Multiple subdivision of an icosahedron using HLSL Geometry Shader

Question

Currently I'm subdividing an icosahedron once by using the following Geometry Shader:

[maxvertexcount(8)]
void gs(triangle VS_OUT gin[3], inout TriangleStream<GS_OUT> s)
{
    //        p1  
    //       /  \
    //      /    \
    //     m0 --- m1
    //    / \    / \
    //   /   \  /   \
    //  p0 -- m2 -- p2

    float3 m0 = .5f * (gin[0].pos_l + gin[1].pos_l),
           m1 = .5f * (gin[2].pos_l + gin[1].pos_l),
           m2 = .5f * (gin[0].pos_l + gin[2].pos_l);

    float3 v[6];
    v[0] = gin[0].pos_l;
    v[1] = m0;
    v[2] = m2;
    v[3] = m1;
    v[4] = gin[2].pos_l;
    v[5] = gin[1].pos_l;

    GS_OUT gout;
    for (int i = 0; i < 5; ++i)
    {
        gout.pos_h = mul(float4(v[i], 1.f), g_mat_wvp);
        s.Append(gout);
    }
    s.RestartStrip();

    gout.pos_h = mul(float4(v[1], 1.f), g_mat_wvp);
    s.Append(gout);

    gout.pos_h = mul(float4(v[5], 1.f), g_mat_wvp);
    s.Append(gout);

    gout.pos_h = mul(float4(v[3], 1.f), g_mat_wvp);
    s.Append(gout);
}

However, I want to specify the subdivision level. Is there any possibility to call the Shader again with the outputed verticies or do I need to follow an other approach?

0xbadf00d 0xbadf00d · Accepted Answer · 2011-10-18T14:45:13

I've solved it in the following way:

[maxvertexcount(128)]
void gs(triangle VS_OUT gin[3], inout TriangleStream<GS_OUT> s)
{
    if (g_subdivs != 0 && g_subdivs <= MAX_SUBDIVISION_LEVEL)
    {
        uint level_count = (uint)pow(2, g_subdivs);
        float3 u = (gin[1].pos_l - gin[0].pos_l) / level_count,
               w = (gin[2].pos_l - gin[0].pos_l) / level_count,
               v = gin[0].pos_l;

        for (int level = level_count; level > 0; --level)
        {            
            GS_OUT gout;
            gout.pos_h = mul(float4(normalize(v), 1.f), g_mat_wvp);
            s.Append(gout);

            float3 p = v;
            uint   vertex_count = level + 1;

            for (uint vertex = 1; vertex < vertex_count; ++vertex)
            {
                gout.pos_h = mul(float4(normalize(p + u), 1.f), g_mat_wvp);
                s.Append(gout);

                gout.pos_h = mul(float4(normalize(p += w), 1.f), g_mat_wvp);
                s.Append(gout);
            }

            v += u;
            s.RestartStrip();
        }
    }
    else
    {
        [unroll]
        for (uint i = 0; i < 3; ++i)
        {
            GS_OUT gout;
            gout.diff  = gin[i].diff;
            gout.pos_h = mul(float4(gin[i].pos_l, 1.f), g_mat_wvp);

            s.Append(gout);
        }
    }
}

Multiple subdivision of an icosahedron using HLSL Geometry Shader

1 Answers