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# How does the pipeline know the index what it need when it draws a quad using the vertices generated by the Geometry Shader?

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4 replies to this topic

### #1swdever  Members

Posted 02 April 2013 - 09:47 PM

Hi! I am learning how the Geometry Shader works in D3D11. I am studying a sample which inputs one point into the Geometry Shader and outputs a quad each time. As shown in the figure below, the sample specifies the vertices generated by the Geometry Shader.(Sorry. I don't know how to upload a image. Hope you can understand the figure.)

v1               v3

------------------

|                 |

|                 |

|                 |

|                 |

------------------

v0                v2

With general method,if we want to draw a quad, we pass four vertices to the VertexBuffer, then create a IndexBuffer, binding them to the pipeline. Then the pipeline uses the data in the IndexBuffer and the VertexBuffer to draw two triangles what make up a quad. But in the sample I am studying, there is no IndexBuffer. The sample just generates these four vertices and append them to a TriangleStream object in the order: v0,v1,v2,v3, at the end of the Geometry Shader. Since D3D11 uses counter-clockwise culling state by default, visible triangles must be drawn in clockwise order. But how does the pipeline determine which three vertices of these four vertices to choose and their order to draw a visible triangle. There is no IndexBuffer in the sample. Maybe the Geometry Shader's output is drawn using a fixed index?Can anybody help me figure it out?

PS: The sample works well.

Edited by swdever, 03 April 2013 - 12:19 AM.

### #2Steve_Segreto  Members

Posted 02 April 2013 - 11:26 PM

So unless you provide adjacency information to your geometry shader, it appears to boil down to this value (http://msdn.microsoft.com/en-us/library/hh404489(v=vs.85).aspx):

FrontCounterClockwise

Type: BOOL

Specifies whether a triangle is front- or back-facing. If TRUE, a triangle will be considered front-facing if its vertices are counter-clockwise on the render target and considered back-facing if they are clockwise. If FALSE, the opposite is true.

Edited by Steve_Segreto, 02 April 2013 - 11:30 PM.

### #3MJP  Moderators

Posted 03 April 2013 - 12:01 AM

The triangles you output from the GS are not indexed at all, since there are no indices. Instead you output one or more triangle strips. The vertices that you append to a TriangleStream form a strip according to the standard rules for a triangle strip topology. To mark the end of a strip, you call RestartStrip. If you want to just output a triangle list you can do so by appending 3 vertices, calling RestartStrip, and then repeating.

So in the case of your quad output from the GS, if you append all 4 verts and then call RestartStrip your 2 triangles will be (v0, v1, v2) and (v1, v2, v3), which means the winding order will be clockwise.

### #4swdever  Members

Posted 03 April 2013 - 12:51 AM

The triangles you output from the GS are not indexed at all, since there are no indices. Instead you output one or more triangle strips. The vertices that you append to a TriangleStream form a strip according to the standard rules for a triangle strip topology. To mark the end of a strip, you call RestartStrip. If you want to just output a triangle list you can do so by appending 3 vertices, calling RestartStrip, and then repeating.

So in the case of your quad output from the GS, if you append all 4 verts and then call RestartStrip your 2 triangles will be (v0, v1, v2) and (v1, v2, v3), which means the winding order will be clockwise.

I'm sorry there is an error in the figure I provided. It's my fault. I have corrected it. If D3D uses the order you provided to draw, that is, (v0, v1, v2) for the first triangle and (v1, v2, v3) for the second triangle. Then the first triangle's winding order will be clockwise and the second triangle's will be counter-clockwise .

I find the diagram of vertex ordering for primitive types in this link.

http://msdn.microsoft.com/en-us/library/bb205124%28v=VS.85%29.aspx

Maybe using the standard rules, the two triangles will be (v0, v1, v2) and (v1, v3, v2)?

### #5swdever  Members

Posted 03 April 2013 - 01:02 AM

So unless you provide adjacency information to your geometry shader, it appears to boil down to this value (http://msdn.microsoft.com/en-us/library/hh404489(v=vs.85).aspx):

FrontCounterClockwise

Type: BOOL

Specifies whether a triangle is front- or back-facing. If TRUE, a triangle will be considered front-facing if its vertices are counter-clockwise on the render target and considered back-facing if they are clockwise. If FALSE, the opposite is true.

So unless you provide adjacency information to your geometry shader, it appears to boil down to this value (http://msdn.microsoft.com/en-us/library/hh404489(v=vs.85).aspx):

FrontCounterClockwise

Type: BOOL

Specifies whether a triangle is front- or back-facing. If TRUE, a triangle will be considered front-facing if its vertices are counter-clockwise on the render target and considered back-facing if they are clockwise. If FALSE, the opposite is true.

So unless you provide adjacency information to your geometry shader, it appears to boil down to this value (http://msdn.microsoft.com/en-us/library/hh404489(v=vs.85).aspx):

FrontCounterClockwise

Type: BOOL

Specifies whether a triangle is front- or back-facing. If TRUE, a triangle will be considered front-facing if its vertices are counter-clockwise on the render target and considered back-facing if they are clockwise. If FALSE, the opposite is true.

So unless you provide adjacency information to your geometry shader, it appears to boil down to this value (http://msdn.microsoft.com/en-us/library/hh404489(v=vs.85).aspx):

FrontCounterClockwise

Type: BOOL

Specifies whether a triangle is front- or back-facing. If TRUE, a triangle will be considered front-facing if its vertices are counter-clockwise on the render target and considered back-facing if they are clockwise. If FALSE, the opposite is true.

I think you don't understand what I have asked. Maybe it because of my poor English. But I found the first link you provided is useful. I have got a deep understand in the circumstance I have encountered. Thank you!

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