D3D中一个明暗处理的问题(D3DSHADE_FLAT) （问题，希望DX解答）

程序生成了一个U形的连续面。大概样子如下图。

令我无法理解的是两个对称的面上明暗竟不一样！

下图是　将U形的模型前后旋转后的效果。同样的位置两个面表现出来的效果不一样？！！

代码如下　：

生成VB:

    int iCount = 6;
    if( FAILED( lpDevice->CreateVertexBuffer( 2*iCount*sizeof(D3DVERTEX),0,D3DVERTEX::FVF,D3DPOOL_SYSTEMMEM,&g_pVB,NULL ) ) )
        return false;

    D3DVERTEX* pVertex;
    if( FAILED( g_pVB->Lock( 0,2*iCount*sizeof(D3DVERTEX),(void**)&pVertex,0 ) ) )
    ...{
        SAFE_RELEASE( g_pVB );
        return false;
    }

    D3DXVECTOR3 vec3;

    int i = 0;
    pVertex[i].p = D3DXVECTOR3( 0,-10,0 );
    pVertex[i].n = D3DXVECTOR3( 0,0,-1 );
    i++;
    pVertex[i].p = D3DXVECTOR3( 0,0,0 );
    pVertex[i].n = D3DXVECTOR3( 0,0,-1 );
    i++;
    pVertex[i].p = D3DXVECTOR3( 10,-10,0 );
    pVertex[i].n = D3DXVECTOR3( 0,0,-1 );
    i++;
    pVertex[i].p = D3DXVECTOR3( 10,0,0 );
    pVertex[i].n = D3DXVECTOR3( 0,0,-1 );
    i++;
    pVertex[i].p = D3DXVECTOR3( 20,-10,0 );
    vec3 = D3DXVECTOR3(1.0f,0,-1.0f);
    D3DXVec3Normalize( &pVertex[i].n ,&vec3 );
    i++;
    pVertex[i].p = D3DXVECTOR3( 20,0,0 );
    vec3 = D3DXVECTOR3(1.0f,0,-1.0f);
    D3DXVec3Normalize( &pVertex[i].n ,&vec3 );
    i++;

    /**////////////////////////////////////////////////////////
    pVertex[i].p = D3DXVECTOR3( 20,-10,10 );
    vec3 = D3DXVECTOR3(1.0f,0,1.0f);
    D3DXVec3Normalize( &pVertex[i].n ,&vec3 );
    i++;
    pVertex[i].p = D3DXVECTOR3( 20,0,10 );
    vec3 = D3DXVECTOR3(1.0f,0,1.0f);
    D3DXVec3Normalize( &pVertex[i].n ,&vec3 );
    i++;
    pVertex[i].p = D3DXVECTOR3( 10,-10,10 );
    pVertex[i].n = D3DXVECTOR3( 0,0,1 );
    i++;
    pVertex[i].p = D3DXVECTOR3( 10,0,10 );
    pVertex[i].n = D3DXVECTOR3( 0,0,1 );
    i++;
    pVertex[i].p = D3DXVECTOR3( 0,-10,10 );
    pVertex[i].n = D3DXVECTOR3( 0,0,1 );
    i++;
    pVertex[i].p = D3DXVECTOR3( 0,0,10 );
    pVertex[i].n = D3DXVECTOR3( 0,0,1 );
    
    g_pVB->Unlock();

 渲染：

        pd3dDevice->SetRenderState( D3DRS_SHADEMODE,D3DSHADE_FLAT );
        
        if( g_pVB )
        ...{
            pd3dDevice->SetStreamSource( 0,g_pVB,0,sizeof(D3DVERTEX) );
            pd3dDevice->SetFVF( D3DVERTEX::FVF );
            pd3dDevice->DrawPrimitive( D3DPT_TRIANGLESTRIP,0,10 );
        }

 

不如哪位高手可以指点一下？！