Hello everyone,

I followed the Chapter "2.3: Simplified High-Quality Anti-Aliased Lines" from the ShaderX7 book. I copy-pasted the shader file from the CD and carefully ported the managing class to XNA. I cannot guarantee that the buffers are correctly filled but I am pretty sure.

Have a look at the first screenshot. The perspective is not correct. I chose my Line texture to be a square to enhance the effect. Looking at the wireframe (second screenshot), you can see that the vertex positions are in fact correct. The error is in the texture lookup.

I assume I cannot upload the whole shader since it's from the CD, but here are the lines that set the texture coordinates:

VS_OUTPUT VS(...)
{
    [...] // Calculate Vertex Positions (works)

    VS_OUTPUT output;
    output.textureUV.x = weights.z;
    output.textureUV.y = weights.w;
    output.textureUV.z = 0.0f;
    output.textureUV.w = 1.0f;
    return output;
}

PS_OUTPUT PS(VS_OUTPUT input)
{
    return g_Color * tex2Dproj( FilterTextureSampler, input.textureUV );
}

where weights contains the texture coordinates in z and w. g_Color is the Line's Color and FilterTextureSampler contains the square texture.

I understand that tex2Dproj divides the coordinates by the w-component. But w = 1.0f everywhere! Can somebody explain what this is supposed to do? Changing "tex2Dproj" to "tex2D" makes no difference. Why can't I just take the usual texture coordinates?

Going on several assumptions at my end (because I can't see your VS_OUTPUT structure and several other pieces of interest), I would say that don't assign textureUV values the way you're doing now.

Change your VS method's signature to pass this parameter "float2 vTexCoord0 : TEXCOORD0"

and then do an assignment to textureUV in below fashion :

output.textureUV = vTexCoord;

After doing this, see if something changes.

I'm afraid to post the whole shader code due to copyright issues. If you have the book, it is printed in chapter 2.3. I wanted to use the original shader to prevent error infusion.

What I can do is post my own adaptation of that code. It uses the original math/shader, but I added hardware instancing, so it should be ok. I also attached the class that manages the buffers.
My main change is that the texture coordinates are now in input.Position.xy instead of weights.zw

I applied your change to both versions, i.e. use float2 instead of float4 for the texture coordinates und tex2D instead of tex2Dproj. The output is the same every time.