Offscreen Picture to Canvas

Getting Started
1

I’m working on a mapping project:
Display raster maps and vector data (Coordinates stored in SQLite)
I don’t know how to display vector data on a canvas
I know how to draw a polygon with g.DrawLine() directly in the Paint method but
I’m not sure using the paint event is a good idea because I may have hundreds of dots, polylines and polygons to display.
What I think of is to draw into a picture offscreen and draw that picture in the paint event of the canvas but how?
The code should be in a method where I access to the database
Thank you for your help
jjc_Mtl

2

Add a picture variable to a module
Initialise it as new picture (w,h)
Draw on the graphics property

When the canvas’ Paint event occurs, use

g.drawpicture myPicture,0,0

to display whatever it looks like at that moment

3

Thank you for your Answer;
Here is what I did:
Create a property (OffScreenPic) for an existing Module (EspaceCarto)
Initialize OffScreenPic in the open event proc. of the main window (the one that includes the canvas);
I initialize a lot of things there:
EspaceCarto.OffScreenPic = New Picture(Fenetre1.Canvas1.Width, Fenetre1.Canvas1.Height)
On this window, there is a button that call a method containing that:

EspaceCarto.OffScreenPic.Graphics.DrawLine(10,10,100,100) EspaceCarto.OffScreenPic.Graphics.DrawString("Test...", 200, 200)
in the Paint event:

g.DrawPicture (EspaceCarto.OffScreenPic, 0, 0)
No bug reported but nothing happens. I’ve try to put the drawing instructions directly in the button action code; no luck.
Did I forgot something? sure but what?
jjc_Mtl

4

invalidate the canvas so that it gets refreshed once you clic the button ?

5

Cool!
that’s it!
Thank you to both of you!
I will post a screen copy of my first map soon.

jjc_Mtl