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UnshavenBastard

OpenGL VBO & Tao.OpenGL, AccessViolation @ glDrawElements

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Hey there. I tried to use VBOs with Tao framework in C#/.NET, there seem to have been threads about my problem on the tao forum, the titles are in google's cache, but I can't access the forum, tao site seems to be heavily compromised ATM? Are there known issues with this? Or I must have made a mistake that's too obvious to see it, haha. If I try to glDrawElements(..), on my Vista laptop it crashes there with an AccessViolationException. Interestingly enough, on my main machine with WinXP, it does not crash, but apparently nothing gets drawn. (my main machine has a DX10+ radeon card, my laptop gfx card is said to support OpenGL 1.5 by the realtech OpenGL extensions viewer software and I can successfully execute tutorial exe's of native win32 tutorials of how to use VBOs) I will only include the VBO compilation & draw call code here to make this less messy ^^ I have tested to draw regular vertex arrays with the same array packing methods & data structures that I omitted here, and the calls are the same just that I don't bind buffer objects, and do specify pointers to client-side array data with glvertexPointer, Texcoordpointer and on glDrawElements... and guess what, it works without problems. So it should be something in the code I will post below. Btw., the test whether the VBO has the size that it should after sending the data never fails. If someone can tell me what's wrong, a thousand thanks, since this has cost me many annoying houre now ;-)
	void transfer_data(int target, object buf, int size, int bufferId, int usage)
	{
		Gl.glBindBuffer( target, bufferId );
		Gl.glBufferData( target, (IntPtr)size, buf,	usage );


		// check if buffer was created correctly
		int actualSize;
		Gl.glGetBufferParameteriv( target, Gl.GL_BUFFER_SIZE, out actualSize);
		if (size != actualSize)
		{
			delete_VBOs();
			throw new Exception("sending data to GPU failed");
		}
	}


	void make_VBOs()
	{

		// pos coord array
		//-------------------
		var vertexTemp = make_pos_coord_array();
		transfer_data(  Gl.GL_ARRAY_BUFFER, vertexTemp, (sizeof(float)*3 * vertexTemp.Length), _vertexVBO,
						Gl.GL_STATIC_DRAW
						);		
		vertexTemp = null;


		// tex coord array
		//-------------------
		var texTemp = make_tex_coord_array();
		transfer_data(  Gl.GL_ARRAY_BUFFER, texTemp, (sizeof(float)*2 * texTemp.Length), _texcoordVBO,
						Gl.GL_STATIC_DRAW
						);		
		texTemp = null;


		// index array
		//-------------------
		var indexTemp = make_index_array();
		transfer_data(	Gl.GL_ELEMENT_ARRAY_BUFFER, indexTemp, (sizeof(uint) * indexTemp.Length), _indexVBO,
						Gl.GL_STATIC_DRAW
						);
		indexTemp = null;

	}



	void render_with_VBOs()
	{
		Gl.glActiveTexture(Gl.GL_TEXTURE0);                     
		Gl.glEnable(Gl.GL_TEXTURE_2D);                          
		Gl.glBindTexture(Gl.GL_TEXTURE_2D, _terr.TextureId);
		
		Gl.glEnableClientState(Gl.GL_TEXTURE_COORD_ARRAY);
		Gl.glEnableClientState(Gl.GL_VERTEX_ARRAY);		


		Gl.glBindBuffer(Gl.GL_ARRAY_BUFFER, _texcoordVBO);		
		Gl.glTexCoordPointer( 2, Gl.GL_FLOAT, 0, 0 );

		Gl.glBindBuffer(Gl.GL_ARRAY_BUFFER, _vertexVBO);
		Gl.glVertexPointer( 3, Gl.GL_FLOAT, 0, 0 );
		
		
		Gl.glBindBuffer(Gl.GL_ELEMENT_ARRAY_BUFFER, _indexVBO);
		
		//
		// CRASH "AccessViolationException" in Vista, and nothing happens & nothing seems to be drawn in WinXP
		//				
		Gl.glDrawElements(Gl.GL_TRIANGLES, 3*_terr.NumTriangles, Gl.GL_UNSIGNED_INT, 0); // offset = 0



		Gl.glBindBuffer(Gl.GL_ELEMENT_ARRAY_BUFFER, 0);
		Gl.glBindBuffer(Gl.GL_ARRAY_BUFFER, 0);		

		Gl.glDisableClientState(Gl.GL_VERTEX_ARRAY);
		Gl.glDisableClientState(Gl.GL_TEXTURE_COORD_ARRAY);
	}



[Edited by - UnshavenBastard on March 20, 2010 12:49:43 PM]

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btw,

I use 4 openGl controls with each a different camera to draw the stuff,
and I used wglShareLists() to share the VBOs between their contexts.

To ensure that this works, I temporarily created display lists and texture objects, and called them on each of the 4 GL windows. This worked, too, without problems.

Just the damn VBOs do crash / not show up, *grrrr* !

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You've hit one of the ugliest parts of the Tao API:
Gl.glDrawElements(Gl.GL_TRIANGLES, 3*_terr.NumTriangles, Gl.GL_UNSIGNED_INT, 0); // offset = 0

The 0 offset is autoboxed into an object. Tao then pins the object, takes its address and passes it down to OpenGL. Ouch.

Use this instead:
Gl.glDrawElements(Gl.GL_TRIANGLES, 3*_terr.NumTriangles, Gl.GL_UNSIGNED_INT, IntPtr.Zero); // offset = 0

OpenTK uses generics and doesn't suffer from this issue. You might wish to consider switching (your existing Tao code will continue to work through OpenTK.Compatibility):
GL.DrawElements(BeginMode.Triangles, 3*_terr.NumTriangles, DrawElementsType.UnsignedInt, 0);


Edit: fixed layout.

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Quote:
Original post by Fiddler
You've hit one of the ugliest parts of the Tao API:
Gl.glDrawElements(Gl.GL_TRIANGLES, 3*_terr.NumTriangles, Gl.GL_UNSIGNED_INT, 0); // offset = 0

The 0 offset is autoboxed into an object. Tao then pins the object, takes its address and passes it down to OpenGL. Ouch.

Use this instead:
Gl.glDrawElements(Gl.GL_TRIANGLES, 3*_terr.NumTriangles, Gl.GL_UNSIGNED_INT, IntPtr.Zero); // offset = 0

[/code]


This doesn't seem to do anything.

I have stumpled upon OpenTK and checke out the documentation that's online a bit, it seems to be quite incomplete, some whole subject areas have only a headline.
I might consider switching, in the future, but for now, if it's only for "beauty" reasons, I have no time for that.


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