How to properly paint in WM_PAINT with GDI+ - c++

Question

I'm new to using GDI+ and I was wondering how to fix the mess of creating and disposing objects that I have. Right now all the program needs to do is handle repeated WM_PAINT messages and update a DrawPie with increased degrees each time.
I have called GdiplusStartup( &gdiplusToken, &gdiplusStartupInput, NULL ); when the window starts up, and GdiplusShutdown( gdiplusToken ); on WM_DESTROY.
Degrees global variable defined near the top:
volatile double degrees = 0;
Here is my WM_PAINT:
case WM_PAINT: {
hdc = BeginPaint( hWnd, &ps );
Graphics g( hdc );
Pen p( Color::Green );
if ( degrees > 0 ) {
if ( degrees == 360 ) {
g.DrawEllipse( &p, 0, 0, 100, 100 );
} else {
g.DrawPie( &p, 0, 0, 100, 100, -90, degrees );
}
}
EndPaint( hWnd, &ps );
break;
}
And here is the function that updates the degrees and updates the window (It's in a separate thread):
void UpdateDegrees() {
for ( ;; ) {
if ( globalHWND != NULL ) {
degrees += 0.1;
InvalidateRect( globalHWND, NULL, TRUE );
UpdateWindow( globalHWND );
}
}
}
If I run it I get a "solid" pie shape like this, which I assume is it just redrawing itself at every angle. It needs to look like this, in other words, clear the graphic before it redraws itself each time. (Sorry, I guess I need 10 rep to post inline images)
I know that I didn't initialize and/or dispose of my graphics correctly, but I honestly have no idea how to do that. Any help would be greatly appreciated! Thanks.

Answer 1

There are lots of ways you can do this, the standard one being to handle the WM_ERASEBKGND message.
For simplicity, you can just fill a white rectangle across the clip rect (which is specified in ps), which will clear the background being painted.
SolidBrush backGroundBrush(Color(255,255,255));
Rect clipRect(ps->rcPaint.left, ps.rcPaint.top, ps->rcPaint.right - ps->rcPaint.left, ps->rcPaint.bottom - ps.rcPaint.top);
g.FillRectangle(&backgroundBrush, Rect(ps->rcPaint.left, ps.rcPaint));