type
TGIFHdr array[6] of Char

data
0 TGIFHdr Hdr

%$IF Hdr='GIF89a';
const
  Ver = 89;
descr ('GIF - GRAPHICS INTERCHANGE FORMAT(sm) Version 89a.',NL)
%$ELSIF Hdr='GIF87a';
const
  Ver=87;
descr ('GIF - GRAPHICS INTERCHANGE FORMAT(sm) Version 87a.',NL)
%$ELSE
assert 0; //fail
%$END

descr ('Info Source: GIF89a.zip at www.wotsit.org',NL)

type
U2 num+(2)

TRGB struc
  Byte R
  Byte G
  Byte B
ends

TSubBlock struc
  Byte L
  raw[@.L] D
ends

TSubBlocks array of TSubBlock ?@.L=0!Byte;

TColorTable(LSz) array[1 shl(@:LSz+1)] of TRGB
TColorTableF(On,LSz) case @:On of
  1: TColorTable(@:LSz)
endc

type bit
TBit num+(1)
TBit2 num+(2)
TBit3 num+(3)

set byteorder rev

TLSDFlags struc
  TBit GlobalC //Global Color Table Flag
  TBit3 ColorRes //Color Resolution
  TBit Sort//Sort Flag
  TBit3 GlobalSize //Size of Global Color Table
ends

TImgFlags struc
  TBit LocalC //Local Color Table Flag
  TBit Interlace //Interlace Flag
  TBit Sort //Sort Flag
  TBit2 Rsrv //Reserved
  TBit3 LocSize //Size of Local Color Table
ends

set byteorder norm

%$IF Ver>=89;

type bit

TGrCtlDisposal enum TBit3 (
  None=0, //No disposal specified. The decoder is not required to 
          //take any action.
  No=1, //Do not dispose. The graphic is to be left in place.
  RestoreBk=2, //Restore to background color. The area used by the
              //graphic must be restored to the background color.
  RestorePrev=3 //Restore to previous. The decoder is required to
                //restore the area overwritten by the graphic with
                //what was there prior to rendering the graphic.
)

set byteorder rev

TGrCtlF struc
  TBit3 Rsrv //Reserved
  TGrCtlDisposal Disposal //Disposal Method
  TBit UserInput //User Input Flag
  TBit Transparent //Transparent Color Flag
ends

set byteorder norm

type

TCtlExtTag enum byte (
  GrCtl=0xF9, //Graphic Control Extension
  Comment=0xFE, //Comment Extension
  PlainText=0x01, //Plain Text Extension
  App=0xFF //Application Extension
);

TGrCtlData struc
  byte Sz
  TGrCtlF F //<Packed Fields>
  U2 Delay //Delay Time (1/100 sec)
  byte TransparentColor //Transparent Color Index
  byte TermB //Block Terminator
ends:assert[@.Sz=4,@.TermB=0]

TStrTbl array of Str ?@[0]=0!Byte;

TPlainTextData struc
  byte Sz
  U2 Left //Text Grid Left Position
  U2 Top //Text Grid Top Position
  U2 W //Text Grid Width
  U2 H //Text Grid Height
  Byte ChW //Character Cell Width
  Byte ChH //Character Cell Height
  Byte TextColor //Text Foreground Color Index
  Byte BkColor //Text Background Color Index
  TStrTbl Text //Plain Text Data
ends:assert[@.Sz=12]

TNetscapeAppData struc
  byte Sz
  byte B1 //Should be 1
  U2 NIter //The number of iterations (0-infinite)
  byte TermB //Block Terminator
ends:assert[@.Sz=3,@.TermB=0,@.B1=1]

TAppData struc
  byte Sz
  array[@.Sz] of Char AppID //Application Identifier (8 Bytes)+
                            //Appl. Authentication Code (3 Bytes)
  case @.AppID of
  'NETSCAPE2.0': TNetscapeAppData
  else TSubBlocks
  endc D
ends:assert[@.Sz=11]

TCtlExtRec struc
  byte ExtMark //Extension Introducer
  TCtlExtTag Tag
  case @.Tag of
    GrCtl: TGrCtlData
    Comment: TStrTbl
    PlainText: TPlainTextData
    App: TAppData
  else TSubBlocks 
  endc D
ends/*:[@.D:Size=@.Sz]*/:assert[@.ExtMark=0x21]

TCtlExtList array of TCtlExtRec ? not @:assert!void;

%$ELSE

type
TCtlExtList void  

%$END

type
//Logical Screen Descriptor.
TLogScrDescr struc
  U2 W //Logical Screen Width
  U2 H //Logical Screen Height
  TLSDFlags F //Packed Fields
  byte BkC //Background Color Index
  byte AspectRatio //Pixel Aspect Ratio
ends

//Image Descriptor
TImgDescr struc
  Byte Sep //Image Separator
  U2 Left //Image Left Position
  U2 Top //Image Top Position
  U2 W //Image Width
  U2 H //Image Height
  TImgFlags F //<Packed Fields>
ends:assert[@.Sep=0x2C]

TImgData struc
  TCtlExtList Ext
  TImgDescr Dsc
  TColorTableF(@.Dsc.F.LocalC,@.Dsc.F.LocSize) CTbl
  Byte LZWMinCode //LZW Minimum Code Size
  TSubBlocks ImgData //Image Data
ends:assert[@.Dsc:assert]

TImgDataTbl array of TImgData ?not @:assert!Byte/*=0x3B*/;

TGifStream struc
  TLogScrDescr LSD
  TColorTableF(@.LSD.F.GlobalC,@.LSD.F.GlobalSize) GColorTbl
  TImgDataTbl Img
ends

data
Hdr:Size TGifStream Stm

Other specifications.

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