You can use bitmaps to draw off-screen. This might sound a little strange, but it is also officially a good thing. However, first you will want an example of how to use a bitmap, I assume. Well, here goes:

procedure Form1.DrawBitmap(const Filename: String; const x,y: Integer); var Bmp: TBitmap; begin if not FileExists(Filename) then begin ShowMessage( + Filename + ); Exit; end; Bmp := TBitmap.Create; try Bmp.LoadFromFile(Filename); Canvas.Draw(x, y, Bmp); finally Bmp.Free; end; end;

 

This function tries to load and display a bitmap (specified by Filename, for example 'myBitmap.bmp') at the given (x,y) coordinates supplied (again, remember that y is 0 at the screen top, and is positive down the screen!).

If you are not sure about creating objects at run-time I strongly advise you to check out the article on using Pointers in Delphi. Anyway, the above procedure creates a bitmap, loads a given picture and draws it to screen. The try..finally block ensures the bitmap is always freed, even if there is an error (for example, an invalid bitmap picture).

One quick point to note about the above function is that it isn't very efficient. It creates and destroys a bitmap every time it gets called, and always checks if the file exists first. A better option if you want to draw the same picture a lot is to declare the TBitmap object as part of your form, creating it and loading the picture in FormCreate, and freeing it in FormDestroy. This would be much more efficient, and is the usual course of action.

   GDI Drawing Functions
TCanvas has several useful drawing functions, which all deal with a TGraphic type. The TGraphic type is simply the base class for a graphic object in Delphi, and some examples are: bitmaps (TBitmap), icons (TIcon), metafiles (TMetafile) and JPEGs (TJPEGImage). All use the same drawing functions, so here is a list for you:

Note: All these functions are methods of TCanvas.

 

Note: All these functions are methods of TCanvas.

NAME	DESCRIPTION	EXAMPLE USE
Draw	Draws a TGraphic to the canvas, exactly as is. Does no stretching	Canvas.Draw(5,10,MyGraphic);
StrechDraw	Draws a TGraphic to the canvas, stretching it to fit the given area	Canvas.StretchDraw( Bounds(0,0,32,32), MyGraphic);
CopyRect	Copies a portion of a TCanvas to another, stretching it if necessary	Canvas.CopyRect( Bounds(0,0,32,32), MyBmp.Canvas, Bounds(0, 0, 640, 480));