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I'm trying to create grass quads and have therefore disabled backface culling when rendering these quads. With each vertex normal set upwards ( 0.f, 1.f, 0.f ), all front faces are lit correctly, but backfaces are black. If I change the the Y normal to -1.f the backfaces light up and the front faces go black.

D3D11_RASTERIZER_DESC desc {};

desc.AntialiasedLineEnable  = false;
desc.CullMode               = D3D11_CULL_NONE;
desc.DepthBias              = 0;
desc.DepthBiasClamp         = 0.f;
desc.DepthClipEnable        = true;
desc.FillMode               = D3D11_FILL_SOLID;
desc.FrontCounterClockwise  = false;
desc.MultisampleEnable      = false;
desc.ScissorEnable          = false;
desc.SlopeScaledDepthBias   = 0.f;

I'm not so sure there's a problem here. Any ideas?

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  • \$\begingroup\$ Are you only applying duffuse lighting? Ambient would need to be applied also \$\endgroup\$
    – ErnieDingo
    Commented Feb 22, 2018 at 8:55
  • \$\begingroup\$ to add also, no, there is no problem here. I am assuming your lighting model is responding correctly due to the normal just being flipped. Could you post your pixel shader to assist in elaborating what your code is doing. \$\endgroup\$
    – ErnieDingo
    Commented Feb 22, 2018 at 21:55

2 Answers 2

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Back faces are facing in the opposite direction to front faces, so yes, their normals should also point in the opposite direction.

You can use the SV_IsFrontFace semantic to detect if you're currently drawing a backfacing primitive.

Alternatively, duplicate the data for front and back faces.

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  • \$\begingroup\$ Do you mean I can check in the shader for backface and append the normal there? And are you suggesting the other option that i cull the backface and render an extra quad as the backface? \$\endgroup\$
    – T. Elliott
    Commented Feb 22, 2018 at 10:49
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Okay I've solved this - I should have provided more information; I'm using normal mapping and it turned out that since I hadn't provided tangent or bitangent vectors for the grass quads, that was causing the problem. Thanks for your assistance.

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