===========================================================================
 BBS: The Abacus * HST/DS * Potterville, MI
Date: 06-08-93 (03:06)             Number: 204
From: RICH GELDREICH               Refer#: NONE
  To: CALVIN FRENCH                 Recvd: NO  
Subj: Gif Loader/4                   Conf: (35) Quick Basi
---------------------------------------------------------------------------
'>>> Start of page 4.

'BSAVE "pic.bas", 0, 64768

'Load images saved with the above code with this:
'DEF SEG= &HA000
'BLOAD "Pic.Bas"
'OUT &H3C8, 0
'FOR a = 0 To 767
'     OUT &H3C9, Peek(a+ 64000)
'NEXT

BEEP: DO: LOOP WHILE INKEY$ <> "": DO: LOOP UNTIL INKEY$ <> ""
END

'Slowly reads one byte from the GIF file...
GetByte: A$ = " ": GET #1, , A$: A = ASC(A$): RETURN

'Moves down one scanline. If the GIF is interlaced, then the number
'of scanlines skipped is based on the current pass.
NextScanLine:
    IF Interlaced THEN
        y = y + PassStep
        IF y >= YEnd THEN
            PassNumber = PassNumber + 1
            SELECT CASE PassNumber
            CASE 1: y = 4: PassStep = 8
            CASE 2: y = 2: PassStep = 4
            CASE 3: y = 1:ŠassStep = 2
            END SELECT
        END IF
    ELSE
        y = y + 1
    END IF
    X = XStart: YBase = y * 320&
RETURN

'Reads a multibit code from the data stream. Look ma, see how
'simple it is now!
GetCode:
    WorkCode = LastChar \ ShiftOut(BitsIn)
  'Loop while more bits are needed.
    DO WHILE CodeSize > BitsIn
'Reads a byte from the LZW data stream. Since the data stream is
'blocked, a check is performed for the end of the current block
'before each byte is fetched.
        IF BlockPointer > BlockSize THEN
          'Retrieve block's length
            GOSUB GetByte: BlockSize = A
            A$ = SPACE$(BlockSize): GET #1, , A$
            BlockPointer = 1
        END IF
      'Yuck, ASC() and MID$() aren't that fast.
        LastChar = ASC(MID$(A$, BlockPointer, 1))
        BlockPointer = BlockPointer + 1
      'Append 8 more bits to the input buffer
        WorkCode = WorkCode OR LastChar * Powersof2(BitsIn)
        BitsIn = BitsIn + 8
    LOOP
  'Take away x number of bits.
    BitsIn = BitsIn - CodeSize
  'Return code to caller.
    Code = WorkCode AND MaxCode
RETURN


'______O_/__________________| SNIP |__________________\_O______
'      O \                  | HERE |                  / O

Have fun Calvin!

    Rich

--- MsgToss 2.0b
 * Origin: Computer Co-Op - Voorhees, NJ | Ted Hare (1:266/29)
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