18.3. Polyline - main line drawing primitive.    



                  Polyline(N,X,Y)

 Note we will use a high level notation for the routines allowing

spaces in names.

    N is number of points (X[1],Y[1]) to (X[N],Y[N])

    N is of type integer and X,Y are variables sized arrays of reals.

 This concept allows efficient drawing of n-1 line segments on most devices at

 the expense of storage required for the array. Is this a real problem?

The arrays may be set up via assignment statements 

         X[i] := minX + (maxX - minX)*i/255

         Y[i] := f(X[i]);

or read in from a data file 

         FOR i:= 1 TO 100 DO

             readln(X[i],Y[i]);


A call to polyline will then plot the line segments defined by the two arrays. 

 Notice that we do not have any specific "Move" primitive. This is done

automatically

- move to (X[1],Y[1]) and then draw to (X[2],Y[2]) and subsequent points.

There is no current position concept - the pen or drawing position is only

relevant when a primitive is executed not before or after.

18.3.1. Polyline Attributes.   

When graphing data it might be nice to differentiate between different plots or between the plot and the axes. 


    
Polyline plots

   
                 Fig. 18.1 : Polyline plots
To do this we need to be able to set the line drawing attribute. This is done via
set polyline index (N)
where the current line drawing attribute is set to N until a subsequent call to set polyline index (M). 

          Index 1   may be solid - default

               2   may be dashed

               3   may be dotted

               4   may be dash-dotted

             set polyline index (1); polyline (N,X1,Y1)

             set polyline index (2); polyline (M,X2,Y2)

What else is appropriate to polyline? (i.e. other attributes).
Line width : device specific but GKS allows us to specify a relative size: we can set a line width scale factor: real number giving the width of the line relative to the width of a standard line on this device.
Lw < 1 thinner than standard line
Lw > 1 thicker than standard line

Colour : colour of line is important and this can be set as can the line width by a call to 


       set polyline representation(WS, PLI, LT, LW, PLCI)


    WS   is workstation in question.

    PLI  is number of polyline index

    LT   is line type

    LW   is line width

    PLCI is polyline colour INDEX

The colour is not defined by a value but by an index into a table of colours: 3 entries for intensities of RED, GREEN, BLUE = colour lookup table. 

(For pens in plotter may be   0 = white

                           1 = blank

                           2 = red      etc.).

Three polyline indices for one workstation could represent solid, dashed and dotted lines with the same three indices giving red, green, and blue lines on a workstation that was a plotter. (NOTE the polyline index could be the same on the different workstations but GKS does allow us the flexibility).

GKS allows the user to setup the colour lookup table for different workstations: set colour representation (WS, CI, R, G, B)

Where CI is colour index and R, G, B are real values between 0 and 1 representing the intensities of the Red, Green and Blue colours. 


                           R   G   B

         what is black?      0   0   0

                white?      1   1   1

                yellow?     1   1   0      Red and Green

                blue?       0   0   1