Cull mode back faces

Question

When i draw a cube using indices, the faces of the cube are skewed but only when CullMode is set to cull back. When i switch it to cull none, it draws the cube perfect. Unfortunately i cant post any code at the time because I'm using my phone, but what are some possible explanations for why the faces of my cube are skewed wjile culling the back? I was thinking the order of my indices may be the problem.

http://i.imgur.com/S1PfxCr.png

VertexType* vertices;
unsigned long* indices;
D3D10_BUFFER_DESC vertexBufferDesc, indexBufferDesc;
D3D10_SUBRESOURCE_DATA vertexData, indexData;
HRESULT result;

m_vertexCount = 8;
m_indexCount = 36;

vertices = new VertexType[m_vertexCount];
if(!vertices)
{
    return false;
}

// Create the index array.
indices = new unsigned long[m_indexCount];
if(!indices)
{
    return false;
}

vertices[0].position = D3DXVECTOR3(-1, 1, -1);
vertices[0].color = D3DXVECTOR4(1, 0, 0, 1);

vertices[1].position = D3DXVECTOR3(1, 1, -1);
vertices[1].color = D3DXVECTOR4(1, 1, 0, 1);

vertices[2].position = D3DXVECTOR3(-1, -1, -1);
vertices[2].color = D3DXVECTOR4(1, 1, 1, 1);

vertices[3].position = D3DXVECTOR3(1, -1, -1);
vertices[3].color = D3DXVECTOR4(0, 1, 0, 1);

vertices[4].position = D3DXVECTOR3(-1, 1, 1);
vertices[4].color = D3DXVECTOR4(0, 1, 1, 1);

vertices[5].position = D3DXVECTOR3(1, 1, 1);
vertices[5].color = D3DXVECTOR4(0, 0, 1, 1);

vertices[6].position = D3DXVECTOR3(-1, -1, 1);
vertices[6].color = D3DXVECTOR4(1, 0, 0, 1);

vertices[7].position = D3DXVECTOR3(1, -1, 1);
vertices[7].color = D3DXVECTOR4(1, 0, 0, 1);

//indices = { 2, 0, 3, 3, 1, 0, 3, 1, 7, 7, 5, 1, 6, 4, 2, 2, 0, 4, 7, 5, 6, 6, 4, 5, 0, 4, 1, 1, 5, 4, 6, 2, 7, 7, 3, 2 };

indices[0] = 2;
indices[1] = 0;
indices[2] = 3;
indices[3] = 3;
indices[4] = 1;
indices[5] = 0;
indices[6] = 3;
indices[7] = 1;
indices[8] = 7;
indices[9] = 7;
indices[10] = 5;
indices[11] = 1;
indices[12] = 6;
indices[13] = 4;
indices[14] = 2;
indices[15] = 2;
indices[16] = 0;
indices[17] = 4;
indices[18] = 7;
indices[19] = 5;
indices[20] = 6; 
indices[21] = 6;
indices[22] = 4;
indices[23] = 5;
indices[24] = 0; 
indices[25] = 4;
indices[26] = 1;
indices[27] = 1;
indices[28] = 5;
indices[29] = 4;
indices[30] = 6;
indices[31] = 2;
indices[32] = 7;
indices[33] = 7;
indices[34] = 3;
indices[35] = 2;

vertexBufferDesc.Usage = D3D10_USAGE_DEFAULT;
vertexBufferDesc.ByteWidth = sizeof(VertexType) *m_vertexCount;
vertexBufferDesc.BindFlags = D3D10_BIND_VERTEX_BUFFER;
vertexBufferDesc.CPUAccessFlags = 0;
vertexBufferDesc.MiscFlags = 0;

vertexData.pSysMem = vertices;

result = device->CreateBuffer(&vertexBufferDesc, &vertexData, &m_vertexBuffer);
if(FAILED(result))
{
    return false;
}

indexBufferDesc.Usage = D3D10_USAGE_DEFAULT;
indexBufferDesc.ByteWidth = sizeof(unsigned long) *m_indexCount;
indexBufferDesc.BindFlags = D3D10_BIND_INDEX_BUFFER;
indexBufferDesc.CPUAccessFlags = 0;
indexBufferDesc.MiscFlags = 0;

indexData.pSysMem = indices;

result = device->CreateBuffer(&indexBufferDesc, &indexData, &m_indexBuffer);

return true;
}

void CubeClass::RenderBuffers(ID3D10Device* device)
{
unsigned int stride;
unsigned int offset;

stride = sizeof(VertexType);
offset = 0;

device->IASetVertexBuffers(0, 1, &m_vertexBuffer, &stride, &offset);

device->IASetIndexBuffer(m_indexBuffer, DXGI_FORMAT_R32_UINT, 0);

device->IASetPrimitiveTopology(D3D10_PRIMITIVE_TOPOLOGY_TRIANGLELIST);

device->DrawIndexed(36, 0, 0);

}

Answer 1

Your indices produce triangle with mixed winding orders. Have a look at the quad which is formed by your vertices 0, 1, 2, 3. When viewed along the z-axis it looks like this (sorry for the ASCII art):

    ^ y
    |
0   |   1
    |
----+-----------> x
    |
2   |   3

Your indices draw 203 which gives a clockwise winding, and then 310 which gives a counter-clockwise winding. Modify your indices to produce only one order.

Answer 2

From experience, when I have an object with faces that do not render, it is because my normals are not facing towards the correct direction. Perhaps setting some normals would help, since you plan on going 3D, this would have to be done anyways.

Here is a reference describing how culling works in D3D. It may well be a winding issue.

Directly from msdn: "To improve rendering performance, you can cull out (or remove) a primitive that faces away from the camera. For single-sided primitives, this saves rendering time because a back-face is not visible. To enable culling, you need to know the winding order of the vertices (typically counter-clockwise). This example will remove any primitive whose back face is facing forward (given a counter-clockwise winding order):..."