Question

I’m (re)learning DirectX and have moved into HLSL coding. Prior to using my custom .fx file I created a skybox for a game with a vertex buffer of quads. Everything worked fine…texture mapped and wrapped beautifully. However now that I have HLSL setup to manage the vertices there are distinctive seams where the quads meet. The textures all line up properly I just cant get rid of this damn seam!

I tend to think the problem is with the texCube…or rather all the texturing information here. I’m texturing the quads in DX…it may just be that I still don’t quite get the link between the two..not sure. Anyway thanks for the help in advance!

Heres the .fx file:

float4x4 World;
float4x4 View;
float4x4 Projection;

float3 CameraPosition;

Texture SkyBoxTexture; 

samplerCUBE SkyBoxSampler = sampler_state 
{ 
   texture = <SkyBoxTexture>; 
   minfilter = ANISOTROPIC; 
   mipfilter = LINEAR; 
   AddressU = Wrap; 
   AddressV = Wrap;
   AddressW = Wrap;
};

struct VertexShaderInput
{
    float4 Position : POSITION0;
};

struct VertexShaderOutput
{
    float4 Position : POSITION0;
    float3 TextureCoordinate : TEXCOORD0;
};

VertexShaderOutput VertexShaderFunction(VertexShaderInput input)
{
    VertexShaderOutput output;

    float4 worldPosition = mul(input.Position, World);
    float4 viewPosition = mul(worldPosition, View);
    output.Position = mul(viewPosition, Projection);

    float4 VertexPosition = mul(input.Position, World);
    output.TextureCoordinate = VertexPosition - CameraPosition;

    return output;
}

float4 PixelShaderFunction(VertexShaderOutput input) : COLOR0
{
    return texCUBE(SkyBoxSampler, normalize(input.TextureCoordinate));
}

technique Skybox
{
    pass Pass1
    {
        VertexShader = compile vs_2_0 VertexShaderFunction();
        PixelShader = compile ps_2_0 PixelShaderFunction();
    }
}

Answer 1

To avoid seams you need to draw your skybox in a single DrawIndexedPrimitive call, preferably using triangle strip. DON’T draw each face as separate primitive transformed with individual matrix or something like that – you WILL get seams. If you for some unexplainable reason don’t want to use single DrawIndexedPrimitive call for skybox parts, then you must ensure that all faces are drawn using same matrix (same world + view + projection matrix used in every call) and same coordinate values for corner vertices – i.e. “top” face should use exactly same vectors (position) for corners that are used by “side” faces.

Another thing is that you should either store skybox as

1. cubemap (looks like that’s what you’re doing) – make just 8 vertices for skybox, draw them as indexed primitive.
2. Or an unwrapped “atlas” texture that has unused areas filled. with border color.

Or – if you’re fine with shaders, you could “raytrace” skybox using shader.