Sorry, it's hard to choose from the context.
here is such window
procedure Plg_Dlg
local oBrh
private oPlg
DEFINE BRUSH oBru
DEFINE DIALOG oPlg PIXEL STYLE nOR(WS_POPUP) BRUSH oBru TRANSPARENT
ACTIVATE DIALOG oPlg
return
I draw on it
oPlg:Line()
oPlg:PolyPolygon(GetDc(oPlg:hWnd),..)
and etc.
Then I want to erase the painting.