in direct2d drawing dimensions change after window resizing - c++

Question

I am a beginner, and I have made a program in direct2d using c++ in visual studio 2015, just a very basic program that draws a white square on a black background , and it does show the square in a small window. But the problem is that if the user maximizes that window the vertical dimension of the drawing becomes elongated and the square becomes rectangle.
This is the screenshot of the window before maximizing it:
enter image description here
And this is the screenshot of the window after maximizing it:
enter image description here
What am I doing wrong ?
here is my code and thanks in advance:
ID2D1HwndRenderTarget* pRT = NULL;
ID2D1Factory* pD2DFactory = NULL;
ID2D1SolidColorBrush* pBlackBrush = NULL;
ID2D1SolidColorBrush* pRedBrush = NULL;
HRESULT hr;
RECT rc;
POINT pt;
void draw(){
pRT->BeginDraw();
pRT->FillRectangle( D2D1::RectF((FLOAT)rc.left,
(FLOAT)rc.top,(FLOAT)rc.right, (FLOAT)rc.bottom),pBlackBrush);
pRT->FillRectangle( D2D1::RectF((FLOAT)pt.x,
(FLOAT)pt.y,pt.x + 50.0f ,pt.y + 50.0f),pRedBrush);
hr = pRT->EndDraw();
}
int APIENTRY wWinMain(/* wWinMain parameters */){
/*
...
window making
...
*/
hr = D2D1CreateFactory(
D2D1_FACTORY_TYPE_SINGLE_THREADED,&pD2DFactory );
// Obtain the size of the drawing area.
GetClientRect(hWnd, &rc);
// Create a Direct2D render target
hr =pD2DFactory->CreateHwndRenderTarget(
D2D1::RenderTargetProperties(),
D2D1::HwndRenderTargetProperties(
hWnd,D2D1::SizeU(rc.right - rc.left, rc.bottom - rc.top)
),
&pRT
);
if (SUCCEEDED(hr)){
pRT->CreateSolidColorBrush(
D2D1::ColorF(D2D1::ColorF::White),
&pRedBrush
);
pRT->CreateSolidColorBrush(
D2D1::ColorF(D2D1::ColorF::Black),
&pBlackBrush
);
}
pt.x = 0;
pt.y = 0;
while (1) {
PeekMessage(&msg, nullptr, 0, 0, PM_REMOVE);
if (msg.message == WM_QUIT) break;
if (!TranslateAccelerator(msg.hwnd, hAccelTable, &msg)){
TranslateMessage(&msg);
DispatchMessage(&msg);
}
draw();
}
}

Answer 1

It seems like you don't resize the render target when your window resizes. I expect you run into similar problems not just from maximizing, but when you resize your window too? See https://learn.microsoft.com/en-us/windows/desktop/LearnWin32/dpi-and-device-independent-pixels#resizing-the-render-target.
Code snippet from the linked document:
void Resize()
{
if (pRenderTarget != NULL)
{
RECT rc;
GetClientRect(m_hwnd, &rc);
D2D1_SIZE_U size = D2D1::SizeU(rc.right, rc.bottom);
pRenderTarget->Resize(size);
InvalidateRect(m_hwnd, NULL, FALSE);
}
}
You'll want to call this by catching WM_SIZE.