Процедуры быстрого получения размера изображения из файлов JPG, GIF, PNG

unit ImgSize;

interface

uses
  Classes;

procedure GetJPGSize(const sFile: string; var wWidth, wHeight: word);
procedure GetPNGSize(const sFile: string; var wWidth, wHeight: word);
procedure GetGIFSize(const sGIFFile: string; var wWidth, wHeight: word);

implementation

uses
  SysUtils;

function ReadMWord(f: TFileStream): word;
  type TMotorolaWord = record
  case byte of
    0: (Value: word);
    1: (Byte1, Byte2: byte);
  end;
var
  MW: TMotorolaWord;
begin
  { It would probably be better to just read these two bytes in normally }
  { and then do a small ASM routine to swap them. But we aren't talking }
  { about reading entire files, so I doubt the performance gain would be }
  { worth the trouble.}
  f.read(MW.Byte2, SizeOf(Byte));
  f.read(MW.Byte1, SizeOf(Byte));
  Result := MW.Value;
end;

procedure GetJPGSize(const sFile: string; var wWidth, wHeight: word);
const
  ValidSig : array[0..1] of byte = ($FF, $D8);
  Parameterless = [$01, $D0, $D1, $D2, $D3, $D4, $D5, $D6, $D7];
var
  Sig: array[0..1] of byte;
  f: TFileStream;
  x: integer;
  Seg: byte;
  Dummy: array[0..15] of byte;
  Len: word;
  ReadLen: LongInt;
begin
  FillChar(Sig, SizeOf(Sig), #0);
  f := TFileStream.Create(sFile, fmOpenRead);
  try
    ReadLen := f.read(Sig[0], SizeOf(Sig));
    for x := Low(Sig) to High(Sig) do
      if Sig[x] <> ValidSig[x] then
        ReadLen := 0;
    if ReadLen > 0 then
    begin
      ReadLen := f.read(Seg, 1);
      while (Seg = $FF) and (ReadLen > 0) do
      begin
        ReadLen := f.read(Seg, 1);
        if Seg <> $FF then
        begin
          if (Seg = $C0) or (Seg = $C1) then
          begin
            ReadLen := f.read(Dummy[0], 3);
            { don't need these bytes }
            wHeight := ReadMWord(f);
            wWidth := ReadMWord(f);
          end
          else
          begin
            if not (Seg in Parameterless) then
            begin
              Len := ReadMWord(f);
              f.Seek(Len-2, 1);
              f.read(Seg, 1);
            end
            else
              Seg := $FF;
            { Fake it to keep looping. }
          end;
        end;
      end;
    end;
  finally
    f.Free;
  end;
end;

procedure GetPNGSize(const sFile: string; var wWidth, wHeight: word);
type TPNGSig = array[0..7] of byte;
const
  ValidSig: TPNGSig = (137,80,78,71,13,10,26,10);
var
  Sig: TPNGSig;
  f: tFileStream;
  x: integer;
begin
  FillChar(Sig, SizeOf(Sig), #0);
  f := TFileStream.Create(sFile, fmOpenRead);
  try
    f.read(Sig[0], SizeOf(Sig));
    for x := Low(Sig) to High(Sig) do
      if Sig[x] <> ValidSig[x] then
        exit;
    f.Seek(18, 0);
    wWidth := ReadMWord(f);
    f.Seek(22, 0);
    wHeight := ReadMWord(f);
  finally
    f.Free;
  end;
end;

procedure GetGIFSize(const sGIFFile: string; var wWidth, wHeight: word);
type
  TGIFHeader = record
    Sig: array[0..5] of char;
    ScreenWidth, ScreenHeight: word;
    Flags, Background, Aspect: byte;
  end;
  TGIFImageBlock    = record
    Left, Top, Width, Height: word;
    Flags: byte;
  end;
var
  f: file;
  Header: TGifHeader;
  ImageBlock: TGifImageBlock;
  nResult: integer;
  x: integer;
  c: char;
  DimensionsFound: boolean;
begin
  wWidth := 0;
  wHeight := 0;
  if sGifFile = '' then
    exit;
  {$I-}
  FileMode := 0; { read-only }
  AssignFile(f, sGifFile);
  reset(f, 1);
  if IOResult <> 0 then {Could not open file }
    exit; { Read header and ensure valid file. }
  BlockRead(f, Header, SizeOf(TGifHeader), nResult);
  if (nResult <> SizeOf(TGifHeader)) or (IOResult <> 0)
  or (StrLComp('GIF', Header.Sig, 3) <> 0) then
  begin { Image file invalid }
    close(f);
    exit;
  end; { Skip color map, if there is one }
  if (Header.Flags and $80) > 0 then
  begin
    x := 3 * (1 shl ((Header.Flags and 7) + 1));
    Seek(f, x);
    if IOResult <> 0 then
    begin { Color map thrashed }
      close(f);
      exit;
    end;
  end;
  DimensionsFound := False;
  FillChar(ImageBlock, SizeOf(TGIFImageBlock), #0);
  { Step through blocks. }
  BlockRead(f, c, 1, nResult);
  while (not EOF(f)) and (not DimensionsFound) do
  begin
    case c of
      ',': { Found image }
      begin
        BlockRead(f, ImageBlock, SizeOf(TGIFImageBlock), nResult);
        if nResult <> SizeOf(TGIFImageBlock) then
        begin { Invalid image block encountered }
          close(f);
          exit;
        end;
        wWidth := ImageBlock.Width;
        wHeight := ImageBlock.Height;
        DimensionsFound := True;
      end;
      'y' : { Skip }
      begin
        { NOP }
      end;
      { nothing else. just ignore }
    end;
    BlockRead(f, c, 1, nResult);
  end;
  close(f);
  {$I+}
end;

end.