DirectX11 wireframe z-fighting help (or why D3D11_RASTERIZER_DESC.DepthBias is an INT?)

Question

I'm trying to use the DepthBias property on the rasterizer state in DirectX 11 (D3D11_RASTERIZER_DESC) to help with the z-fighting that occurs when I render in wireframe mode over solid polygons (wireframe overlay), and it seems setting it to any value doesn't change anything to the result. But I noticed something strange... the value is defined as a INT rather than a FLOAT. That doesn't make sense to me, but it still doesn't happen to work as expected. How do we properly set that value if it is a INT that needs to be interpreted as a UNORM in the shader pipeline?

Here's what I do:

1. Render all geometry
2. Set the rasterizer to render in wireframe
3. Render all geometry again

I can clearly see the wireframe overlay, but the z-fighting is horrible. I tried to set the DepthBias to a lot of different values, such as 0.000001, 0.1, 1, 10, 1000 and all the minus equivalent, still no results... obviously, I'm aware when casting the float as integer, all the decimals get cut... meh?

D3D11_RASTERIZER_DESC RasterizerDesc;
ZeroMemory(&RasterizerDesc, sizeof(RasterizerDesc));
RasterizerDesc.FillMode = D3D11_FILL_WIREFRAME;
RasterizerDesc.CullMode = D3D11_CULL_BACK;
RasterizerDesc.FrontCounterClockwise = FALSE;
RasterizerDesc.DepthBias = ???
RasterizerDesc.SlopeScaledDepthBias = 0.0f;
RasterizerDesc.DepthBiasClamp = 0.0f;
RasterizerDesc.DepthClipEnable = TRUE;
RasterizerDesc.ScissorEnable = FALSE;
RasterizerDesc.MultisampleEnable = FALSE;
RasterizerDesc.AntialiasedLineEnable = FALSE;

As anyone figured out how to set the DepthBias properly? Or perhaps it is a bug in DirectX (which I doubt) or again maybe there's a better way to achieve this than using DepthBias?

Thank you!

Answer 1

http://msdn.microsoft.com/en-us/library/windows/desktop/cc308048(v=vs.85).aspx

Depending on whether your depth buffer is UNORM or floating point varies the meaning of the number. In most cases you're just looking for the smallest possible value that gets rid of your z-fighting rather than any specific value. Small values are a small bias, large values are a large bias, but how that equates to a shift numerically depends on the format of your depth buffer.

As for the values you've tried, anything less than 1 would have rounded to zero and had no effect. 1, 10, 1000 may simply not have been enough to fix the issue. In the case of a D24 UNORM depth buffer, the formula would suggest a depth bias of 1000 would offset depth by: 1000 * (1 / 2^24), which equals 0.0000596, a not very significant shift in z-buffering terms.

Does a large value of 100,000 or 1,000,000 fix the z-fighting?