Calculate look at vector after mouse scrolled

mouserotationlookat.cpp

D3DXVECTOR3 CPR_Player::GetNewDirFromInput()
{
	D3DXVECTOR3 newDir;

	D3DXVECTOR2 deltaMousePos = ( GetMousePosition() - m_startMousePos );
	deltaMousePos.x *= MOUSE_SENSITIVITY;
	deltaMousePos.y *= MOUSE_SENSITIVITY;
	deltaMousePos.x = ( abs( deltaMousePos.x ) > MOUSE_THRESHOLD ) ? deltaMousePos.x : 0.0f;
	deltaMousePos.y = ( abs( deltaMousePos.y ) > MOUSE_THRESHOLD ) ? deltaMousePos.y : 0.0f;
	deltaMousePos.y = CLAMP( deltaMousePos.y, D3DXToRadian( -89.9f ), D3DXToRadian( 89.9f ) );

	D3DXMATRIX rotmatYaw;
	D3DXMatrixRotationY( &rotmatYaw, deltaMousePos.x );
	D3DXVec3TransformCoord( &newDir, &m_startDir, &rotmatYaw );

	D3DXMATRIX rotmatPitch;
	D3DXVECTOR3 pitchVector;
	D3DXVECTOR3 up( 0.0f, 1.0f, 0.0f );
	D3DXVec3Cross( &pitchVector, &up, &newDir );
	D3DXMatrixRotationAxis( &rotmatPitch, &pitchVector, deltaMousePos.y );
	D3DXVec3TransformCoord( &newDir, &newDir, &rotmatPitch );

	return newDir;
}