Raster images use many colored pixels or individual building blocks to form a complete image. JPEGs, GIFs and PNGs are common raster image types. Almost all of the photos found on the web and in print catalogs are raster images.
Because raster images are constructed using a fixed number of colored pixels, they can’t be dramatically resized without compromising their resolution. When stretched to fit a space they weren’t designed to fill, their pixels become visibly grainy and the image distorts. This is why altered photos may appear pixilated or low resolution. Therefore, it is important that you save raster files at precisely the dimensions needed to eliminate possible complications.
Vector images, alternatively, allow for more flexibility. Constructed using mathematical formulas rather than individual colored blocks, vector file types such as EPS, AI and PDF* are excellent for creating graphics that frequently require resizing. Your company logo and brand graphics should be created as a vector and saved as a master file so you can use it with smaller items such as your business card and letterhead, but also on larger surfaces, such as your corporate jet. When necessary, always create a JPG or PNG for use on the web from this master vector file. Just be sure to save the new raster file in the exact dimensions needed.
*A PDF is generally a vector file. However, depending how a PDF is originally created, it can be either a vector or a raster file. Whether you opt to flatten the layers of your file or choose to retain each one will determine the image type.
Graphics Interchange Format (GIF) is one of the file formats used to display on the internet. It is mainly used on websites. It is indexed-colored graphics and images in HTML documents. Indexed color means that it will only display a maximum of 256 colors. For this reason, GIF is NOT a good format for saving photographic type images with many colors. GIF is good for saving images with flat blocks of color such as logos or simple illustrations. Another very important feature of GIF images is that is allows you to preserve transparency. It uses an LZW-compressed format designed to minimize file size. Not used for many web needs because of quality considerations. Best use = web sites and other digital needs.
JPG or JPEG
Joint Photographic Experts Group (JPEG) format is used to display photographs and other continuous-tone images on the web. JPEG format supports CMYK, RGB (millions of colors), and Grayscale color modes. Unlike GIF format, JPEG retains all color information in an RGB image but compresses file size by selectively discarding data. This is known as lossy compression, and can result in a loss of quality if a high level of compression is applied. In most image editing programs you can specify how much compression/loss of quality you want. Generally, if you choose the maximum quality option, your image will be indistinguishable from your original photograph. However, you can also save your files with lower quality settings that still produces a reasonably good image (or at least good enough for the web) image. Another important difference between GIF and JPG, is that JPG does not preserve transparency. Best use = Personal photograph collection, web sites and other digital needs.
Portable Network Graphics (PNG) is a little bit like the best of both worlds. It was developed based on GIF, for lossless compression and for display of images on the web. Unlike GIF, PNG supports 24?bit images and produces background transparency without jagged edges; however, some older web browsers do not support PNG images. PNG format supports RGB, Indexed Color, Grayscale, and Bitmap mode images. PNG also preserves transparency. Best use = web sites and other digital needs.
A TIF (or TIFF) is a large raster file. It has no loss in quality and therefore is primarily used for images used in printing. On the web, because of load time, you generally want to use smaller images such as JPG or PNG. Best use = images and photographs for high quality print.
An EPS file is a vector file of a graphic, text or illustration. Because it is vector it can easily be resized to any size it needs to be. An EPS file can be reopened and edited. Best use = master logo files and graphics and print designs.
An AI file is a proprietary, vector file type created by Adobe that can only be created or edited with Adobe Illustrator. It is most commonly used for creating logos, illustrations and print layouts. Best use = creating logos, graphics, illustrations.