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# Specifying the order of a triangle strip?

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Hey all, I'm working on a .map parser. I've learned quite a bit from Stefan's article here: http://folk.uio.no/stefanha/ However, I've run into a little snag and I'm looking for a way to rectify it. From what I can gather, DirectX draws its triangle strips like this (when you're drawing a quad): 2 3 0 1 If you follow my sexah ASCII art. However, when I try to simply create my verticies for my quad in the same order I'm getting them from the CMF, I end up with 'broken quads'. You know the kind I mean, they look like: ____ | / |__ That kind of stuff. I've found that if I juggle the verticies around, I can get the verticies to draw just fine. Basically, I get the verticies in the order A B C D and I change that order to D A C B, the polygons all end-up fine. However, this juggling is pretty silly and will only complicate things as I add support for triangles and polygons more complex then 3 or 4 points. So I figured I'd fix the problem now, before I continue. So, going back to my original assumption, we can see the quad is drawn like this: C B D A So is there a way to change my polygon ordering, so that I start at A, move up to B, left to C, then finish my other triangle to form the quad with D? Near as I can tell, thats still CCW it just starts at a different vertex. This would make the order for all of my other polygons work, I would hope. Do I maybe want soemthing other then a triangle strip? So any ideas? [edited by - GroZZleR on February 7, 2004 12:13:43 AM]

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You''re going to apply this to a terrain map right? I wouldn''t recommend triangle strips for such. You should use Triangle Lists. Also, I didn''t quite understand your explanation, but I''m not sure you comprehend triangle strips completely. Forgive me if I''m wrong. I''ll give a brief synopsis just in case.

Triangle strips take the first two points in the vertex buffer or index buffer, call those two points of the starting triangle. Then the third point encountered makes the first full triangle. Then the fourth point defines a triangle between the second and third points. Then the fifth point defines a triangle between the third and fourth points, and so forth. So, you can''t draw the quad as A,B,C,D as in:

C B
D A

It has to be A,B,D,C or A,D,B,C or B,C,A,D or etc... Does that make sense? Is this what you were asking or am I confused?

Also, the vertex declaration "Direction" (i.e. clockwise or counter) is defined by the first three vertices in a Triangle Strip. Make sure you have culling set appropriately.

Chris

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I don't think we're on the same wavelength and probably just confused eachother more =P

The map is going to be used in many different ways, but definitely not a height map if thats what you mean by terrain. More Half-Life / Quake style levels.

Alright.

I parse the map file, and I get 4 points like this:
-1, 0.5, 1 (A)
-0.5, 0.5, 1 (B)
-0.5, 0.5, -1 (C)
-1, 0.5, -1 (D)

But when I slap the verticies into my Quad, I get the 'broken polygon' as discussed earlier.

By re-arranging the verticies, to this:
-1, 0.5, -1 (D)
-1, 0.5, 1 (A)
-0.5, 0.5, -1 (C)
-0.5, 0.5, 1 (B)

My Quad is drawn just fine. Bottom Left, Bottom Right, Top Left, Top Right.

What I'd like to do, is render the polygon in the same direction I'm given it though. So not re-arrange them, but take the original points and render from there. That way, all my verticies for future objects (triangles and others) are rendered in the correct order as well.

I'm using a triangle strip because that seemed to make the most sense to me. I'll have to check out triangle lists I guess.

I wouldn't mind a solution for either primitive =)

[edited by - GroZZleR on February 8, 2004 1:13:15 AM]

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Use an index buffer with a triangle list.

Assuming the quad is like the one below

1------2
| |
| |
| |
3------4

Youd have to set the vertex and index buffers like so:
Vertex Buffer: 1, 2, 3, 4
Index Buffer: 1, 2, 3, 3, 2, 4

and then call DrawIndexedPrimitive(0,2,D3DPT_TRIANGLELIST)

The logic behind this is to maintain readability while allowing vertex reuse. Basically, the index buffer specifies which vertices are to be drawn and in what order. Once you start using it youll scoff at triangle strips and fans.

Thats the logic but you might want to check on the syntax and the way DX orders its polys.

-- psamty

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I apologize for my poor ASCII art skills

i meant:

1------2
|xxxxxx|
|xxxxxx|
|xxxxxx|
3------4

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Oops... a little blooper on the syntax too....

DrawIndexedPrimitive(D3DPT_TRIANGLELIST, 0, 0, 0, 4, 0, 2)

Look into the SDK docs for an explanation of each of the arguments.

Creating an index buffer is much like creating a vertex buffer, look at the function CreateIndexBuffer()

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It looks like the actual problem is that the verts are ordered such that they should be drawn using a tri-strip, but you''re telling D3D to draw a quad, and you will have to re-order to get that to work. Instead, try drawing the four verts as a strip in the order that they''re specified, and see what happens.

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quote:
Original post by Anonymous Poster
It looks like the actual problem is that the verts are ordered such that they should be drawn using a tri-strip, but you're telling D3D to draw a quad, and you will have to re-order to get that to work. Instead, try drawing the four verts as a strip in the order that they're specified, and see what happens.

I am currently using a strip and without reordering the polygon comes out malformed. Unless I've misunderstood what you mean.

---

Thanks psamty10, I'll try using an Index Buffer for it. Won't this be adding a little bit of overhead though? Or is it just an insignificant amount?

[edited by - GroZZleR on February 8, 2004 1:20:48 PM]

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Somethings amiss. Nothing fails, but I''m only getting 1 triangle in the quad, not two.

I''m not sure if its my code or not, I''m sure it is, so here''s a little code:

bool CQuad :: CreateIndexBuffer(void){	VOID *pBufferIndices = 0x00;	if(FAILED(m_pD3DDevice->CreateIndexBuffer(6 * sizeof(WORD), 0, D3DFMT_INDEX16, D3DPOOL_MANAGED, &m_pIndexBuffer, NULL)))	{		MessageBox(NULL, "CreateIndexBuffer", "CREATE INDEX BUFFER BROKE", MB_OK);		return false;	}	// WORD pIndices[] = { 1, 2, 3, 3, 2, 4 };	WORD pIndices[] = { 0, 1, 2, 2, 1, 3 };	if(FAILED(m_pIndexBuffer->Lock(0, 6 * sizeof(WORD), &pBufferIndices, 0)))	{		MessageBox(NULL, "Lock", "CREATE INDEX BUFFER BROKE", MB_OK);		return false;	}	// Copy stored indices to index buffer	memcpy(pBufferIndices, pIndices, 6 * sizeof(WORD));	if(FAILED(m_pIndexBuffer->Unlock()))	{		MessageBox(NULL, "Unlock", "CREATE INDEX BUFFER BROKE", MB_OK);		return false;	}	return true;}

Heres how I draw it:
void CQuad :: Render(void){	/*	// No Texture, Probably Don''t Need This...	m_pD3DDevice->SetTextureStageState(0, D3DTSS_COLOROP, D3DTOP_SELECTARG2);	m_pD3DDevice->SetTextureStageState(0, D3DTSS_COLORARG1, D3DTA_TEXTURE);	m_pD3DDevice->SetTextureStageState(0, D3DTSS_COLORARG2, D3DTA_CURRENT);	*/	//Select the material to use	m_pD3DDevice->SetMaterial(&m_matMaterial);	// Set Us Up.	m_pD3DDevice->SetFVF(QUAD_D3DFVF_CUSTOMVERTEX);	m_pD3DDevice->SetStreamSource(0, m_pVertexBuffer, 0, sizeof(QUAD_CUSTOMVERTEX));	m_pD3DDevice->SetIndices(m_pIndexBuffer);	// Draw!    //m_pD3DDevice->DrawPrimitive(D3DPT_TRIANGLESTRIP, 0, 2);	if(FAILED(m_pD3DDevice->DrawIndexedPrimitive(D3DPT_TRIANGLELIST, 0, 0, 4, 0, 2)))		MessageBox(NULL, "FAILED TO DRAW PRIMITIVE", "AHHH", MB_OK);	return;}

Nothing fails, and the VB and stuff is fine. Unless I have to alter the VB in someway to compensate for the IB, but the example from Andy Pike didn''t show anything like that so maybe I''m wrong. The VB was working fine with the DrawPrimitive call when I re-arranged the verticies, so I don''t think thats the problem.

When I get verticies like these:
-1, 0.5, 1 (A)
-0.5, 0.5, 1 (B)
-0.5, 0.5, -1 (C)
-1, 0.5, -1 (D)

And I order them, it does come out to
A B
C D

As psamty10''s example shows, so everything is in synch. I just can''t figure out whats wrong =(

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Hey woah,

I'm searching the forms for an answer to my question and I came across this thread, it's like the same problem I'm having... However, it's not answered. I tried the stuff above too and I've got the same problem as Grozzler.

Anyone still looking into this problem?!?!?!

- Newb Programmer: Geek++

EDIT: spelling

[edited by - GeekPlusPlus on February 8, 2004 11:45:59 PM]

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