A Gif decoder/p2

 BBS: Inland Empire Archive
Date: 11-23-92 (22:39)             Number: 283
From: RICH GELDREICH               Refer#: NONE
  To: HARVEY PARISIEN               Recvd: NO  
Subj: A Gif decoder/p2               Conf: (2) Quik_Bas

<<Part 2 Starts Here>>
                X = X + 1: IF X = XEnd THEN GOSUB NextScanLine
            NEXT

            IF NextCode < 4096 THEN
                Prefix(NextCode) = LastCode
                Suffix(NextCode) = LastPixel
                NextCode = NextCode + 1
                IF NextCode > MaxCode AND CodeSize < 12 THEN
                    CodeSize = CodeSize + 1
                    MaxCode = MaxCode * 2 + 1
                END IF
            END IF
            LastCode = Code
        END IF
    END IF
LOOP UNTIL DoneFlag OR Code = EOSCode
BEEP
A$ = INPUT$(1)
END

GetByte: A$ = " ": GET #1, , A$: A = ASC(A$): RETURN

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: PassStep = 2
            END SELECT
        END IF
    ELSE
        Y = Y + 1
    END IF
    X = XStart: Ybase = Y * 320&: DoneFlag = Y > 199
RETURN
GetCode:
    IF BitsIn = 0 THEN GOSUB ReadBufferedByte: LastChar = A: BitsIn = 8
    WorkCode = LastChar \ ShiftOut(BitsIn)
    DO WHILE CodeSize > BitsIn
        GOSUB ReadBufferedByte: LastChar = A
        WorkCode = WorkCode OR LastChar * Powersof2(BitsIn)
        BitsIn = BitsIn + 8
    LOOP
    BitsIn = BitsIn - CodeSize
    Code = WorkCode AND MaxCode
RETURN
ReadBufferedByte:
    IF BlockPointer > BlockSize THEN
        GOSUB GetByte: BlockSize = A
        A$ = SPACE$(BlockSize): GET #1, , A$
        BlockPointer = 1
    END IF
    A = ASC(MID$(A$, BlockPointer, 1)): BlockPointer = BlockPointer + 1
RETURN

<<-Cut Here->>

    There you go!

    This is a more efficient version of my first GIF decoder... It can
decode 320x200x256 GIFs at an acceptable speed(MUCH faster than my
original decoder), but there are still a few optimizations I left out to
keep it simple.

    Rich

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