Cary Ravitz

    This  article will cover how to draw lines on a bit map, i.e.  convert
lines from vector format to raster format.  Drawing one pel wide lines
efficiently has been  covered in detail in many  places,  but for  the real
world this is not sufficient.  Lines need  emphasis  so that they can be
distinguished or highlighted. This can be accomplished by using wide lines,
shading patterns, dashed lines, and color.  The algorithms and code presented
here are simple techniques to accomplish this. They are not meant to be the
fastest available  algorithms, but  to provide usable, modifiable, straight
forward code.

WIDE LINES
    There  are two (at least)  ways to attack the problem  of drawing wide
lines.  One way is to draw  multiple one pel lines.  Another way is to draw the
line once  with wide dots.  I have chosen the second way here because it makes
things simpler when dealing with dashed lines.
    I have not used Bresenham's method for calculating the points  along a
line. The routine that draws a dot will be a much bigger computational hit that
calculating the dot locations, so simplicity is better here.
    The basic algorithm for drawing a  line  from x1,y1 to  x2,y2 is shown
here:

  if distance_from_x1_to_x2 > distance_from_y1_to_y2 then
    for x = x1..x2 draw a dot at x,y_value_at_x
  else
    for y = y1..y2 draw a dot at x_value_at_y,y

    For each  step  along  a  line, one  floating  point addition and  one
floating  point  to  integer conversion  is  done, but  there  are  no
multiplies or divides.


SHADING
    Shading is accomplished by masking the dots that are ored into the bit map.
If  you mask a shading pattern  into the dots then, as a line is created with
dots that overlap by one pel, the patterns will interfere and become solid.
The trick that  we need is to tie the  shading mask location to the  bit  map,
not  the dot.  Then  when dots overlap, the shading  will not  interfere.  So
we will wait until after the dot  is shifted to align with  the bit map in the
x direction (m:=dot[w,i] shr l; in DRAWDOT)  before  applying  the  shade mask.
The  dot is  never shifted to align in the y direction and instead the starting
scan line in the mask is chosen to match the dot.

DASHED LINES
    Since each point on a  line  is drawn as a large dot,  the line may be
dashed  by simply leaving out series of dots.  If we keep track of how long
the  line  is as we draw each point on it, and compare this to a true/false
pattern, when  the pattern is  true we draw a dot, and when it is false we move
on to the next point.

COLOR
    Drawing  in eight  colors (including  the  background) is a  matter of
using  mulitple color planes.  Eight basic colors -  red, green, blue, cyan,
magenta, yellow,  white, and black - can be represented in three color planes -
either red/green/blue or yellow/magenta/cyan.

MIXING COLORS WITH SHADING
    You can use shading to mix the eight basic colors. For example, draw a line
solid in one color plane and checkerboard in another color plane. This  mixes
the two colors and greatly expands the available colors on eight color devices.

SAMPLE CODE
    A sample program that demonstrates these techniques is available from
COMPUTER LANGUAGE's CompuServe forum and BBS network. Included in the program
listing is a  sample program that invokes  VGA 640x480 16  color  mode  and
draws  a number  of  lines  with  various attributes.  The code  is  mostly in
Turbo  Pascal,  with  DRAWDOT in assembler (DRAWDOT's constants are in the
Pascal code).

Independent software developer Cary Ravitz is the author of the Ravitz Editor
and other shareware products.