Playing with DepthBias render state do not give any results...

I still assume it's a z-fighting problem. And I assume you're already using LessEqual for the ZFunc (this should be the default).

Oh my, I just realized this: Depth Bias (Direct3D 9). The devil's in the details:

The bias is not applied to any line and point primitive. However, this bias needs to be applied to triangles drawn in wireframe mode.

Alternative: Setup your WorldViewProjection to manually create a depth bias, e.g. use WVP * Matrix.Translation(0,0,-0.001f) or something. Edit: For the lines only!

Nice suggestion, but not executable in my case. I draw lines and triangles from 1 buffer and WVP matrix is the same for both. I can't change something only for lines or only for triangles. Changes will affect on all primitives.

In addition to trying out suggestions from this, like I said before, you should also try to introduce some sampler state in your shader now.

[quote name='Yura' timestamp='1357203186' post='5017018']
My shader is the same. I didn't change anything.
[/quote]

That's why I was saying that please change it a bit and use a sampler state with some basic filtering, like LINEAR.

The way I see that area is going white around your lines is most probably because of aliasing. So once aliasing is out of the way a clearer picture should come out.

And my bad that I kept overlooking the VFACE comment of yours. It makes sense now, why to use one kind of input than the other.

My shader is the same. I didn't change anything.

That's why I was saying that please change it a bit and use a sampler state with some basic filtering, like LINEAR.

The way I see that area is going white around your lines is most probably because of aliasing. So once aliasing is out of the way a clearer picture should come out.

And my bad that I kept overlooking the VFACE comment of yours. It makes sense now, why to use one kind of input than the other.

With sampler state I have some problems. I have no idea how to implement it. I found some basic information about samplers, filters etc, but I didn't get it... Maybe you have some useful links or code samples?

I can't change something only for lines or only for triangles. Changes will affect on all primitives.

effect.SetValue("worldViewProj", worldViewProj);
effect.BeginPass(0);

    d3dDevice.Indices = triangles;
    d3dDevice.SetStreamSource(0, vertices, 0, Marshal.SizeOf(typeof(Vertex)));
    d3dDevice.DrawIndexedPrimitive(PrimitiveType.TriangleList, 0, 0, totalPointsCount, 0, totalPointsCount / 2);
    
    d3dDevice.Indices = lines;          
    effect.SetValue("worldViewProj", slightlyCloserWorldViewProj);
    effect.CommitChanges();    // <- needed when changing shader constants during a running pass	!
    d3dDevice.DrawIndexedPrimitive(PrimitiveType.LineList, 0, 0, totalPointsCount, 0, totalPointsCount);

effect.EndPass();

I can't change something only for lines or only for triangles. Changes will affect on all primitives.

You have two draw calls, so in-between you can change render states and shader constants:

effect.SetValue("worldViewProj", worldViewProj);
effect.BeginPass(0);

    d3dDevice.Indices = triangles;
    d3dDevice.SetStreamSource(0, vertices, 0, Marshal.SizeOf(typeof(Vertex)));
    d3dDevice.DrawIndexedPrimitive(PrimitiveType.TriangleList, 0, 0, totalPointsCount, 0, totalPointsCount / 2);
    
    d3dDevice.Indices = lines;          
    effect.SetValue("worldViewProj", slightlyCloserWorldViewProj);
    effect.CommitChanges();    // <- needed when changing shader constants during a running pass	!
    d3dDevice.DrawIndexedPrimitive(PrimitiveType.LineList, 0, 0, totalPointsCount, 0, totalPointsCount);

effect.EndPass();

slightlyCloserWorldViewProj is working with formula WVP * Matrix.Translation(0, 0, -0.001f);

Thanks a lot!

But, If I'll change my surface to another one, which has Z values with opposite sign, does it mean that my line grid will become invisible (will be absorbed)?