Whether we’re talking about drawings, photographs, clip art or logos, graphics are a welcome addition to any print piece. In combination with text, graphics can make content easier to understand, more likely to be retained and more likely to grab the attention of readers compared to plain text. In print publishing, graphics are universally two dimensional, though the web also allows for 3D and other multimedia images.
How are graphics created?
There are two different ways to create graphics: mathematically using geometric objects (lines, points, curves and polygons) as building blocks; these are known as vector graphics – and pixel by pixel using a grid called a raster image or bitmap. All scanned images and digital photographs are bitmap images, while vector images are, for the most part, generated from illustrations made with drawing or technical drawing software.
Common bitmap file formats include JPG or JPEG (Joint Photographic Experts Group); GIF (Graphics Interchange Format); TIF or TIFF (Tagged Image File Format); PCX (Windows Paint); PSD (Adobe Photoshop); BPM (Bitmap); and PICT (MacOS pixel image format). Adobe Photoshop, Corel Paintshop Pro and Microsoft Paint all work with bitmap files.
Common vector file formats include AI (Adobe Illustrator); CDR (Corel Draw); and EPS (Encapsulated Postscript). Software you might use for vector drawing include Macromedia Freehand, Corel Draw, and Adobe Illustrator.
Metafiles fall under the umbrella of vector files, even though they contain both vector and bitmap information – a vector object filled with a bitmap pattern. WMF (Windows Metafile) and CGM (Computer Graphics Metafile) are examples of this format.
GIF and JPG are the most common graphics formats online, while print projects may use vector or bitmap images. Most printers prefer EPS for vector images and the TIF format for bitmap images.
Characteristics of bitmap images
A bitmap is made up of many individual pixels; tiny dots which each have their own color. Their small size makes it nearly impossible for the unaided eye to see them individually – but taken together, they make up an image with a seamless gradation of color. Bitmap images may use any color, but there are four basic ways that color is used in these images:
- Line art: These images use only black and white.
- Grayscale: These images use black, white and shades of gray.
- Multi-tone: These images use shades of two or more colors; the most common type of multi-tone image is duotone, which usually includes black along with one other color.
- Full color: Full color images assign colors to each pixel using a color space like CMYK (Cyan Magenta Yellow Black) or RGB (Red Green Blue).
Bitmaps do not need to contain a bit of information about the color for each individual pixel. It just contains information which tells the computer when to display a new color as it is scanned along a row. Because of this, images with a lot of solid color will have a smaller file size.
One of the disadvantages of bitmap images is that they are resolution dependent. Resolution refers to the number of pixels needed to create an image, generally expressed as dots per inch (DPI) or pixels per inch (PPI). Since computer screens and printing equipment have different resolution requirements for image rendering, an image that looks great on the web may not have a high enough resolution to produce a high quality print.
Resolution dependence also makes it hard to resize bitmap images without affecting their quality. Reducing a bitmap image involves down-sampling, which removes pixels, making the image less sharp. Enlarging means up-sampling, which introduces new pixels through interpolation based on the colors of surrounding pixels – which can also result in a blurry image.
Characteristics of vector images
Vector images are made up of geometric objects which are defined mathematically. Rather than a bitmap which describes which colors go where, a vector image describes a series of connected points using Bezier curves.
Vector objects may be outlined, filled or colored, enlarged, reduced, stretched or rotated without affecting its resolution. Vector objects may also be changed independent of the other objects in the image. Perhaps the most commonly used types vector images are fonts.
Vector graphics are resolution independent, which makes them a good candidate for logos and other images which are frequently resized. Their file sizes are also generally significantly smaller than those of bitmap images. However, there is one important downside to vector images, which is that it is very difficult (though not impossible) to create photorealistic images using this method.
What’s the best image file format?
This all depends on what your image will be used for. If you need graphics for the web, then JPG or GIF is probably the best choice. However, if you need to create images for a print project, then you’ll want to use either TIF format for bitmap images or EPS for vector images. TIF is the most commonly used format in professional printing applications, is supported by Windows, MacOS and Linux operating systems and supports CMYK, RGB and other common color spaces and works with a number of different compression formats. For vector images, EPS is a Postscript file which is specialized to make it easier for these images to be embedded in Postscript documents. Additionally, EPS files can include previews for screen display.