INSTALL @lib$+"SORTLIB"
      Sort% = FN_sortinit(0,0)

      Width% = 200
      Height% = 200

      VDU 23,22,Width%;Height%;8,16,16,128
      *display c:\lenagrey

      DIM hist%(255), idx%(255)
      FOR i% = 0 TO 255 : idx%(i%) = i% : NEXT

      REM Build histogram:
      FOR y% = 0 TO Height%-1
        FOR x% = 0 TO Width%-1
          l% = FNgetpixel(x%,y%) AND &FF
          hist%(l%) += 1
        NEXT
      NEXT y%

      REM Sort histogram:
      C% = 256
      CALL Sort%, hist%(0), idx%(0)

      REM Find median:
      total% = SUM(hist%())
      half% = 0
      FOR i% = 0 TO 255
        half% += hist%(i%)
        IF half% >= total%/2 THEN
          median% = idx%(i%)
          EXIT FOR
        ENDIF
      NEXT

      REM Display black & white version:
      FOR y% = 0 TO Height%-1
        FOR x% = 0 TO Width%-1
          l% = FNgetpixel(x%,y%) AND &FF
          IF l% > median% THEN
            PROCsetpixel(x%,y%,255,255,255)
          ELSE
            PROCsetpixel(x%,y%,0,0,0)
          ENDIF
        NEXT
      NEXT y%
      END

      DEF PROCsetpixel(x%,y%,r%,g%,b%)
      COLOUR 1,r%,g%,b%
      GCOL 1
      LINE x%*2,y%*2,x%*2,y%*2
      ENDPROC

      DEF FNgetpixel(x%,y%)
      LOCAL col%
      col% = TINT(x%*2,y%*2)
      SWAP ?^col%,?(^col%+2)
      = col%