Re: clipping region of a device context.

"Mark Salsbery [MVP]" <MarkSalsbery[MVP]@newsgroup.nospam>
Fri, 31 Aug 2007 13:00:24 -0700
If a clipping region is set on a DC, drawing will only occur within that

Windows that redraw the entire client area while resizing probably have the
CS_HREDRAW and/or CS_VREDRAW window class style(s).

You can mark an area of a window for updating with the Invalidate___() APIs.
On the next WM_PAINT message, if you use BeginPaint() to get a DC, the DC
will have a clipping region selected into it so drawing will only be done in
the area of the window marked invalid. You can also handle window update
regions yourself if you want the best performance.

Your example RectVisible call just checks to see if any of the object being
drawn is in the clipping region before doing any actual drawing. This can
improve performance by eliminating unnecessary drawing.


Mark Salsbery
Microsoft MVP - Visual C++

"marcomb" <> wrote in message

can someone explaing me what's a clipping region in a device context?
if i draw for example different circles in a window client area when i
resize the window these objects disappear because the whole client area is
invalidated by windows, so what represents the clipping region?The whole
client area?
And what about

if(pDC->RectVisible(pElement->GetBoundRect()))//obtain enclosing shape
pElement->Draw(pDC); // ...draw it

this function checks if the enclosing rect of a shape, for example a
overlay the clipping region...what it exactly means?
Maybe th enclosing rect must be contained in the window client area that

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