Amstrad CPC Raytracer

1 BORDER 0:MODE 0
2 st=TIME/300
3 INK 0,0:INK 1,6:INK 2,2:INK 3,18:INK 4,24:INK 5,26
5 dx=0:dy=0:GOSUB 1000
10 FOR x%=0 TO 639 STEP 4
20 FOR y%=0 TO 399 STEP 2
130 dx=(x%-320)/580
131 dy=(y%-200)/580
140 GOSUB 1000
300 PLOT x%,y%,col%
500 NEXT
505 GOSUB 3000:LOCATE 1,1:PEN 5:PRINT USING "#####,.##s";et
510 NEXT
515 WHILE 1:WEND
520 STOP
1000 mt=1E+10
1020 a=2*(dx*dx+dy*dy+1)
1100 RESTORE 9000
1101 READ ns%
1102 FOR s%=1 TO ns%
1110 READ cx,cy,cz,sr,sc%
1211 b=2*(dx*cx+dy*cy+cz)
1212 c=(cx*cx+cy*cy+cz*cz)-sr
1220 d=b*b-2*a*c
1230 IF d<0 THEN GOTO 1500
1231 d=SQR(d)
1240 t=(b+d)/a
1241 IF t>0 AND t<mt THEN col%=sc%:mt=t
1250 t=(b-d)/a
1251 IF t>0 AND t<mt THEN col%=sc%:mt=t
1500 NEXT
1999 IF mt=1E+10 THEN col%=0
2000 RETURN
3000 et=(TIME/300)-st
3010 RETURN
9000 DATA 4
9001 DATA 0,-1,4,1,1
9002 DATA 2,0,4,1,2
9003 DATA -2,0,4,1,3
9004 DATA 0,-5001,0,25000000,4