5MODE7 10REM Read a JPEG header to extract t he 20REM useful information 30: 40INPUT"JPEG file to read: "f$ 50F=OPENINf$ 60DIMmarkerSOF(15) 70FORloop=0TO15:markerSOF(loop)=&C0+l oop:NEXT:REM Array of command codes 80markerSOF(4)=255:REM Evil people,th is one doesn't fit the pattern 90markerSOI=&D8:REM Start of image 100markerEOI=&D9:REM End of image 110markerSOS=&DA:REM Start of stream 120markerCOM=&FE:REM Comment 130IFBGET#F<>255 ORBGET#F<>markerSOI T HENPRINT"Not a JPEG file":CLOSE#F:END 140: 150REPEAT 160M%=FNgetmarker 170FORloop=0TO15 180 IFmarkerSOF(loop)=M% THENPROCdecod emark(M%):M%=0:loop=16 190NEXT 200IFmarkerSOS=M% ORmarkerEOI=M% THENM %=TRUE 210IFmarkerCOM=M% THENPROCshowcomment: M%=0 220IFM%<>TRUE ANDM%<>0 THENM%=FNskipun known 230UNTILM%=TRUE 240CLOSE#F 250END 260: 270DEFFNtwobyte 280LOCALbyte%:byte%=(BGET#F*256):=byte %+BGET#F 290: 300DEFPROCshowcomment 310LOCALloop% 320length%=FNtwobyte:FORloop%=1TO(leng th%-2):VDU BGET#F:NEXT 330PRINT 340ENDPROC 350: 360DEFPROCdecodemark(M%) 370LOCALloop%,flag% 380length%=FNtwobyte 390precision%=BGET#F 400height%=FNtwobyte 410width%=FNtwobyte 420components%=BGET#F 430PRINT"Compression method: "; 440IFM%=markerSOF(0) THENPRINT"Baselin e":flag%=TRUE 450IFM%=markerSOF(1) THENPRINT"Extende d sequential":flag%=TRUE 460IFM%=markerSOF(2) THENPRINT"Progres sive":flag%=TRUE 470IFM%=markerSOF(3) THENPRINT"Lossles s";:flag%=TRUE 480IFM%=markerSOF(5) THENPRINT"Differe ntial sequential":flag%=TRUE 490IFM%=markerSOF(6) THENPRINT"Differe ntial progressive":flag%=TRUE 500IFM%=markerSOF(7) THENPRINT"Differe ntial lossless":flag%=TRUE 510IFM%=markerSOF(8) THENPRINT"Extende d sequential, arithmetic coding":flag%=T RUE 520IFM%=markerSOF(10) THENPRINT"Progre ssive, arithmetic coding":flag%=TRUE 530IFM%=markerSOF(11) THENPRINT"Lossle ss, arithmetic coding":flag%=TRUE 540IFM%=markerSOF(13) THENPRINT"Differ ential sequential, arithmetic coding":fl ag%=TRUE 550IFM%=markerSOF(14) THENPRINT"Differ ential progressive, arithmetic coding":f lag%=TRUE 560IFM%=markerSOF(15) THENPRINT"Differ ential lossless, arithmetic coding":flag %=TRUE 570IFflag%=FALSE THENPRINT"Unknown" 580PRINT"Size: ";width%" x ";height% 590IFlength%<>(8+(components%*3)) THEN PRINT"Bad SOF marker":CLOSE#F:END 600FORloop%=1TOcomponents% 610 dummy=BGET#F:REM Component ID code 620 dummy=BGET#F:REM H, V sampling fac tors 630 dummy=BGET#F:REM Quantization tabl e number 640NEXT 650ENDPROC 660: 670DEFFNgetmarker 680LOCALbyte%,length%:length%=TRUE 690REPEAT:byte%=BGET#F:length%=length% +1:UNTILbyte%=&FF 700REPEAT:byte%=BGET#F:UNTILbyte%<>&FF 710IFlength%<>0 THENPRINT"Too many byt es found in JPEG header":=TRUE 720=byte% 730: 740DEFFNskipunknown 750LOCALbyte% 760byte%=FNtwobyte:IFbyte%<2 THENPRINT "Bad pointer length in JPEG header":=TRU E 770PTR#F=PTR#F+(byte%-2) 780=FALSE