In addition to all of the functions described above, the blitter can draw
patterned lines.  The line draw mode is selected by setting bit 0
(LINEMODE) of  BLTCON1 , which changes the meaning of some other bits in
BLTCON0  and  BLTCON1 .  In line draw mode, the blitter can draw lines up
to 1024 pixels long, it can draw them in a variety of modes, with a
variety of textures, and can even draw them in a special way for simple
 area fill .

Many of the blitter registers serve other purposes in line-drawing mode.
Consult  Appendix A  for more detailed descriptions of the use of these
registers and control bits.  You should also see Appendix C for the
 new limits  on line-drawing mode in the Enhanced Chip Set (ECS).

In line mode, the blitter draws a line from one point to another, which
can be viewed as a vector.  The direction of the vector can lie in any of
the following eight octants.  (In the following diagram, the standard
Amiga convention is used, with x increasing towards the right and y
increasing down.) The number in parenthesis is the octant numbering; the
other number represents the value that should be placed in bits 4 through
2 of  BLTCON1 .

                               |
                               |
                      \   (2)  |  (1)   /
                       \       |       /
                        \   3  |  1   /
                         \     |     /
                          \    |    /
                     (3)   \   |   /   (0)
                            \  |  /
                        7    \ | /    6
                              \|/
                  -------------*-------------
                              /|\
                        5    / | \    4
                            /  |  \
                     (4)   /   |   \   (7)
                          /    |    \
                         /     |     \
                        /   2  |  0   \
                       /       |       \
                      /   (5)  |  (6)   \
                               |
                               |

              Figure 6-8: Octants for Line Drawing


Line drawing based on octants is a simplification that takes advantage of
symmetries between x and -x, y and -y. The following Table lists the
octant number and corresponding values:


      Table 6-3:  BLTCON1  Code Bits for Octant Line Drawing


                BLTCON1  Code Bits      Octant #
               -------------------       --------
                     4  3  2
                     -  -  -
                     1  1  0               0
                     0  0  1               1
                     0  1  1               2
                     1  1  1               3
                     1  0  1               4
                     0  1  0               5
                     0  0  0               6
                     1  0  0               7


We initialize  BLTCON1  bits 4 through 2 according to the above Table.
Now, we introduce the variables dx and dy and set them to the absolute
values of the difference between the x coordinates and the y coordinates
of the endpoints of the line, respectively.

dx = abs(x2 - x1) ;
dy = abs(y2 - y1) ;

Now, we rearrange them if necessary so dx is greater than dy.

if (dx < dy)
        {
        temp = dx ;
        dx = dy ;
        dy = temp ;
        }

Alternately, set dx and dy as follows:

dx = max(abs(x2 - x1), abs(y2 - y1)) ;
dy = min(abs(x2 - x1), abs(y2 - y1)) ;

These calculations have the effect of "normalizing" our line into octant
0; since we have already informed the blitter of the real octant to use,
it has no difficulty drawing the line.

We initialize the A  pointer register  to 4 * dy - 2 * dx. If this value
is negative, we set the sign bit (SIGNFLAG in  BLTCON1 ), otherwise we
clear it.  We set the A   modulo register  to 4 * (dy - dx) and the B
 modulo register  to 4 * dy.

The A  data register  should be preloaded with $8000. Both word  masks 
should be set to $FFFF. The A  shift  value should be set to the x
coordinate of the first point (x1) modulo 15.

The B  data register  should be initialized with the line texture pattern,
if any, or $FFFF for a solid line. The B  shift  value should be set to
the bit number at which to start the line texture (zero means the last
significant bit.)

The C and D  pointer registers  should be initialized to the word
containing the first pixel of the line; the C and D  modulo registers 
should be set to the width of the bitplane in bytes.

The SRCA, SRCC, and DEST bits of  BLTCON0  should be set to one, and the
SRCB flag should be set to zero.  The OVFLAG should be cleared. If only a
single bit per horizontal row is desired, the ONEDOT bit of  BLTCON1 
should be set; otherwise it should be cleared.

The logic function remains.  The C  DMA channel  represents the original
source, the A channel the bit to set in the line, and the B channel the
pattern to draw.  Thus, to draw a line, the function
        _
   AB + A

is the most common.  To draw the line using exclusive-or mode, so it can
be easily erased by drawing it again, the function
     _   _
   ABC + AC

can be used.

We set the blit height to the length of the line, which is dx + 1. The
width must be set to two for all line drawing.  (Of course, the  BLTSIZE 
register should not be written until the very end, when all other
registers have been filled.)

 Register Summary for Line Mode