increasingly used to display screen on screen graphics for apps and websites.
What file format
Vector graphics may exist as a mathematical formula in a native vector file format such as a scalable vector graphic (SVG) file, but these files are most often used for on screen display. For print it is standard for vector graphics to be contained within a document format rather than existing on their own. Two common document formats that may contain vector information are PDF files and EPS files. However, both of these document formats are also capable of carrying raster graphics simultaneously. Adobe design software files such those produced from Photoshop (.psd), InDesign (.indd) and Illustrator (.ai) may also contain both types of graphics.
How do I know which format I have?
Determining whether your graphics are vectors or rasters is simple. First open your document format in an appropriate program and zoom in on an area of your graphic. If your graphics are raster, eventually the pixels making up the graphic will become visible. If your graphic is a vector, pixels will never be visible regardless of how far you zoom. In a vector graphic the edges of your art will remain just as sharp at standard view as they will be throughout the zoom range.