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Declaration: property OnBeforeRenderData: TRenderEvent;
{ TRenderEvent = procedure (Sender: TObject; const Canvas: TCanvas) of object; }

The event OnBeforeRenderData provides a hook for adding user defined graphics to the map. The event OnBeforeRenderData occurs before the gridlines, and the landmarks are drawn. The parameter Canvas provides access to the canvas of the data area. Please note that the state of the canvas (e.g. the color of its pen, or the fill mode of the brush) may depend on previous drawing activities.

Hint 1: User defined graphics may be positioned either relative to the window or relative to the map image. In order to draw relative to the map image you have to use the property TopLeft and subtract it from the pixel coordinates of the canvas (see example below).

Hint 2: In order to avoid unwanted size effects regarding characters displayed on canvases of different resolution (i.e. the screen and a printer) you should never directly assign the font size within the event. Use SetCanvasFontSizeScaled instead. So, for example, the statement Canvas.Font.Size := 12; should be replaced by SetCanvasFontSizeScaled (Canvas, 12);

Example: Following is an example on how to implement user defined graphics on top of a TGeoMap named "GM" (but below any landmarks or calibration points). It displays two lines, the red one is at a fixed position relative to the map window, the blue one moves along with the map if the map is panned.

procedure TForm1.GMBeforeRenderData(Sender: TObject; const Canvas: TCanvas);
Canvas.Pen.Color := clRed;
Canvas.Pen.Color := clBlue;

Last Update: 2012-Oct-25