Region scene with 2 swappable buffers redraw algorithm

Port.cpp

void
Port::Update(BRegion& updateRgn)
{
	if (
		!fBackSurf->bitmap.IsSet() ||
		fBackSurf->bitmap->Width() != fWidth ||
		fBackSurf->bitmap->Height() != fHeight
	) {
		fBackSurf->bitmap.SetTo(new AppServer::Bitmap(fWidth, fHeight, B_RGBA32, (bitmap_flags)0), true);
		fBackSurf->canvas.reset(fPainterAddOn->NewBitmapCanvas(fBackSurf->bitmap.Get()));
		fBackSurf->canvas->State().externalInfo = fCanvasExtInfo;

		// Newly allocated buffer has undefined contents, so we need to force filling it.
		fOldUpdateRgn.Set(clipping_rect{0, 0, fWidth - 1, fHeight - 1});
	}

	updateRgn.MakeEmpty();

	_CalcVisible();

	for (Frame* frame = fFrames.Last(); frame != nullptr; frame = fFrames.GetPrevious(frame)) {
		IntPoint offset(frame->fCurState.pos.x - frame->fOldState.pos.x, frame->fCurState.pos.y - frame->fOldState.pos.y);
		bool isMoved = offset.x != 0 || offset.y != 0;

		BRegion oldVisible = frame->fOldState.visible;
		oldVisible.OffsetBy(offset.x, offset.y);

		BRegion dirty = frame->fCurState.visible;
		dirty.Exclude(&oldVisible);

		if (isMoved)
			updateRgn.Include(&frame->fCurState.visible);
		else
			updateRgn.Include(&dirty);

		BRegion copy = oldVisible;
		copy.IntersectWith(&frame->fCurState.visible);

		if (!isMoved)
			copy.IntersectWith(&fOldUpdateRgn);

		Canvas& c = *fBackSurf->canvas;

		// copy
		if (copy.CountRects() > 0) {
			c.PushState();
			c.ClipToRegion(false, copy);
			c.DrawBitmap(fFrontSurf->bitmap.Get(), (bitmap_drawing_options)0, fFrontSurf->bitmap->Bounds(), fFrontSurf->bitmap->Bounds().OffsetByCopy(-offset.x, -offset.y));
			c.PopState();
		}

		// draw
		if (dirty.CountRects() > 0) {
			c.PushState();
			c.ClipToRegion(false, dirty);
			c.State().SetOrigin(frame->fCurState.pos);
			c.PushState();
			frame->fView->Draw(c, dirty);
			c.PopState();
			c.PopState();
		}
	}

	for (Frame* frame = fFrames.Last(); frame != nullptr; frame = fFrames.GetPrevious(frame)) {
		frame->fOldState.pos = frame->fCurState.pos;
		frame->fOldState.visible = frame->fCurState.visible;
	}

	fOldUpdateRgn = updateRgn;

	std::swap(fBackSurf, fFrontSurf);
}


void
Port::_CalcVisible()
{
	BRegion remainingVisible;
	remainingVisible.Set(clipping_rect{0, 0, fWidth - 1, fHeight - 1});

	for (Frame* frame = fFrames.Last(); frame != nullptr; frame = fFrames.GetPrevious(frame)) {
		frame->fCurState.visible = remainingVisible;
		frame->fCurState.visible.IntersectWith(&frame->fShape);
		remainingVisible.Exclude(&frame->fShape);
	}
}