# OpenGL VBO with ELEMENT_ARRAY causes OpenGL segfault

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Hey all, I'm trying to get a heightfield running with VBOs and everytime I try to render the terrain my app crashes and the debugger tells me there was a segfault in libGLcore.so (linux - ubuntu feisty). Here's how I set up my vbo, heights and indices are stored in std::vector, indices as int, heights as a set of three float. Without using indices it works seamlessly..
glGenBuffers(1,&m_VBO);
glBindBuffer(GL_ARRAY_BUFFER,m_VBO);
glBufferData(GL_ARRAY_BUFFER,mc_heights.size()*3*sizeof(float),&mc_heights[0],GL_STATIC_DRAW);

//Index Buffer
glGenBuffers(1,&m_VBOi);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER,m_VBOi);

//as im filling my index data later on I first call this
glBindBufferARB(GL_ARRAY_BUFFER_ARB, 0);
glBindBufferARB(GL_ELEMENT_ARRAY_BUFFER_ARB, 0);

//after filling index buffer
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER,m_VBOi);
glBufferData(GL_ELEMENT_ARRAY_BUFFER,mc_indices.size()*sizeof(int),&mc_indices[0],GL_STATIC_DRAW);
glBindBufferARB(GL_ELEMENT_ARRAY_BUFFER_ARB, 0);


And when I render I do this:
glBindBuffer(GL_ARRAY_BUFFER,m_VBO);

glEnableClientState(GL_VERTEX_ARRAY); //i'm not sure about that, though
glVertexPointer(3, GL_FLOAT, 0, 0);

glBindBuffer(GL_ELEMENT_ARRAY_BUFFER,m_VBOi);
glIndexPointer(GL_INT, 0, 0);

glDrawElements(GL_TRIANGLE_STRIP, mc_indices.size(), GL_INT, 0);

glDisableClientState(GL_VERTEX_ARRAY);

glBindBufferARB(GL_ARRAY_BUFFER_ARB, 0);
glBindBufferARB(GL_ELEMENT_ARRAY_BUFFER_ARB, 0);


Am I missing anything? I'm not really sure about the EnableClientState(VertexArray)-thing, is this needed? And if it is, is there an aquivalent for elementArrays, too that I need to use? Ah, and perhaps you should know that it only crashes if I call the render method, even if I comment out the DrawElements-line. Thanks in advance!

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1: Why are you mixing names with and without the ARB suffix? Stick to one, and don't mix.

2: The index array must be unsigned. Read the documentation for glDrawElements and see what the valid constants for the third parameter are.

3: While looking in the documentation, take a look at what glIndexPointer does. It does nothing related to the index data for indexed vertex arrays, which you seems to think, and is most likely totally useless in this case as you're probably using RGBA color mode, not indexed colors.

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1: yup, sorry.. I removed all ARBs

2: k, I changed it so my index array is now unsigned int

3: how do I have to tell OpenGL which indices to use then?

glEnableClientState(GL_VERTEX_ARRAY);glBindBuffer(GL_ARRAY_BUFFER,m_VBO);glVertexPointer(3, GL_FLOAT, 0, 0);                 //glBindBuffer(GL_ELEMENT_ARRAY_BUFFER,m_VBOi);//mc_indices is of the type std::vector<unsigned int>glDrawElements(GL_TRIANGLE_STRIP, mc_indices.size(), GL_UNSIGNED_INT, &mc_indices[0]);glDisableClientState(GL_VERTEX_ARRAY);               glBindBuffer(GL_ARRAY_BUFFER, 0);//glBindBuffer(GL_ELEMENT_ARRAY_BUFFER,0);

The problem is now, that the whole app makes my computer do nothing until I restart the X-Server if I use glBindBuffer(ElementArray,mVBOi); , if I dont, I just see nothing (I'm not rendering anything apart from the terrain).

Is that because I then have both, the indices given to glDrawElements and the ELEMENT_ARRAY_BUFFER? Do I have to pass 0 to draw elements for the indices so it uses the ELEMENT_ARRAY?

I cant find any good ressource on indexed VBOs on the net..

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Try this source code:

(I assume that mc_heights is std::vector<float> and mc_indices is std::vector<unsigned int>)

glGenBuffersARB(1,&m_VBO); glBindBufferARB(GL_ARRAY_BUFFER_ARB,m_VBO);glBufferDataARB(GL_ARRAY_BUFFER_ARB,mc_heights.size()*sizeof(float),&mc_heights[0],GL_STATIC_DRAW_ARB);glBindBufferARB(GL_ARRAY_BUFFER_ARB,0);glGenBuffersARB(1,&m_VBOi);glBindBufferARB(GL_ELEMENT_ARRAY_BUFFER_ARB,m_VBOi);glBufferDataARB(GL_ELEMENT_ARRAY_BUFFER_ARB,mc_indices.size()*sizeof(unsigned int),&mc_indices[0],GL_STATIC_DRAW_ARB);glBindBufferARB(GL_ELEMENT_ARRAY_BUFFER_ARB, 0);

and for drawing:
glEnableClientState(GL_VERTEX_ARRAY);glBindBufferARB(GL_ARRAY_BUFFER_ARB,m_VBO);glBindBufferARB(GL_ELEMENT_ARRAY_BUFFER_ARB,m_VBOi);glVertexPointer(3, GL_FLOAT, 0, 0);glDrawElements(GL_TRIANGLE_STRIP, mc_indices.size(), GL_UNSIGNED_INT, 0);glBindBufferARB(GL_ARRAY_BUFFER_ARB, 0);glBindBufferARB(GL_ELEMENT_ARRAY_BUFFER_ARB, 0);glDisableClientState(GL_VERTEX_ARRAY);

btw glIndexPointer is for drawing in palletized color format (not RGB). It has nothing to do with vertex indices for glDrawElements funcion.

Quote:
 Original post by Caste3: how do I have to tell OpenGL which indices to use then?

You provide offset in bytes in your indices buffer in last argument of glDrawElements function.

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Thanks guys, now I dont get any errors any more.. But I cant see anything (=.. But I guess theres something wrong with my camera setup..

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