Learning OpenGL GLSL - VAO buffer problems?

Question

I've just started digging through OpenGL and GLSL, and now stumbled on something I can't get my head around this one!? I've stepped back to loading a simple cube and using a simple shader on it, but the result is triangles drawn incorrectly and/or missing. The code I had working perfectly on meshes, but was attempting to move to using VAOs so none of the code for storing the vertices and indices has changed.

What I have for creating the VAO and buffers is this

//Create the Vertex array object
glGenVertexArrays(1, &vaoID);

// Finally create our vertex buffer objects
glGenBuffers(VBO_COUNT, mVBONames);

glBindVertexArray(vaoID);

    // Save vertex attributes into GPU
    glBindBuffer(GL_ARRAY_BUFFER, mVBONames[VERTEX_VBO]);

    // Copy data into the buffer object
    glBufferData(GL_ARRAY_BUFFER, lPolygonVertexCount*VERTEX_STRIDE*sizeof(GLfloat), lVertices, GL_STATIC_DRAW);

    glEnableVertexAttribArray(pos);
    glVertexAttribPointer(pos, 3, GL_FLOAT, GL_FALSE, VERTEX_STRIDE*sizeof(GLfloat),0);

    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, mVBONames[INDEX_VBO]);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, lPolygonCount*sizeof(unsigned int), lIndices, GL_STATIC_DRAW);

glBindVertexArray(0);

And the code for drawing the mesh.

glBindVertexArray(vaoID);
glUseProgram(shader->programID);

GLsizei lOffset = mSubMeshes[pMaterialIndex]->IndexOffset*sizeof(unsigned int);
const GLsizei lElementCount = mSubMeshes[pMaterialIndex]->TriangleCount*TRIAGNLE_VERTEX_COUNT;
glDrawElements(GL_TRIANGLES, lElementCount, GL_UNSIGNED_SHORT, reinterpret_cast<const GLvoid*>(lOffset));

// All the points are indeed in the correct place!?
//glPointSize(10.0f);
//glDrawElements(GL_POINTS, lElementCount, GL_UNSIGNED_SHORT, 0);

glUseProgram(0);
glBindVertexArray(0);

Eyes have become bleary looking at this today so any thoughts or a fresh set of eyes would be greatly appreciated.

Answer 1

The size of your GL_ELEMENT_ARRAY_BUFFER looks wrong:

lPolygonCount*sizeof(unsigned int)

Based on the rest of your code it's highly probable that should be:

lElementCount*sizeof(unsigned short)

You're also binding mVBONames[INDEX_VBO] as your GL_ELEMENT_ARRAY_BUFFER, which is going to overwrite the data you previously loaded to your GL_ARRAY_BUFFER - perhaps you meant to use mVBONames[INDEX_IBO] or something similar instead?

Other than that your VAO code looks good.

Answer 2

The element array is not part of the VAO. Only the vertex attribute arrays get saved to it. Make sure to bind the element array buffer before the draw call.

glBindVertexArray(vao);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, index_vbo);
glDrawElements(...);