REM triangle filling routine for acorn atom. quite effective
REM from a letter to Greg, c. 1985
REM ATOM BASIC PROGRAM TO FILL TRIANGLES
REM
DIM XX(3), YY(3), DD(3), GG(3): S=256
XX1=10:    YY1=15
XX2=250:   YY2=100
XX3=120:   YY3=190
S = #87: E=#80
E!4=#FFFEFCF8
 !E=#F0E0C080
S!4=#0103070F
 !S=#1F3F7FFF
L = #91, R= L+1,V=L+2, W=L+3,M=L+4, G=L+5,T=L+6,B=L+7
DIM LL9, PRINT $1
F.N=1 TO 2: DIM P(-1): REM or P=#2800
:LL1  LDA L: LSR A: LSR A: LSR A: TAY: STA G
      LDA L: AND @07: TAX
      LDA S,X: STA M
      LDA R: LSR A: LSR A: LSR A: SEC SBC G
      BEQ LL4
      STA G
      LDA M: AND W: ORA (B),Y: STA T
      LDA M: AND V: EOR T: STA (B),Y
      INY: DEC G
      BEQ LL3
:LL2  LDA (B),Y: ORA W: EOR V: STA (B),Y
      INY: DEC G
      BNE LL2
:LL3  LDA R: AND @07: TAX: LDA E,X
      STA M: AND W: ORA (B),Y: STA T
      LDA M: AND V: EOR T: STA(B),Y
      RTS
:LL4  LDA R: AND @7: TAX: LDA E,X: AND M
      STA M: AND W: ORA (B),Y: STA T
      LDA M: AND V: EOR T: STA(B),Y:
      RTS
]
N. PRINT $6: PRINT $7: CLEAR 4
?w = SETMASK
?V = invertmask
!B = 38880-32*YY1
DD3 = YY3-YY1: GG3=S*(XX3-XX1)/DD3
F = S*XX1: H=F

bottomTriangle
DD1 = YY2-YY1: GG1=S*(XX2-XX1)/DD1
?l=F/S: ?R=H/S: WAIT: LINK LL1: F=F+GG3: H=H+GG1
!B= !B-32: DD1=DD1-1: IF DD1<>0 GOTO bottomTriangle

topTriangle
DD2 = YY3-YY1: GG2=S*(XX3-XX1)/DD1
?l=F/S: ?R=H/S: WAIT: LINK LL1: F=F+GG3: H=H+GG1
!B= !B-32: DD2=DD2-1: IF DD2<>0 GOTO topTriangle
RETURN