Synchronizing Thread for image processing Delphi

Question

I must processing one image in main form , but the processing speed is low for inc speed i use thread...

My thread code :

type
  TPaintThread = class(TThread)
    Source,Mask :TBitmap ;
    image : TImage;
    public
       procedure SetAll(src,msk:TBitmap;img:TImage);
    private
     procedure  DoWritePix;
    var
       mBit : TBitmap ;

    protected
      procedure Execute; override;
  end;

implementation

procedure TPaintThread.SetAll(src: TBitmap; msk: TBitmap; img: TImage);
begin
      Source := src ;
      mask := msk ;
      img := img ;
      mBit := TBitmap.Create ;
end;

procedure TPaintThread.DoWritePix;
begin
  image.Picture.Bitmap := mBit ;
end;

procedure TPaintThread.Execute;
var
    i: Integer;
    j: Integer;
begin
  mBit.Width := Source.Width ;
  mBit.Height := Source.Height ;
  for i := 1 to Source.Width do
    for j := 1 to Source.Width do
    begin
      // my processing event
    end;
    // result := mBit ;
    // write on form image 
    Synchronize(DoWritePix);
end;

and I'm use it in on timer :

procedure TForm1.tmr1Timer(Sender: TObject);
var
    pThread  : TPaintThread ;
begin
  pThread := TPaintThread.Create(True) ;
  pThread.SetAll(MyBmp,mask,img1);
  pThread.Resume ;
  pThread.FreeOnTerminate := True ;
end;

but I have error in DoWritePix in run time :

First chance exception at $005A81DE. Exception class $C0000005 with message 'access violation at 0x005a81de: read of address 0x000001b8'. Process myexe.exe (6032)
First chance exception at $754E9617. Exception class EAccessViolation with message 'Access violation at address 005A81DE in module 'myexe.exe'. Read of address 000001B8'. Process myexe.exe (6032) 

My question: Is that way correct for edit image in main form or not ? if not what is correct way access and write on thread? if yes how I can solve problem ?

Answer 1

This code is wrong:

procedure TPaintThread.SetAll(src: TBitmap; msk: TBitmap; img: TImage);
begin
  Source := src ;
  mask := msk ;
  img := img ; // OOPS!
  mBit := TBitmap.Create ;
end;

When you write img := img; you are doing nothing – that is a no-op. You meant to write:

image := img;

And that's why image is nil in DoWritePix which explains the access violation.

Don't be helpless when you face a runtime error. Run the code under the debugger and let the debugger tell you which variable is not initialised.