Load Jpg/Gif/Bitmap and convert to Bitmap

Question

Hi, I have to load an image from an XML file. There is no information in the XML file about whether the image is JPG/GIF/BMP. After loading the image, I need to convert it to Bitmap.

Does anyone have any clue how to convert images to Bitmap without knowing the actual file format? I'm using Delphi 2007/2009

Thank you.

Answer 1

Delphi 2009 comes with built in support for JPEG, BMP, GIF and PNG.

For earlier versions of Delphi you may need to find third party implementations for PNG and GIF, but in Delphi 2009 you simply add the Jpeg, pngimage and GIFImg units to your uses clause.

If the file has an extension you can use the following code, as noted by others the TPicture.LoadFromFile looks at the extensions registered by the inherited classes to determine which image to load.

uses
  Graphics, Jpeg, pngimage, GIFImg;

procedure TForm1.Button1Click(Sender: TObject);
var
  Picture: TPicture;
  Bitmap: TBitmap;
begin
  Picture := TPicture.Create;
  try
    Picture.LoadFromFile('C:\imagedata.dat');
    Bitmap := TBitmap.Create;
    try
      Bitmap.Width := Picture.Width;
      Bitmap.Height := Picture.Height;
      Bitmap.Canvas.Draw(0, 0, Picture.Graphic);
      Bitmap.SaveToFile('C:\test.bmp');
    finally
      Bitmap.Free;
    end;
  finally
    Picture.Free;
  end;
end;

If the file extension is not known one method is to look at the first few bytes to determine the image type.

procedure DetectImage(const InputFileName: string; BM: TBitmap);
var
  FS: TFileStream;
  FirstBytes: AnsiString;
  Graphic: TGraphic;
begin
  Graphic := nil;
  FS := TFileStream.Create(InputFileName, fmOpenRead);
  try
    SetLength(FirstBytes, 8);
    FS.Read(FirstBytes[1], 8);
    if Copy(FirstBytes, 1, 2) = 'BM' then
    begin
      Graphic := TBitmap.Create;
    end else
    if FirstBytes = #137'PNG'#13#10#26#10 then
    begin
      Graphic := TPngImage.Create;
    end else
    if Copy(FirstBytes, 1, 3) =  'GIF' then
    begin
      Graphic := TGIFImage.Create;
    end else
    if Copy(FirstBytes, 1, 2) = #$FF#$D8 then
    begin
      Graphic := TJPEGImage.Create;
    end;
    if Assigned(Graphic) then
    begin
      try
        FS.Seek(0, soFromBeginning);
        Graphic.LoadFromStream(FS);
        BM.Assign(Graphic);
      except
      end;
      Graphic.Free;
    end;
  finally
    FS.Free;
  end;
end;

Answer 2

You can't use TPicture.LoadFromFile if you don't know what format the graphic has, as this method uses the file extension to determine which of the registered graphic formats needs to be loaded. There's a reason that there is no matching TPicture.LoadFromStream method.

An external library which can examine data and determine the graphic format at runtime would be the best solution. You could use the efg page as a starting point of your research.

A quick and dirty solution is to try the few formats you need to handle until one succeeds:

function TryLoadPicture(const AFileName: string; APicture: TPicture): boolean;
const
  GraphicClasses: array[0..3] of TGraphicClass = (
    TBitmap, TJPEGImage, TGIFImage, TPngImage);
var
  FileStr, MemStr: TStream;
  ClassIndex: integer;
  Graphic: TGraphic;
begin
  Assert(APicture <> nil);
  FileStr := TFileStream.Create('D:\Temp\img.dat', fmOpenRead);
  try
    MemStr := TMemoryStream.Create;
    try
      MemStr.CopyFrom(FileStr, FileStr.Size);
      // try various
      for ClassIndex := Low(GraphicClasses) to High(GraphicClasses) do begin
        Graphic := GraphicClasses[ClassIndex].Create;
        try
          try
            MemStr.Seek(0, soFromBeginning);
            Graphic.LoadFromStream(MemStr);
            APicture.Assign(Graphic);
            Result := TRUE;
            exit;
          except
          end;
        finally
          Graphic.Free;
        end;
      end;
    finally
      MemStr.Free;
    end;
  finally
    FileStr.Free;
  end;
  Result := FALSE;
end;

Edit:

The GraphicEx library has an example convert that uses

GraphicClass := FileFormatList.GraphicFromContent(...);

to determine the graphic format. This seems very similar to the VB6 way of doing this that you mention. Maybe you can use this library for your purpose.

Answer 3

I've found a simpler way! It loads JPG/GIF/BMP etc. files automatically without even knowing/checking the file format, and convert that accordingly. It worked for me perfectly.

Sharing it here :)

Uses
Classes, ExtCtrls, Graphics, axCtrls;

Procedure TForm1.Button1Click(Sender: TObject);
Var
     OleGraphic               : TOleGraphic;
     fs                       : TFileStream;
     Source                   : TImage;
     BMP                      : TBitmap;
Begin
     Try
          OleGraphic := TOleGraphic.Create; {The magic class!}

          fs := TFileStream.Create('c:\testjpg.dat', fmOpenRead Or fmSharedenyNone);
          OleGraphic.LoadFromStream(fs);

          Source := Timage.Create(Nil);
          Source.Picture.Assign(OleGraphic);

          BMP := TBitmap.Create; {Converting to Bitmap}
          bmp.Width := Source.Picture.Width;
          bmp.Height := source.Picture.Height;
          bmp.Canvas.Draw(0, 0, source.Picture.Graphic);

          image1.Picture.Bitmap := bmp; {Show the bitmap on form}
     Finally
          fs.Free;
          OleGraphic.Free;
          Source.Free;
          bmp.Free;
     End;
End;