How to capture a certain region of screen - direct3d

Question

Ihave to record a certain area of a screen and save the frames (20-30fps) of the video being played. Currently I'm able to capture the screen at the required frame rate using the following code, However I'm not really sure about how I can capture a certain region.
HRESULT SavePixelsToFile32bppPBGRA(UINT width, UINT height, UINT stride, BYTE, *pixels, LPWSTR filePath, const GUID &format)
{
if (!filePath || !pixels)
return E_INVALIDARG;
HRESULT hr = S_OK;
IWICImagingFactory *factory = nullptr;
IWICBitmapEncoder *encoder = nullptr;
IWICBitmapFrameEncode *frame = nullptr;
IWICStream *stream = nullptr;
GUID pf = GUID_WICPixelFormat32bppPBGRA;
BOOL coInit = CoInitialize(nullptr);
CoCreateInstance(CLSID_WICImagingFactory, nullptr, CLSCTX_INPROC_SERVER, IID_PPV_ARGS(&factory));
factory->CreateStream(&stream);
stream->InitializeFromFilename(filePath, GENERIC_WRITE);
factory->CreateEncoder(format, nullptr, &encoder);
encoder->Initialize(stream, WICBitmapEncoderNoCache);
encoder->CreateNewFrame(&frame, nullptr); // we don't use options here
frame->Initialize(nullptr); // we dont' use any options here
frame->SetSize(width, height);
frame->SetPixelFormat(&pf);
frame->WritePixels(height, stride, stride * height, pixels);
frame->Commit();
encoder->Commit();
RELEASE(stream);
RELEASE(frame);
RELEASE(encoder);
RELEASE(factory);
if (coInit) CoUninitialize();
return hr;
}
HRESULT Direct3D9TakeScreenshots(UINT adapter, int count)
{
HRESULT hr = S_OK;
IDirect3D9 *d3d = nullptr;
IDirect3DDevice9 *device = nullptr;
IDirect3DSurface9 *surface = nullptr;
D3DPRESENT_PARAMETERS parameters = { 0 };
D3DDISPLAYMODE mode;
D3DLOCKED_RECT rc;
UINT pitch;
SYSTEMTIME st;
BYTE *shot = nullptr;
// init D3D and get screen size
d3d = Direct3DCreate9(D3D_SDK_VERSION);
d3d->GetAdapterDisplayMode(adapter, &mode);
parameters.Windowed = TRUE;
parameters.BackBufferCount = 1;
parameters.BackBufferHeight = mode.Height;
parameters.BackBufferWidth = mode.Width;
parameters.SwapEffect = D3DSWAPEFFECT_DISCARD;
parameters.hDeviceWindow = NULL;
// create device & capture surface
d3d->CreateDevice(adapter, D3DDEVTYPE_HAL, NULL, D3DCREATE_SOFTWARE_VERTEXPROCESSING, &parameters, &device);
device->CreateOffscreenPlainSurface(mode.Width, mode.Height, D3DFMT_A8R8G8B8, D3DPOOL_SYSTEMMEM, &surface, nullptr);
// compute the required buffer size
surface->LockRect(&rc, NULL, 0);
pitch = rc.Pitch;
surface->UnlockRect();
// allocate screenshots buffers
shot = new BYTE[pitch * mode.Height];
// get the data
device->GetFrontBufferData(0, surface);
// copy it into our buffers
surface->LockRect(&rc, NULL, 0);
CopyMemory(shot, rc.pBits, rc.Pitch * mode.Height);
surface->UnlockRect();
WCHAR file[100];
wsprintf(file, L"C:/Frames/cap%i.png", count);
SavePixelsToFile32bppPBGRA(mode.Width, mode.Height, pitch, shot, file, GUID_ContainerFormatPng);
if (shot != nullptr)
{
delete shot;
}
RELEASE(surface);
RELEASE(device);
RELEASE(d3d);
return hr;
}
I know I can open the saved images and crop them and then again save them, but that will take up extra time. Is there a way to select a desired region