TPaintBox to JPEG/BMP - Buffer

Question

I have a paintbox and I draw a TBitmap like this:

procedure MyForm.PaintBoxPaint(Sender: TObject); // ONPAINT
begin
 PaintBox.Canvas.Lock;
 MyBitMap.Canvas.Lock;
 PaintBox.Canvas.Draw(0, 0, MyBitMap);
 PaintBox.Canvas.UnLock;
 MyBitMap.Canvas.UnLock;
end;

I would like to "map"/"copy" a JPEG or BMP from the PaintBox. There are some challenges tho that I have to think about first. The first thing is that I want to save a JPEG or BMP to a file that supports a WidePath/WideFileName. I use Delphi 7 and the .SaveToFile procedures only supports AnsiPaths/AnsiFileNames. The next thing is that I want to make a copy that is independent from the "MainGUI Thread". So the GUI should be still active and drawing to the paintbox while it saves the copy in the background (so NO TTimer). The last thing is the actual filename of the "copy". I would like to set a counter that goes from 1.jpg ... to 2.jpg ... to N.jpg The problem is the counting of an Integer that can cause access violations due to every function tries to increment the counter.

My Idea was the following:

A structure for the saving Thread:

type
 PTRTSaveImage = ^TSaveImage;
 TSaveImage = record
 Number : Integer;
 Pic    : TBitMap;
end;

The actual SaveImageThread:

function SaveImageToHDD ( p : pointer ) : Integer; stdcall;
var
 jpg      : TJpegImage;
 jpgStr   : TStringStream;
 _infos   : TSaveImage;
begin
 CopyMemory(@_infos, p, SizeOf(_infos));
 jpg    := TJpegImage.Create;
 jpgStr := TStringStream.Create ('');
 jpg.assign (_infos.Pic);
 _infos.Pic.Free;
 jpg.SaveToStream(jpgStr);
 jpg.Free;
 StrToFile ('C:\' + inttostr(_infos.Number) + '.jpg',0,jpgStr.DataString); // for WidePath/WideFileName Support.
 jpgStr.Free;
end;

That's how I call the Thread:

procedure MyForm.PaintBoxPaint(Sender: TObject); // ONPAINT
var
 Saving : PTRTSaveImage;
 BackUp : TBitMap;
begin
 PaintBox.Canvas.Lock;
 MyBitMap.Canvas.Lock;
 PaintBox.Canvas.Draw(0, 0, MyBitMap);
 BackUp := TBitMap.Create;
 BackUp.Assign (MyBitMap); // Immediate copy of the actual drawing! 
 Saving   := PTRTSaveImage(LocalAlloc(LPTR, SizeOf(TSaveImage)));
 Saving^.Pic    := BackUp;
 Saving^.Number := Counter;
 inc (Counter);
 PaintBox.Canvas.UnLock;
 MyBitMap.Canvas.UnLock;
end;

it takes a few pictures just fine but then the debugger shows me an exception:

Debugger Fault Notification Project C:....exe faulted with message: 'application-defined exception (code 0x0eedfade) at 0x759d9617'. Process Stopped. Use Step or Run to continue.

Is there any "better way" to do this? Saving and drawing at the same time...?!

Thanks for your help.

EDIT:

Then I also thought about just making 1 thread as a replacement for a TTimer that goes like this:

function SaveImages ( p : TMyForm ) : Integer; stdcall
var
 jpg    : TJpegImage; 
 jpgStr : TStringStream;
begin
while true do begin   // ---> constant LOOP that saves pictures in the intervall
 sleep (1000);        // Intervall
 jpg    := TJpegImage.Create;
 jpg.assign (p.MyBitMap);
 jpgStr := TStringStream.Create ('');
 jpg.SaveToStream(jpgStr);
 jpg.Free;
 StrToFile ('C:\' + inttostr(p.Counter) + '.jpg',0,jpgStr.DataString); // for WidePath/WideFileName Support.  
 inc (p.Counter);  
 jpgStr.Free;
end;
end;

but the same error/exception occurs.

OP:Is there any "better way" to do this? Saving and drawing at the same time...?!. It might be better explain exactly what "this" is trying to achieve..why do you need to save the entire paintbox canvas every second? You are going to create a lot of pictures... — Despatcher
exactly. The paintbox almost redraws every second (or even faster). I need the pictures to collect so I can convert them to an AVI file. BMP is faster but very big. I prefer a slower process (compression) rather than big filesize. — Benjamin Weiss

Despatcher Despatcher · Accepted Answer · 2012-06-19T16:41:13

AVI file...I did wonder... I would create a new Paintbox control derived from TPaintBox. (You may have done that already?)

Add a counter property and code to it that captures the canvas using BitBlit to a Bmp then creates a thread to convert and save the bitmap as a jpg, and increments the counter when it it starts. (You will need to use Synchronize for that call). Name it say CaptureCanvas.

Finally add a method called say afterChange or afterTimePeriod that calls captureCanvas. Make sure you have a try ... finally that ensures the bitmap and Jpeg are destroyed if anything goes wrong with the saving. Its then all in one place(inside TNewPaintbox, and if it gets behind then the images being created already have their index and you won't save them out of order.... just an idea :)

Sorry Meant to add the form code would then simplify to:

Form1.Paintboxpaint()
begin
.
.
PaintBox.Canvas.Draw(x, y, bitmap);
Paintbox.AfterChange;
.

Sorry for brevity, editing the answer and can't see the original post

TPaintBox to JPEG/BMP - Buffer

1 Answers