How to prevent Canvas to erase background?

General
#1

Hi,

I’m programming a game using the canvas on macOS and experiencing problems. I tried a simple test-app:

I’m drawing a circle that moves and should leave a trail.

The only way to draw to a canvas is the paint event so there I draw the circle and increment its x and y-positions.

From a timer I call canvas1.fresh(false) and the ball moves as expected.

But it does not leave a trail, the old drawings of the ball are erased.

I know, I could draw into a picture and draw the picture in the paint event, but I don’t want to (big screen, big canvas = slow).

How do I preserve the background? I remember Xojo behaved different years ago.

Thanks for your input!

Greetings
Nils

#2

Create a picture, same size as the canvas.
Draw on the picture
In the paint event of the canvas, use .drawpicture to draw the picture onto the canvas

#3

Better yet: put the picture a backdrop, and draw directly to it. No need to draw again in paint.

#4

Hi Jeff,

thank you for your answer, know this method, but as stated earlier, this is to slow (very big screen, very big canvas) so I just wanted to update what changed.

I thought the canvas.eraseBackground=false setting would prevent that — what is this setting for?

#5

Use the areas() parameter of the Paint event to only paint the region you’ve just updated.

[code]Sub Paint(g As Graphics, areas() As REALbasic.Rect) Handles Paint
If areas.LastRowIndex > -1 Then
For n As Integer = 0 To areas.LastRowIndex
g.DrawPicture(p, areas(n).Left, areas(n).top, areas(n).width, areas(n).height, areas(n).Left, areas(n).top, areas(n).width, areas(n).height)
Next
Else
g.DrawPicture(p, 0, 0)
End If

End Sub
[/code]

https://www.dropbox.com/s/rnuri1rv5vj2ft7/TestPaintAreas.xojo_binary_project?dl=0

#6

Hi Michael,

this works, thank you. I’ll will compare the speed of the method with the other options.

#7

Hi Julian,

that works very well, I try this out too. In the game I’m using a list which tiles have changed and only they are redrawn, I could pass this info on to the canvas areas()

#8