Jpeg save to base64 in TThread - delphi

Question

I have a some problem with Delphi.
I was write two simple functions for make the screenshot, convert it to jpeg and decode into base64 stream.
And its works good if i make it on main stream program. But if i create a TThread class and start this function on Execute, windows freezes and i can only reboot my pc.
By making several attempts, I found that hangs PC through procedure JpegImg.SaveToStream(Input);
And if i don't convert Bitmap to jpeg, its works good, and i get the image string.
Help please.
Here a code
procedure TEvReader.ScreenShot(DestBitmap : TBitmap) ;
var DC : HDC;
begin DC := GetDC (GetDesktopWindow) ;
try
DestBitmap.Width := GetDeviceCaps (DC, HORZRES) ;
DestBitmap.Height := GetDeviceCaps (DC, VERTRES) ;
BitBlt(DestBitmap.Canvas.Handle, 0, 0, DestBitmap.Width, DestBitmap.Height, DC, 0, 0, SRCCOPY) ;
finally
ReleaseDC (GetDesktopWindow, DC) ;
end;
end;
function TEvReader.Base64FromBitmap(Bitmap: TBitmap): string;
var
Input: TBytesStream;
Output: TStringStream;
JpegImg:TJPEGImage;
begin
Input := TBytesStream.Create;
try
JpegImg:=TJPEGImage.Create;
JpegImg.Assign(Bitmap);
JpegImg.SaveToStream(Input); {here a problem.When i replace "JpegImg" to "Bitmap" all works good }
Input.Position := 0;
Output := TStringStream.Create('', TEncoding.ASCII);
try
Soap.EncdDecd.EncodeStream(Input, Output);
Result := Output.DataString;
finally
Output.Free;
end;
finally
Input.Free;
end;
end;
procedure TOutThread.Execute;
var
bmp:TBitmap;
strrr:String;
begin
bmp:=TBitmap.Create;
mObj.ScreenShot(bmp);
strrr := mObj.Base64FromBitmap(bmp);
Form2.Memo4.Text := strrr;
end;

Answer 1

TJPEGImage is not thread safe. While issue with thread safe drawing mentioned in http://qc.embarcadero.com/wc/qcmain.aspx?d=55871 is somewhat fixed in Delphi XE6 (by exposing Canvas property you have to lock yourself), in your case it will probably not help much.
You have to synchronize TJPEGImage handling with main thread.
Also in your code you have created some memory leaks since you have never released JpgImg and Bmp objects.
Try with following code:
procedure TEvReader.ScreenShot(DestBitmap: TBitmap);
var
DC: HDC;
begin
DC := GetDC(GetDesktopWindow);
DestBitmap.Canvas.Lock;
try
DestBitmap.Width := GetDeviceCaps(DC, HORZRES);
DestBitmap.Height := GetDeviceCaps(DC, VERTRES);
BitBlt(DestBitmap.Canvas.Handle, 0, 0, DestBitmap.Width, DestBitmap.Height, DC, 0, 0, SRCCOPY);
finally
DestBitmap.Canvas.Unlock;
ReleaseDC(GetDesktopWindow, DC);
end;
end;
function TEvReader.Base64FromBitmap(Bitmap: TBitmap): string;
var
Input: TBytesStream;
Output: TStringStream;
JpegImg: TJPEGImage;
begin
Input := TBytesStream.Create;
try
JpegImg := TJPEGImage.Create;
try
TThread.Synchronize(nil,
procedure
begin
JpegImg.Assign(Bitmap);
JpegImg.SaveToStream(Input);
end);
finally
JpegImg.Free;
end;
Input.Position := 0;
Output := TStringStream.Create('', TEncoding.ASCII);
try
Soap.EncdDecd.EncodeStream(Input, Output);
Result := Output.DataString;
finally
Output.Free;
end;
finally
Input.Free;
end;
end;
procedure TOutThread.Execute;
var
mObj: TEvReader;
bmp: TBitmap;
strrr: string;
begin
mObj := TEvReader.Create;
bmp := TBitmap.Create;
try
mObj.ScreenShot(bmp);
strrr := mObj.Base64FromBitmap(bmp);
finally
bmp.Free;
mObj.Free;
end;
Synchronize(nil,
procedure
begin
Form2.Memo4.Text := strrr;
end);
end;