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【Delphi】返回Gif图片的尺寸
作者:jellyvice / 发布于2013/12/3/ 1362
function GetGIFSize(const FileName: string): Types.TSize;
type
 // GIF header record
 TGIFHeader = packed record
  Sig: array[0..5] of AnsiChar;   // signature bytes
  ScreenWidth, ScreenHeight: Word; // logical screen width and height
  Flags: Byte;           // various flags
  Background: Byte;         // background colour index
  Aspect: Byte;           // pixel aspect ratio
 end;
 // GIF image block header record
 TGIFImageBlock = packed record
  Left, Top: Word;   // image top left
  Width, Height: Word; // image dimensions
  Flags: Byte;     // flags and local colour table size
 end;
const
 cSignature: PAnsiChar = 'GIF'; // gif image signature
 cImageSep = $2C;        // image separator byte
var
 FS: Classes.TFileStream;   // stream onto gif file
 Header: TGIFHeader;      // gif header record
 ImageBlock: TGIFImageBlock;  // gif image block record
 BytesRead: Integer;      // bytes read in a block read
 Offset: Integer;       // file offset to seek to
 B: Byte;           // a byte read from gif file
 DimensionsFound: Boolean;   // flag true if gif dimensions have been read
begin
 Result.cx := 0;
 Result.cy := 0;
 if (FileName = '') or not SysUtils.FileExists(FileName) then
  Exit;
 FS := Classes.TFileStream.Create(
  FileName, SysUtils.fmOpenRead or SysUtils.fmShareDenyNone
 );
 try
  // Check signature
  BytesRead := FS.Read(Header, SizeOf(Header));
  if (BytesRead <> SizeOf(TGIFHeader)) or
   (SysUtils.StrLComp(cSignature, Header.Sig, 3) <> 0) then
   // Invalid file format
   Exit;
  // Skip colour map, if there is one
  if (Header.Flags and $80) > 0 then
  begin
   Offset := 3 * (1 shl ((Header.Flags and 7) + 1));
   if Offset >= FS.Size then
    Exit;
   FS.Seek(Offset, Classes.soFromBeginning);
  end;
  DimensionsFound := False;
  FillChar(ImageBlock, SizeOf(TGIFImageBlock), #0);
  // Step through blocks
  FS.Read(B, SizeOf(B));
  while (FS.Position < FS.Size) and (not DimensionsFound) do
  begin
   if B = cImageSep then
   begin
    // We have an image block: read dimensions from it
    BytesRead := FS.Read(ImageBlock, SizeOf(ImageBlock));
    if BytesRead <> SizeOf(TGIFImageBlock) then
     // Invalid image block encountered
     Exit;
    Result.cx := ImageBlock.Width;
    Result.cy := ImageBlock.Height;
    DimensionsFound := True;
   end;
   FS.Read(B, SizeOf(B));
  end;
 finally
  FS.Free;
 end;
end;
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