Hi all,

I'm trying to 'extrude' 4 sides of a cube, from a linestrip with 4 vertices.

After playing around I think I'm almost there, but I'm missing something crucial.

This is what I do:

- create 4 vertices (VTX buffer)

- draw them as a linestrip

- execute my Geometry Shader, taking line[2] as input and a trianglestream as output

The result so far is that 2 of the 4 sides of the extracted cube (without 'caps'/ top-bottom) are drawn correct.

It might be something with the normals, not sure.

Here's the code, can someone help me out?

[maxvertexcount(4)]
void GSStrip(line VertexOut gin[2], inout TriangleStream<GeoOut> stream)
{
	/* 4 vertices as input, 1 linestrip

	1	.-----------.	2		0 = -1, 0, -1
		|			|			1 = -1, 0,  1
		|			|			2 =  1, 0,  1
		|			|			3 =  1, 0, -1
	0	.-----------.	3

	create 4 new vertices 'above' 0 to 3 and draw sides of the cube using trianglestrips
	the input linestrip is dimension[2] -> meaning the GS processes 'per line' / 2 connected vertices

	2 .---------. 3
	  |			|		Y = up (2 and 3 are XZ == 0 and 1)
	  |			|
	0 .---------. 1

	Strip1: 2, 3, 0
	Strip2: 3, 0, 1

	*/
	float4 v[4];
	v[0] = float4(gin[0].CenterW, 1.0f);
	v[1] = float4(gin[1].CenterW, 1.0f);

	v[2] = v[0];
	v[2].y = 15.0f;

	v[3] = v[1];
	v[3].y = 15.0f;

	GeoOut gout;
	[unroll]

	for(int i = 0;i<4;++i)
	{
		gout.PosH     = mul(v[i], gViewProj);
		gout.PosW     = v[i].xyz;
		gout.NormalW  = float3(-1.0f, 0.0f, 0.0f);
		gout.Tex      = gTexC[i];
		
		stream.Append(gout);
	}
//	stream.RestartStrip();
}

// in the code, creating the vtx buffer
	vertices[0].Pos = XMFLOAT3(-5.0, 0.0f, -5.0f);
	vertices[1].Pos = XMFLOAT3(-5.0, 0.0f, 5.0f);
	vertices[2].Pos = XMFLOAT3(5.0, 0.0f, 5.0f);
	vertices[3].Pos = XMFLOAT3(5.0, 0.0f, -5.0f);

Been thinking about it, could it be that the Geometry shader executes per 2 vertices, not knowing the others.

That could explain that sharing vertices will not work.

If so, wouldn't a linestrip be able to solve this in the first place?

I've tried disabling culling, now 3 out of 4 sides turn up.

The same result I achieve when drawing using 2 triangle lists (and a RestartStrip in between), with maxvertexcount[6].

Fixing the winding order for the strip also gives we 3 out of 4 sides of the cube now.

One conclusion of this is that the drawing order can indeed be an issue, I've tacked this now (using the initial Strip approach).

There's just one thing I don't understand, where's the 4th side?

Maybe I need to have the 1st vertex twice in the vtx buffer and use 5 vertices (ending with the one I started with).

Update: this does work. It seems all good now.

Now I should be able to change the 4 (/5) vertices into a list of vertices making up a circle. Using the same GS, that will finish the exercise I'm doing :)

Thanks for helping out