Storing and repainting a rectangular region of interest

Hello,

I have the following problem:
I have to integrate cairo into an existing rendering application so I need to stick to the applications API as close as possible.
It requires me to implement two functions which work on rectangles:
The first gets a rectangle specification (x,y,width,height) and needs to save that exact area of the painted surface somehow, the second just restores the last saved rectangle.

My (not working) implementation looks like this:

void CairoEngine::SaveRegion(ptvlib::CRectI region)
{
    cairo_surface_t* saved_surface = cairo_image_surface_create(CAIRO_FORMAT_RGB24, region.Width(), region.Height());
    cairo_t* saved_context = cairo_create(saved_surface);

    cairo_set_source_surface(saved_context, win32_surface, -region.left, -region.top);
    cairo_set_operator(saved_context, CAIRO_OPERATOR_SOURCE);
    cairo_paint(saved_context);
    cairo_surface_flush(saved_surface);
    cairo_destroy(saved_context);

    saved_regions.push(std::make_pair(region, saved_surface));
}

void CairoEngine::ReplayRegion()
{
    if(saved_regions.empty()) return;
    ptvlib::CRectI region = saved_regions.top().first;
    cairo_surface_t* saved_surface = saved_regions.top().second;

    cairo_save(win32_context);
    cairo_set_operator(win32_context, CAIRO_OPERATOR_SOURCE);
    cairo_set_source_surface(win32_context, saved_surface, region.left, region.bottom);
    cairo_paint(win32_context);
   cairo_surface_flush(win32_surface);
    cairo_surface_destroy(saved_surface);
    cairo_restore(win32_context);

    saved_regions.pop();
}

Any hints what I'm doing wrong?

Cheers,
--
David Weiß

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