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Report From: Delphi-BCB/VCL/Graphics [ Add a report in this area ]

Report #: 94085

Status: Reported

Allow metafile to support successive creation of canvas without loosing image

Project: Delphi

Build #: 14.0.3593.25826

Version: 14.2

Submitted By: David Tetard

Report Type: Suggestion / Enhancement Request

Date Reported: 5/21/2011 3:54:14 AM

Severity: Infrequently encountered problem

Last Updated: 5/22/2011 5:25:16 PM

Platform: All platforms

Internal Tracking #:

Resolution: None

Resolved in Build: : None

Duplicate of: None

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Description

Hi,

I'd like to be able to draw on a metafile using a succession of TMetafileCanvas without loosing previous drawing.
This is best explained using the code example below:

procedure TForm6.Image1Click(Sender: TObject);
Var
   MF : TMetafile;
   MFC : TMetafileCanvas;
begin
     MF := TMetafile.Create;
     Try
          MF.SetSize(250, 250);

          // First drawing using first canvas:
          MFC := TMetafileCanvas.Create(MF, 0);
          Try
               MFC.TextOut(10, 10, 'xxxx');
          Finally
               MFC.Free;
          End;

          // Second drawing using second canvas:
          MFC := TMetafileCanvas.Create(MF, 0);
          Try
               MFC.TextOut(50, 50, 'yyyy');
          Finally
               MFC.Free;
          End;

          Image1.Picture.Assign(MF);
     Finally
          MF.Free;
     End;
end;

In this case, the creation fo the second canvas erases the previous drawing and you only get the drawn "yyyy" text. What I'd like is to be able to do this and have both drawing on the resulting MF.

NB: Of course, this example is not useful in itself (you could draw both texts using the same canvas) but in cases of a metafile that's a composite of several complex drawings done by several methods, it's good to be able to pass only a TMetafile and have each routine create its own canvas and draw on the metafile rather than have to pass both metafile and canvas as parameters, as in:

Requested solution:

Procedure DrawBackground(MF : TMetafile);
begin
...
end;

procedure DrawXAxis(MF : TMetafile);
begin
...
end;

...

procedure DrawGraph;
var
    MF : TMetafile;
begin
     MF := TMetafile.Create;
     Try
          MF.SetSize(...);
         DrawBackground(MF);
         DrawXAxis(MF);
         ...
    Finally
         MF.Free;
    End;
End;

Currently, the only option is the following that's more cumbersome in some cases:

Procedure DrawBackground(MF : TMetafile; MFC : TMetafileCanvas);
begin
...
end;

procedure DrawXAxis(MF : TMetafile; MFC : TMetafileCanvas);
begin
...
end;

...

procedure DrawGraph;
var
    MF : TMetafile;
MFC : TMetafileCanvas;
begin
     MF := TMetafile.Create;
     Try
          MF.SetSize(...);
         MFC := TMetafileCanvas.Create(MF, 0);
         Try
              DrawBackground(MF, MFC);
              DrawXAxis(MF, MFC);
              ...
         Finally
              MFC.Free;
          End;
    Finally
         MF.Free;
    End;
End;

Steps to Reproduce:

1- Create VCL form
2- Add TImage on it.
3- In OnClick of image, copy code in description.

Run app, click image, and observe the image. No xxx text displayed, meaning that the second canvas erased the work of the first one.

Workarounds

None

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