Electronic Images
Computer graphic images can be stored in two types vector and raster.
Vector graphics - Graphics defined by mathematical descriptions of shapes, each of with may be assigned an outline with a given width and color. Also known as object-based graphics.
- Outline type such as PostScript or TrueType are actually examples of vector graphics.
Raster graphics - Raster graphics are defined in terms of pixels. In particular, raster graphics are defined by bitmaps which can be up to 32 bits in depth (for CMYK files).
- This makes possible the representation of millions of colors. The simplest bit map is only one bit in depth, corresponding to only 2 levels of gray.
- There is a natural relationship between vector graphics and line art on one hand and between raster graphics and continuous tone imagery on the other.
- However, with current desktop technology it is easy to combine line art and continuous tone images in the same document.
- Furthermore, line art is no longer restricted to solid colors and tints, i.e. blends may be used.
- Raster images can yield very large files. A 4x6 photographic scanned at 1200 dpi yields a 104MB RGB file and a 139MB CMYK file. Thus, scaling and rotating are slow.
- Scaling and rotating of vector graphics is fast since the operations are defined mathematically instead of pixel by pixel
- Whether displayed or printed, vector graphics still needs to be rasterized. The monitor is a 24-bit raster device while the imagesetter is a 1-bit raster device
- Nevertheless, for real time processing and storage, vector graphics are better for certain types of images.
- Rasterization of lines or fonts may become aliased (or pixelized) when displayed or printed on a low resolution device. (Some software includes antialias features to reduce this effect.)
- Vector and Raster objects may sometimes need to be converted into one another. Raster to vector conversion requires an autotrace routine that finds edges of objects. Vector to raster conversion is done all the time in display and RIPs (Raster Image Processing).
- It is a simple round-off problem in terms of an integer number of device pixels. However, once converted conversion back to vector loses all mathematical information about its origin.