平面投影

void RenderShadow(){
    Device->SetRenderState(D3DRS_STENCILENABLE,    true);
    Device->SetRenderState(D3DRS_STENCILFUNC,      D3DCMP_EQUAL);
    Device->SetRenderState(D3DRS_STENCILREF,       0x0);
    Device->SetRenderState(D3DRS_STENCILMASK,      0xffffffff);
    Device->SetRenderState(D3DRS_STENCILWRITEMASK, 0xffffffff);
    Device->SetRenderState(D3DRS_STENCILZFAIL,     D3DSTENCILOP_KEEP);
    Device->SetRenderState(D3DRS_STENCILFAIL,      D3DSTENCILOP_KEEP);
    Device->SetRenderState(D3DRS_STENCILPASS,      D3DSTENCILOP_INCR); // increment to 1

    // position shadow
    D3DXVECTOR4 lightDirection(0.707f, -0.707f, 0.707f, 0.0f);
    D3DXPLANE groundPlane(0.0f, -1.0f, 0.0f, 0.0f);

    D3DXMATRIX S;
    D3DXMatrixShadow(
        &S,
        &lightDirection,
        &groundPlane);

    D3DXMATRIX T;
    D3DXMatrixTranslation(
        &T,
        TeapotPosition.x,
        TeapotPosition.y,
        TeapotPosition.z);

    D3DXMATRIX W = T * S;
    Device->SetTransform(D3DTS_WORLD, &W);

    // alpha blend the shadow
    Device->SetRenderState(D3DRS_ALPHABLENDENABLE, true);
    Device->SetRenderState(D3DRS_SRCBLEND, D3DBLEND_SRCALPHA);
    Device->SetRenderState(D3DRS_DESTBLEND, D3DBLEND_INVSRCALPHA);

    D3DMATERIAL9 mtrl = D3d::InitMtrl(D3d::BLACK, D3d::BLACK, D3d::BLACK, D3d::BLACK, 0.0f);
    mtrl.Diffuse.a = 0.5f; // 50% transparency.

    // Disable depth buffer so that z-fighting doesn't occur when we
    // render the shadow on top of the floor.
    Device->SetRenderState(D3DRS_ZENABLE, false);

    Device->SetMaterial(&mtrl);
    Device->SetTexture(0, 0);
    Teapot->DrawSubset(0);

    Device->SetRenderState(D3DRS_ZENABLE, true);
    Device->SetRenderState(D3DRS_ALPHABLENDENABLE, false);
    Device->SetRenderState(D3DRS_STENCILENABLE,    false);
}