User Space - Device independent coordinate system where pages are created. (Origin at lower left).
- Limitless and any fraction of the 1/72” unit can be specified (the precision is only limited by that of the underlying arithmetic).
PostScript objects are vector except for embedded bit maps.
Vector drawing is based on the graphic primitives, straight lines, arcs and curves.
- Lines are based on two coordinates
- arcs are based on a portion of a circle
- curves are based on Bezier splines (end points and control points) A path is a virtual line which is not actually drawn until a stroke or fill operation is performed. Characters defined as series of straight and curved paths.
Encapsulated PostScript - EPS file, special kind of PostScript object that can be embedded into a page. Can contain text, graphics or other EPS files. Not sent output device, no device information and no showpage command. Includes
- Header identifying it as EPS
- Bounding Box defining area of user space
- Optional bitmap preview (can be PICT, TIFF or EPSI). EPS files may be application specific.
Images - The image operator defines
- The image format (height, width, spatial resolution and number of colors)
- Coordinates in user space where the image will be placed. • The color values for each pixel.
- How the color values are to be translated into the current color space.
Bitmapped images are given a bounding box that defines the position in user space of the image in addition to the values for each pixel.
- Images can be scaled and rotated.
- Usually the color values for pixels will be stored in a separate Tiff or EPS file.
- Level 2 provides for handling color specification and output. Setcolorspace supports the device dependent RGB, CMYK, grayscale and HSB and the device independent CIE spaces as well as special color spaces.
- Level 2 can support up to 12 bits per color.
The Printer Driver - generates PostScript code when “print” is chosen from an application. Translates from computer’s native format. Downloads fonts and dictionaries.
- Driver handles devices using printer description files.
- Code can be output as ASCII or binary. Binary is faster.
- Can be “printed to file” to create a file which can be edited or previewed by a PostScript viewer or using Adobe Acrobat.
PostScript Errors-Errors vary from failure to print to just the wrong fonts being printed. The most common errors are
- The requested fonts not being available
- Interpreter memory limits being exceeded
How to fix font problems is clear enough, but memory on the interpreter is more difficult to predict or deal with.Memory errors are signaled by the issuance of limitcheck errors. These are less likely with Level 2, but are difficult to predict.Problems can be reduced by
- Splitting up very long paths.
- Cropping in image editing programs (e.g. Photoshop) instead of using cropping paths in the page-layout program.
PostScript Limitations-Although it is very powerful, PostScript does have some limitations. Among these are
- Inflexibility of opaque painting model which makes difficult trapping and adding of transparent color.
- The lack of vignettes (graduated blends to background) which makes necessary construction of tint gradations with separate objects.
- Lack of page independence which makes imposition difficult.
- Difficulty of editing a document once it has been translated to PostScript.
- Inability of printer drivers to generate error-free code.
- Frequency of execution errors.