# OpenGL VBO with ELEMENT_ARRAY causes OpenGL segfault

## Recommended Posts

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!

##### Share on other sites
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.

##### Share on other sites
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..

##### Share on other sites
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.

##### Share on other sites
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..

## Create an account

Register a new account

• ### Forum Statistics

• Total Topics
627757
• Total Posts
2978950
• ### Similar Content

• Hello! As an exercise for delving into modern OpenGL, I'm creating a simple .obj renderer. I want to support things like varying degrees of specularity, geometry opacity, things like that, on a per-material basis. Different materials can also have different textures. Basic .obj necessities. I've done this in old school OpenGL, but modern OpenGL has its own thing going on, and I'd like to conform as closely to the standards as possible so as to keep the program running correctly, and I'm hoping to avoid picking up bad habits this early on.
Reading around on the OpenGL Wiki, one tip in particular really stands out to me on this page:
For something like a renderer for .obj files, this sort of thing seems almost ideal, but according to the wiki, it's a bad idea. Interesting to note!
So, here's what the plan is so far as far as loading goes:
Set up a type for materials so that materials can be created and destroyed. They will contain things like diffuse color, diffuse texture, geometry opacity, and so on, for each material in the .mtl file. Since .obj files are conveniently split up by material, I can load different groups of vertices/normals/UVs and triangles into different blocks of data for different models. When it comes to the rendering, I get a bit lost. I can either:
Between drawing triangle groups, call glUseProgram to use a different shader for that particular geometry (so a unique shader just for the material that is shared by this triangle group). or
Between drawing triangle groups, call glUniform a few times to adjust different parameters within the "master shader", such as specularity, diffuse color, and geometry opacity. In both cases, I still have to call glBindTexture between drawing triangle groups in order to bind the diffuse texture used by the material, so there doesn't seem to be a way around having the CPU do *something* during the rendering process instead of letting the GPU do everything all at once.
The second option here seems less cluttered, however. There are less shaders to keep up with while one "master shader" handles it all. I don't have to duplicate any code or compile multiple shaders. Arguably, I could always have the shader program for each material be embedded in the material itself, and be auto-generated upon loading the material from the .mtl file. But this still leads to constantly calling glUseProgram, much more than is probably necessary in order to properly render the .obj. There seem to be a number of differing opinions on if it's okay to use hundreds of shaders or if it's best to just use tens of shaders.
So, ultimately, what is the "right" way to do this? Does using a "master shader" (or a few variants of one) bog down the system compared to using hundreds of shader programs each dedicated to their own corresponding materials? Keeping in mind that the "master shaders" would have to track these additional uniforms and potentially have numerous branches of ifs, it may be possible that the ifs will lead to additional and unnecessary processing. But would that more expensive than constantly calling glUseProgram to switch shaders, or storing the shaders to begin with?
With all these angles to consider, it's difficult to come to a conclusion. Both possible methods work, and both seem rather convenient for their own reasons, but which is the most performant? Please help this beginner/dummy understand. Thank you!

• I want to make professional java 3d game with server program and database,packet handling for multiplayer and client-server communicating,maps rendering,models,and stuffs Which aspect of java can I learn and where can I learn java Lwjgl OpenGL rendering Like minecraft and world of tanks

• A friend of mine and I are making a 2D game engine as a learning experience and to hopefully build upon the experience in the long run.

-What I'm using:
C++;. Since im learning this language while in college and its one of the popular language to make games with why not.     Visual Studios; Im using a windows so yea.     SDL or GLFW; was thinking about SDL since i do some research on it where it is catching my interest but i hear SDL is a huge package compared to GLFW, so i may do GLFW to start with as learning since i may get overwhelmed with SDL.
-Questions
Knowing what we want in the engine what should our main focus be in terms of learning. File managements, with headers, functions ect. How can i properly manage files with out confusing myself and my friend when sharing code. Alternative to Visual studios: My friend has a mac and cant properly use Vis studios, is there another alternative to it?

• Both functions are available since 3.0, and I'm currently using glMapBuffer(), which works fine.
But, I was wondering if anyone has experienced advantage in using glMapBufferRange(), which allows to specify the range of the mapped buffer. Could this be only a safety measure or does it improve performance?
Note: I'm not asking about glBufferSubData()/glBufferData. Those two are irrelevant in this case.
• By xhcao
Before using void glBindImageTexture(    GLuint unit, GLuint texture, GLint level, GLboolean layered, GLint layer, GLenum access, GLenum format), does need to make sure that texture is completeness.

• 11
• 10
• 10
• 23
• 14