Tom,
As Joe said, the Paint event should be ready to draw the whole canvas
as needed. As Phil and Joe pointed out, and another Joe pointed out
to me offlist, sometimes you may need to draw directly to
canvas.graphics for performance reasons.
Here's what I do to work with my own FlickerFreeCanvas... All other
canvases do all of their drawing in their Paint events, not outside.
Sometimes, this might be bad for performance. In those cases, I cache
objects to be drawn as picture properties of the canvas. These are
computed on demand, and nil'd out when the canvas is resized or
otherwise need the cached pictures updated.
So a paint event might look like:
// draw cached pictures first
if picBackground1 = nil then
MakeBackground1
end if
if picBackground1 <> nil then
g.DrawPicture picBackground1, 0,0
end if
...
// draw dynamic content here
An example of something I'm able to do with this general approach is
adjust the brightness or color level of a full screen picture using a
slider and getting very good responsiveness. So I don't think you
have to draw from outside to achieve good performance. At least, I
haven't seen a case where it was required. The bonus with this
approach is that if you use a drop-in replacement for canvas that
squashes flicker on Windows, your code just works, because it only
draws inside the Paint event.
Hope that helps.
-Brad
On Feb 15, 2006, at 12:39 PM, Tom Buchler wrote:
Brad,
Thanks. I'm trying to get my head around this.
The functionality I'm building involves editing music notation with
lyrics. So at various moments I'm drawing lines for music staffs,
musical notes, positioned text with features (bold, etc.), scroll
bar etc. In some code, I need to determine text width as it will
appear in the canvas' current font. Sometimes I draw the whole
page. Sometimes I just change the background color for a small area
to indicate highlighted notes. Sometimes I redraw one note and its
surrounding staff lines. Sometimes I need to redraw the entire
canvas because another window is no longer obscuring part of my
control, and I don't what part of the canvas was obscured.
What do I put in the paint event so to achieve that?
I can imagine marking the list of objects I'm displaying with a
"Changed" property so that a monolithic redraw would only modify
parts of the backimage before BLTing it to the canvas. In that
scenario, how do I know that Paint wasn't triggered by another
window that was just moved from obscuring the canvas?
-Tom
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