When loading a JPEG from a file, sometimes it will load fine, but contain a corrupt JPEG image. When an attempt is made to draw the image, an exception will be raised. Quite often, the programmer may try to suppress errors with a procedure such as:

procedure TForm .btnOpenDefaultClick(Sender: TObject);
begin
  if OpenPictureDialog .Execute then
  try
    Image .Picture.LoadFromFile(OpenPictureDialog .FileName);
  except end;
end;

But, unexpectedly, JPEG's sometimes display an exception message and it appears that the creator of the JPEG unit hard-coded a ShowMessage to display the exception. The problem is that the

LoadFromFile

The way around this problem is to explicitly control when any possible errors happen. You can capture any errors that happen in the loading of the image, and/or in the drawing of the image.

Below is an example of how to do this. The project consists of a unit (called MainFrm.pas). It has a TImage and an OpenPictureDialog placed on the form. Two buttons are also placed on it, one named

btnOpenCatch

btnOpenDefault

btnOpenCatchClick

except

btnOpenDefaultClick

To create a corrupt JPEG that displays this error, open a JPEG image in a hex editor and change the first few bytes or the last few bytes.

unit MainFrm;

interface

uses Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs, ExtDlgs, ExtCtrls, StdCtrls, JPEG;

type TForm = class(TForm) btnOpenCatch: TButton; Image : TImage; OpenPictureDialog : TOpenPictureDialog; btnOpenDefault: TButton; procedure btnOpenCatchClick(Sender: TObject); procedure btnOpenDefaultClick(Sender: TObject); private { Private declarations } public { Public declarations } end;

var Form : TForm ;

implementation

{$R *.DFM}

procedure TForm .btnOpenCatchClick(Sender: TObject); var Picture: TPicture; Bitmap: TBitmap; begin if OpenPictureDialog .Execute then begin Picture := TPicture.Create; try // Try to load the picture from a file. // I'm wrapping it in a try..except to capture // any exceptions that happen in loading the image from // a stream. try Picture.LoadFromFile(OpenPictureDialog .FileName); except on E: EInvalidGraphic do begin ShowMessage('Capturing Load Error: ' + E.Message); Exit; // Exit the procedure end; end; Bitmap := TBitmap.Create; try try // Assign the Graphic to the Bitmap; any drawing errors/exceptions // will happen here, and will be captured and shown to the user Bitmap.Assign(Picture.Graphic); except // Eat invalid graphic messages (which may be raised // when a jpeg is being drawn) on E: EInvalidGraphic do begin ShowMessage('Capturing Assign (Drawing) Error: ' + E.Message); Exit; // Exit the procedure end; end; // Now draw the picture on the screen's TImage Image .Picture.Bitmap.Assign(Bitmap); finally Bitmap.Free; end; finally Picture.Free; end; end; // OpenPictureDialog .Execute end;

procedure TForm .btnOpenDefaultClick(Sender: TObject); begin if OpenPictureDialog .Execute then try Image .Picture.LoadFromFile(OpenPictureDialog .FileName); except end; end;

end.

Another thing to notice is that the OpenPictureDialog does not contain this fix. It will display some JPEG error messages when you click on a corrupt JPEG and it attempts to display a preview image.