±12       Inconvex Vector Objects
±13-------------------------------------
±12by Aragorn/Avalon

±11After  a while you probably get bored
just working with plain convex vector
routines,    because   of   the   few
opportunities they give you to design
objects.  So what you have to do then
is  to  fix your routine a little bit
so  that it's able to handle inconvex
objects.   And that should open a new
horizon  for  you  and let you create
much more interesting objects.

First of all you need a normal hidden
line  vector  routine  to  start  out
with.   And  as you know, in a convex
routine  you  draw the lines directly
in  the  screenbuffer, and that works
fine  since  there is no intreference
with  more  than one surface over the
same  area.   However,  when  you are
working  with  inconvex objects, this
overlapping  will  occur.   What  you
have  to  do  then is to draw all the
lines  for  one surface in a separate
buffer,  then  fill  the  buffer, and
blit  it out on the screen.  Then you
clear  the  buffer,  and  do  it over
again  until  you  have taken care of
all  the surfaces.  Now, that is just
a  really simple way of saying it, so
let's  get  a little deeper into what
needs to be done...

In addition to your screenbuffers you
will  need  another buffer big enough
for   one  bitplane  in  screen  size
(usually [256*40]=10240 bytes).  This
is  where  you'll  draw  and fill all
your   surfaces   separately   before
blitting  them  onto the screen where
the  complete  object will be created
and  shown.  Since the surface buffer
(the  one explained above) is only in
one  bitplane,  you'll  have to label
every  surface  in  the  declarations
with  the bitplanes you want it to be
shown in.  Here is an example on such
a declaration:

±12dc.w	%10,0,1,1,2,2,3,3,0,-1

±11Here,  the  first  word  shows  which
bitplanes  the  surface will be shown
in.   Then  comes the coordinates the
surface  is  drawn  between (from,to,
from,to...)  and  the last word marks
end  of surface.  Then, after filling
the  surface  it's  time  to blit it.
Let's  say  you  have  the first word
"%10"  in  d0.   You  simply test the
bits  in d0 to find where to blit the
surface.   And  remember to clear the
area  which the surface covers in the
bitplanes  where  it  is  not blitted
out!  Let's look at an example:

±13btst	#0,d0	; Draw Bpl1? [No]
beq	.No1	; If No, Skip It!
(BLIT SURFACE IN 1. BITPLANE)
btst	#1,d0	; Draw Bpl2?
bne	.No1	; If No, Clear It!
(CLEAR SURFACE'S AREA IN 2. BITPLANE)
.No1:
btst	#1,d0	; Draw Bpl2? [Yes]
beq	.No2	; If No, Skip It!
(BLIT SURFACE IN 2. BITPLANE)
btst	#0,d0	; Draw Bpl1? [No]
bne	.No2	; If No, Clear It!
(CLEAR SURFACE'S AREA IN 1. BITPLANE)
.No2:

±11The  answers  in the brackets will go
for  the  "%10" combination.  Now all
you  need  to  do  is  to  clear  the
surface-buffer  and  repeat the cycle
until  all  the  surfaces  have  been
processed.   And  then your object is
complete!

"Now  wait  a minute", you might say;
"how   can   I  make  sure  that  the
surfaces are blitted out in the right
order?".   Well,  since  there  is no
routine  that  sorts  the surfaces as
they  rotate  and  gets  them  in the
right   order,  we'll  have  to  pre-
arrange  the  surfaces.  That is done
down   in   the   declarations.   The
easiest  way  of explaining how to do
that  is  just  to  switch around the
orders  the  surfaces are declared in
until  it  works.   But here are some
guidelines that you might find handy:

±13*±11 Declare the surfaces that are over,
  in front of, or outside the others
  at the end.
±13*±11 Declare the surfaces that are shown
  in all bitplanes last, or first.


±12Well, that is all I have to say about
inconvex   vector   objects  in  this
article.    Good   luck   with   your
experiments!

±13                    Aragorn/Avalon...ç