Appendixes 4. An Introduction to PostScript ® CVG Lab

Introduction PostScript® is known as a “page description” programming language. → it is commonly used to specify how a page should be printed. An important feature of PostScript: -. Its device independence. The usual way to work with PostScript is to use a text processor to fashion a PostScript, and then to submit the file to a PostScript printer. -. The printer can be set to interpret the script and make a picture rather than simply print the script as text.

Introduction Ghostscript -. Provide a convenient alternative. -. Freely available and operates on a variety of PC and workstation platforms. -. Interprets a PostScript script -. Displays the picture on the PC monitor. -. Can make debugging a script much easier → immediately see the results of a script

About the PostScript Language Scripts in PostScript are readable by humans 1.Some Preliminares Different character appearing in a sript take on different meanings. -. Comments. All characters from ‘%’ to the end of the current line are comments and are ignored by the interpreter. -. Case. Case is significant. -. White space. Characters such as spaces, tabs, or newlines leave spaces on the script page, and are collectively called “white space.”

About the PostScript Language  Numbers in PostScript -. Numbers are written in the usual way, either with or without a decimal point. 2.PostScript Is “Stack based” -. PostScript is similar to the language Forth in that it maintains a stack of objects called the operand stack. 34 –5.2 % push 34 than –5.2 onto the stack 12 % push 12 on top (meaning the stack is empty) ,2 12

About the PostScript Language 3.Some Stack Operators: pop, dup, exch, and clear -. pop 12 → — -. dup 12 → exch exch → clear clear →

4.More Advanced Stack Operators -. n index counts down n items into the stack, and pushes a copy of the nth item on to the stack: index → index → 6 6 % same as dup -. n copy n copy pops the n and then pushes a copy of the top n elements of the stack: copy → copy → About the PostScript Language

-. num shifts roll num shifts roll pops shifts and num, and then does a circular shift of the top num elements shifts times roll → roll → roll → count count the number of items on the stack and pushes that value onto the stack count →

5.Some Arithmetic Operators -. Add: add → subtract: sub → Multiply: mul → Division: div→ Integer Division: idiv→ Modulo: mod → mod → and, or, exclusive or, complement (&, |, ^, and !) -. abs, neg, floor, ceiling, truncate, round, sqrt, ln, log, cos, sin exp, atan  Random-number Generation About the PostScript Language

Graphics Operators in PostScript PostScript has operators that make it easy to draw lines, circles, Bezier curves, and many other figures. 1.Coordinate Systems and Transformations

Graphics Operators in PostScript 2.Path Construction Verbs -. Move to pops the top two items off of the stack and set the CP accordingly x y moveto → — newpath 1 1 moveto 1 3 lineto 3 3 lineto 3 1 lineto closepath stroke 2 2 moveto 4 2 lineto 4 0 lineto 2 0 lineto closepath fill showpage

Graphics Operators in PostScript 3.Arcs of Circles -. Acrs of circles are drawn in PostScript by using one of the two verbs arc and acrn. center.x center.y rad start_angle end_angle arc → — center.x center.y rad start_angle end_angle arcn → —

Graphics Operators in PostScript 4.Used for Painting Verbs -. stroke -. fill -. setlinewidth -. setgray -. setrgbcolor -. clip -. setlinecap -. setlinejoin

Graphics Operators in PostScript 5.Coordinate Transformations

2.Automatic dodging-and burning Bringing “up” selected dark regions or bringing “down” selected light regions to avoid loss of detail. Dodging-and-burning is typically applied over an entire region bounded by large contrasts. -. Chose a center-surround function derived from Blommaert’s model for brightness perception. -. This function is constructed using circularly symmetric Gaussian profiles of the form: Algorithm

-. The center-surround function defined by; Algorithm |V(x, y, s m )|<є … Computed for the sole purpose of establishing a measure of locality for each pixel, which amounts to finding a scale s m of appropriate size. … This scale may be different for each pixel, and the procedure for its selection is the key to the success of our dodging-and-burning technique. … It is also a deviation from the original Blommaert model.

The luminance of a dark pixel in a relatively bright region will satisfy L<V1, so this operator will decrease the display luminance Ld, thereby increasing the contrast at that pixel. “dodging” A pixel in a relatively dark region sill be compressed less. “burned” Algorithm