SOLVED
Ok I solved my problem. Here is the code if anyone is interested:
VOID CAMERA2D::ZoomIn(FLOAT const& MouseX, FLOAT const& MouseY) {
// Get the setting of the current view port.
D3DVIEWPORT9 ViewPort;
this->Direct3DDevice->GetViewport(&ViewPort);
// Convert the screen coordinates of the mouse to world space coordinates.
D3DXVECTOR3 VectorOne;
D3DXVECTOR3 VectorTwo;
D3DXVec3Unproject(&VectorOne, &D3DXVECTOR3(MouseX, MouseY, 0.0f), &ViewPort,
&this->ProjectionMatrix, &this->ViewMatrix, &WorldMatrix);
D3DXVec3Unproject(&VectorTwo, &D3DXVECTOR3(MouseX, MouseY, 1.0f), &ViewPort,
&this->ProjectionMatrix, &this->ViewMatrix, &WorldMatrix);
// Calculate the resulting vector components.
float WorldZ = 0.0f;
float WorldX = ((WorldZ - VectorOne.z) * (VectorTwo.x - VectorOne.x)) /
(VectorTwo.z - VectorOne.z) + VectorOne.x;
float WorldY = ((WorldZ - VectorOne.z) * (VectorTwo.y - VectorOne.y)) /
(VectorTwo.z - VectorOne.z) + VectorOne.y;
// Move the camera into the screen.
this->Position.z = this->Position.z * 0.9f;
D3DXMatrixLookAtLH(&this->ViewMatrix, &this->Position, &this->Target, &this->UpDirection);
// Calculate the world space vector again based on the new view matrix,
D3DXVec3Unproject(&VectorOne, &D3DXVECTOR3(MouseX, MouseY, 0.0f), &ViewPort,
&this->ProjectionMatrix, &this->ViewMatrix, &WorldMatrix);
D3DXVec3Unproject(&VectorTwo, &D3DXVECTOR3(MouseX, MouseY, 1.0f), &ViewPort,
&this->ProjectionMatrix, &this->ViewMatrix, &WorldMatrix);
// Calculate the resulting vector components.
float WorldZ2 = 0.0f;
float WorldX2 = ((WorldZ2 - VectorOne.z) * (VectorTwo.x - VectorOne.x)) /
(VectorTwo.z - VectorOne.z) + VectorOne.x;
float WorldY2 = ((WorldZ2 - VectorOne.z) * (VectorTwo.y - VectorOne.y)) /
(VectorTwo.z - VectorOne.z) + VectorOne.y;
// Create a temporary translation matrix for calculating the origin offset.
D3DXMATRIX TranslationMatrix;
D3DXMatrixIdentity(&TranslationMatrix);
// Calculate the origin offset.
D3DXMatrixTranslation(&TranslationMatrix, WorldX2 - WorldX, WorldY2 - WorldY, 0.0f);
// At the offset to the cameras world matrix.
this->WorldMatrix = this->WorldMatrix * TranslationMatrix;
}
Maybe someone has even a better solution than mine.