jpeg文件头里，哪些是保存图象大小信息的呢？

[hjx000]

jpeg文件头里，哪些是保存图象大小信息的呢？

[hjx000]

也就是如何通过jpeg文件头来判断文件的存储大小？

[S.F.]

可以参考：
http://www.dhxy.com/school/filetype/2004082519375535767.html [简单jpeg编码程序]
http://www.588188.com/netbook/tech/fileformat/html/fileformat.ohchina.18.htm [JPEG 简易文档 V2.1]

[sdzeng]

转贴自超级猛料
----------------------------------
获取JPEG的宽度、高度等信息
{ Copyright Kingron 2002 }
function GetJPEGSize(FileName: string; var Width, Height: WORD): Boolean;
const
JPEG_FLAG_BEGIN = $D8FF;
JPEG_FLAG_END = $D9FF;
JPEG_FRAME = $C0FF;
var
FS: TFileStream;
Flag1, Flag2: WORD;
B: Byte;

procedure SeekForFrame;
type
TSegHeader = packed record
Flag: WORD;
LenHi, LenLo: Byte;
end;
var
Seg: TSegHeader;
begin
with Seg, FS do
repeat
Read(Seg, SizeOf(Seg));
if Flag <> JPEG_FRAME then
Position := Position + MakeWord(LenLo, LenHi) - 2;
until (Position >= Fs.Size) or (Seg.Flag = JPEG_FRAME);
end;

begin
FS := TFileStream.Create(FileName, fmOpenRead);
try
{ JPEG 文件开头必须为 FF D8,文件尾必须为 FF D9 }
FS.Read(Flag1, SizeOf(Flag1));
FS.Position := FS.Size - 2;
FS.Read(Flag2, SizeOf(Flag2));
Result := (Flag1 = JPEG_FLAG_BEGIN) and (Flag2 = JPEG_FLAG_END);
if not Result then exit; { 不是合法的JPEG文件则退出 }

FS.Position := 2;
SeekForFrame; { 寻找JPEG的Frame段，即图像数据区 }
FS.Read(B, SizeOf(B)); { Frame段段头后第一个Byte为数据精度 }

FS.Read(B, SizeOf(B)); { 高度高字节 }
WordRec(Height).Hi := B;
FS.Read(B, SizeOf(B)); { 高度低字节 }
WordRec(Height).Lo := B;

FS.Read(B, SizeOf(B)); { 宽度高字节 }
WordRec(Width).Hi := B;
FS.Read(B, SizeOf(B)); { 宽度低字节 }
WordRec(Width).Lo := B;
finally
FS.Free;
end;
end;

procedure TForm1.Button1Click(Sender: TObject);
var
W, H: Word;
begin
with OpenDialog1 do
if Execute then
if GetJPEGSize(FileName, W, H) then
Caption := Format('Yes,W:%d,H:%d', [W, H])
else Caption := 'No';
end;