merge paint results in thread bitmap painting - multithreading

Question

I want to speed up painting a bitmap, therefore I designed a class like BITMAP THREAD CLASS. Once the individual painting of a partial image is finished I want to merge all image in the Thread.done procedure
My code goes like this
type
TbmpthreadForm = class(TForm)
.....
THreadImage: TImage;
procedure Button_threadstartClick(Sender: TObject);
procedure FormCreate(Sender: TObject);
procedure FormClose(Sender: TObject; var Action: TCloseAction);
private
{ Private-Deklarationen }
procedure ThreadDone(Sender: TObject);
public
{ Public-Deklarationen }
fserver, fdatabasename, ftablename: String;
global_thread_counter: Integer;
XPixel, YPixel: Integer;
Masterbitmap: TBitmap;
end;
var
bmpthreadForm: TbmpthreadForm;
implementation
{$R *.dfm}
procedure TbmpthreadForm.ThreadDone(Sender: TObject);
begin
dec(global_thread_counter);
MyStatusBar.SimpleText := 'Thread Count ->' + IntToStr(global_thread_counter);
Masterbitmap.Canvas.Lock;
with (Sender as TPaintBitmapThread) do
begin
bitblt(Masterbitmap.Canvas.handle, 0, 0, XPixel, YPixel,
bitmap.Canvas.handle, 0, 0, srcand);
THreadImage.Picture.Bitmap.Assign(Masterbitmap);
// lets see local tthread intermediate results and save it to HD
THreadImage.Picture.Bitmap.SaveToFile('c:\temp\myimage' + IntToStr(Index)
+ '.bmp');
end;
Masterbitmap.Canvas.UnLock;
if (global_thread_counter = 0) then
begin
...
end;
end;
procedure TbmpthreadForm.Button_threadstartClick(Sender: TObject);
var
.....
begin
index_max := 2000000;
threadCounter := 10;
Indexdelta := round(index_max / threadCounter);
///
///
....
Masterbitmap.Width := XPixel;
Masterbitmap.Height := YPixel;
for i := 0 to threadCounter - 1 do
begin
n := i * Indexdelta;
m := (i + 1) * Indexdelta;
// just a test sql string ....
sqlstr := 'select * from Mytable where objectindex <' + IntToStr(m) +
' and Objectindex >' + IntToStr(n);
aPaintBitmapThread := TPaintBitmapThread.Create(XPixel, YPixel, ...... , fserver, fdatabasename, ftablename,
sqlstr, i);
aPaintBitmapThread.OnTerminate := ThreadDone;
Memo1.Lines.Add('start thread->' + IntToStr(i));
inc(global_thread_counter);
end;
end;
The Thread.done design follows a previous topic here on SO ( reference question
As the resulting image/Masterbitmap looks a bit different from run to run , I guess my approach is not thread safe design for copy Thread bmp content into the masterbitmap in the VCL mainform,
I can not see any error in my code, what is wrong ????
additional question
Q1 : fbitmap inside TPaintBitmapThread is created inside the Thread.create procedure, for TAdoconnection I found the comment, it should be created inside the thread.execute. Must this also be done the the bitmap ?
Q2 : the attached image shows the expected result of an image(Bitmap) by an Thread and the actual image results (as seen by the THreadImage.Picture.Bitmap.SaveToFile command)

Answer 1

bitblt(Masterbitmap.Canvas.handle, 0, 0, XPixel, YPixel,
bitmap.Canvas.handle, 0, 0, srcand);
you explicitly called Masterbitmap.Canvas.Lock, however you didn't call bitmap.Canvas.Lock (so you can loose the canvas handle anytime within this call...)
Additionally, you need to consider thread safety within GDI itself: Sharing of any GDI objects between different threads should be avoided at all cost. For example if you select a bitmap into two different device contexts at the same time (but in different threads) you may run into problems within GDI itself...
Please note that older versions of delphi don't protect against sharing of cached handles (Font, Brush and Pen handles are cached within a global list. This was fixed in an XE3 service pack if I remember correctly).
In short: I'd consider to avoid TCanvas and TBitmap completely if you really need multi-threading. (It's much easier to be multi-threading safe that way)