ALGORITHMS
Algorithm for drawing a line
(C) Ars
To display a straight line connecting
any two points can be used
algorithm Brezenhema. Its meaning is to
following: the line is regarded as a set
segments of two types: those that
located diagonally, and those who
arranged horizontally or vertically.
For lines with a slope of more than 1 direct
vertical segments, otherwise
they are horizontal. The first task of the algorithm
is to determine the slope. Then
calculated smoothing factor
which checks that a certain number of
straight segments had a greater length than
others. Finally, the cycle turns
output the diagonal and straight segments.
The equalizing factor in turn takes
is positive, then negative
values, noting what type of segment
displayed.
LINE procedure displays a straight line
by 2 points, the coordinates are given in
Registers DE (top) and BC (the end).
PIXEL procedure takes a point with
coordinates in ref. DE (D-Y, E-X).
LM1 LD H, E
JR PM1
LM2 LD L, C
JR PM2
LM3 LD H, A; dY = 0, ie direct
LD (ST_XY), HL; horizontal segments
EXX; experimental
LD C, L; greater distance
LD L, H; reg.C in less than a
JR PM3; Reg. L
LINE EXX
LD DE, # 1514; Codes DEC D and INC D
LD BC, # 1D1C; codes and DEC E INC E
EXX
LD A, B
SUB D; check y.> Yn?
EXX; if yes, then we take
JR NC, LM1; code INC D, otherwise
NEG; take the module races
LD H, D; Toyan and code DEC D
PM1 EXX
LD H, A; got y.-Yn
LD A, C
SUB E; X. verification> Xn?
EXX; if yes, then we take
JR NC, LM2; code INC E, otherwise
NEG; take the module races
LD L, B; Toyan and code DEC E
PM2 LD (DI_XY), HL; entry codes
EXX
LD L, A; received X.-Xn
SUB H; determine the type of
EXX; straight segments
LD A, 0
JR NC, LM3
LD L, A; dX = 0, ie direct
PM0 LD (ST_XY), HL; vertical segments
EXX; locally
LD C, H; see ca. to LM3
PM3 LD B, A; A = 0!
LD H, A; A = 0!
ADD HL, HL; double the lesser
LD (ST_CO +1), HL; distance
SBC HL, BC; calculate:
OR A; 2 x less than minus
SBC HL, BC; 2 x greater
LD (DI_CO +1), HL
ADD HL, BC; loop counter in
LD A, C; Reg. A = more
PIXEL PUSH HL; rasstoyaiyu
BIT 7, H; HL-factor expression
PUSH DE; smoothing if
EX AF, AF '; H <0 (t.e H> 127)
EX DE, HL; segment at the line
LD A, H; my way -
AND 7; diagonal
OR # 40; in register DE -
LD D, A; current coordinates
RR H; coordinates
RR H
RR H
LD A, H
AND # 18
OR D
LD D, A
LD A, H
AND 7
RRCA
RRCA
RRCA
LD E, A; calculation bytes
LD A, L; on the screen, where
AND 7, will output
LD C, A; Xia current point
LD A, L
SRL A
SRL A
SRL A
OR E
LD E, A
LD HL, DATA_P; calculation of the pixel
LD A, L; for the point
ADD A, C
LD L, A
LD A, (DE)
OR (HL)
LD (DE), A; draw a point
EX AF, AF '
POP DE
POP HL
JR Z, DI_XY; the conclusion of direct
ST_XY DW 0; segments and
ST_CO LD BC, 0; conversion factor
JR DECDIST; alignment
DI_XY DW 0; output diagonal
DI_CO LD BC, 0; segments and
DECDIST ADD HL, BC; conversion factor
DEC A
JP P, PIXEL; next point
RET
DATA_P DB 128,64,32,16,8,4,2,1
The disadvantage of this procedure is
that the max length of the calculated line - 127
points. Data Block DATA_P should not
located on the border between the two sectors!
