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 Access Violation after using AutoCrop

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T O P I C    R E V I E W
JonRobertson Posted - Oct 20 2011 : 17:01:59
I'm wondering if I'm doing something wrong. I've implemented scanning using ImageEn and it works great. Then I went back and added a call to AutoCrop after the call to Acquire:

ImageEnMView.MIO.Acquire();
ImageEnMView.Proc.AutoCrop(10, CreateRGB(0, 0, 0));

Now I get an Access Violation when freeing ImageEnMView. Simplified stack:

  hyieutils.TIEBitmap.FreeAllMaps
  hyieutils.TIEBitmap.FreeImage(True)
  hyieutils.TIEBitmap.Destroy
  imageenproc.TImageEnProc.Destroy
  iemview.TImageEnMView.Destroy

At this point, when FreeAllMaps is called, fScanlinesToUnMapPtr is nil. So accessing fScanlinesToUnMapPtr.Count causes an Access Violation. Best I can tell, the problem is that TImageEnProc.fIEBitmap is being freed twice.

TImageEnMView.GetImageEnProc creates TImageEnProc and attaches itself to fImageEnProc. At that point,
fImageEnProc.FIEBitMapCreated = True
fImageEnProc.fBitMap is not assigned
fImageEnproc.fIEBitmap is assigned

Then fSelectedItem = -1 so SelectedImage is assigned zero. This causes TImageEnProc.OnBitmapChange to be called.

Both fImageEnView and fIEBitmap are assigned, so fIEBitmap is assigned frmImageEnView.IEBitmap.

Now, fImageEnView should own frmImageEnView.Bitmap. But TImageEnProc.fIEBitmapCreated is not set to False.

When I free the ImageEnMView that I created, the embedded TImageEnProc is destroyed as well. The bottom of its constructor is 

  if fIEBitmapCreated then
    FreeAndNil(fIEBitmap);

Since fIEBitmapCreated is still True, the bitmap is freed twice.

I added a line to OnBitmapChange to set fIEBitmapCreated := False. But I'm not sure this is the correct fix:

procedure TImageEnProc.OnBitmapChange(Sender: TObject; destroying: boolean);
begin
  if destroying then
  begin
    fImageEnView := nil;
  end
  else if assigned(fImageEnView) then
  begin
    if assigned(fIEBitmap) then
    begin
      fIEBitmap := fImageEnView.IEBitmap;
      fBitmap := nil; // both fBitmap and fIEBitmap aren't allowed if not encapsulated
      fIEBitmapCreated := False; // ADDED THIS LINE!
    end
    else if assigned(fBitmap) then
    begin
      fBitmap := fImageEnView.Bitmap;
      if fIEBitmapCreated then
        fIEBitmap.EncapsulateTBitmap(fBitmap, true);
    end;

  end;
end;
3   L A T E S T    R E P L I E S    (Newest First)
fab Posted - Oct 25 2011 : 11:56:22
It is ok.
JonRobertson Posted - Oct 25 2011 : 09:41:36
Thanks. I implemented a loop in case there were multiple pages. Is there anything wrong with doing this:

ImageEnMView.MIO.Acquire();

for iLoop := 0 to ImageEnMView.ImageCount - 1 do begin
  ImageEnMView.SelectedImage := iLoop;
  ImageEnMView.Proc.AutoCrop(10, CreateRGB(0, 0, 0));
end;
fab Posted - Oct 24 2011 : 00:23:27
Please make sure that an image is selected before call ImageEnMView.Proc... methods, because these methods refers to the currently selected image.
You should write, for example:
ImageEnMView.MIO.Acquire();
if ImageEnMView.ImageCount > 0 then
begin
  // select last image
  ImageEnMView.SelectSeek(iskLast);
  ImageEnMView.Proc.AutoCrop(10, CreateRGB(0, 0, 0));
end;