Question

I’ve been bashing my head on the wall this afternoon persuading my openGLES2.0 code to perform correctly when I move from using VBO to VAO / VBO. Basically I’m working my way through Apple’s “expert” advice on openGLES and moving to using Vertex Array Objects was top of the list …

I’ve reviewed the similar question and response here but that didn’t seem to help me, other than re-assure me that other people run into similar problems 🙁

My scenario is that I have approximately 500 rectangular textures moving around the screen. The code all works fine without VAO, but when I define USE_VAO (my constant) it’s crashing on the first draw elements call. I’m obviously not understanding VAO properly … but I can’t see the error of my ways!

The setupBeforeRender method is called as the last part of my setup before entering the render loop.

-(void) setupBeforeRender {

glClearColor(0.6, 0.6, 0.6, 1);
glViewport(0, 0, self.frame.size.width, self.frame.size.height);
glEnable(GL_DEPTH_TEST);

glUniform1i(_textureUniform, 0);
glActiveTexture(GL_TEXTURE0); 

glEnableVertexAttribArray(_positionSlot);
glEnableVertexAttribArray(_colorSlot);
glEnableVertexAttribArray(_texCoordSlot);


glGenBuffers(1, &_indexBuffer);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, _indexBuffer);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(Indices), Indices, GL_STATIC_DRAW);

}

And here’s the render method

- (void)render:(CADisplayLink*)displayLink {

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

// Model view matrix and projection code removed for clarity

GLsizei stride = sizeof(Vertex);
const GLvoid* colourOffset = (GLvoid *) sizeof(float[3]);
const GLvoid* textureOffset = (GLvoid *) sizeof(float[7]);

 for (my objectToDraw in objectToDrawArray)
 {

    if (objectToDraw.vertexBufferObject == 0)
    {

        #ifdef USE_VAO

        glGenVertexArraysOES(1,&_vao);
        glBindVertexArrayOES(_vao);

        glVertexAttribPointer(_positionSlot, 3, GL_FLOAT, GL_FALSE, stride, 0);
        glVertexAttribPointer(_colorSlot, 4, GL_FLOAT, GL_FALSE, stride, colourOffset);
        glVertexAttribPointer(_texCoordSlot, 2, GL_FLOAT, GL_FALSE,stride, textureOffset);    

        objectToDraw.vertexBufferObject = [objectToDraw createAndBindVBO];
        objectToDraw.vertexArrayObject = _vao;
        glBindBuffer(GL_ARRAY_BUFFER, 0);
        glBindVertexArrayOES(0);

        #else

        objectToDraw.vertexBufferObject = [objectToDraw createAndBindVBO];

        #endif
    }

   // Texture binding removed for clarity     

    #ifdef USE_VAO

    // This code crashes with EXC_BAD_ACCESS on the glDrawElements  

    glBindVertexArrayOES(objectToDraw.vertexArrayObject);
    glDrawElements(GL_TRIANGLES, sizeof(Indices) / sizeof(Indices[0]), GL_UNSIGNED_SHORT,0);
    glBindVertexArrayOES(0);                

    #else        

    // This path works fine. So turning VAO off works :(

    glBindBuffer(GL_ARRAY_BUFFER, storyTile.vertexBufferObject);
    glVertexAttribPointer(_positionSlot, 3, GL_FLOAT, GL_FALSE, stride, 0);
    glVertexAttribPointer(_colorSlot, 4, GL_FLOAT, GL_FALSE, stride, colourOffset);
    glVertexAttribPointer(_texCoordSlot, 2, GL_FLOAT, GL_FALSE, stride, textureOffset);

    glDrawElements(GL_TRIANGLES, sizeof(Indices) / sizeof(Indices[0]), GL_UNSIGNED_SHORT,0);

    #endif

} // End for each object

[_context presentRenderbuffer:GL_RENDERBUFFER];

}

Finally, my create and bind VBO method looks like this;

-(GLuint) createAndBindVBO {

const float* rgba = CGColorGetComponents([self.colour CGColor]);

Vertex Vertices[] = {
    {{0, 1, 0}, {1, 0, 1, 1}, {0,1}},
    {{0, 0, 0}, {1, 0, 1, 1}, {0,0}},
    {{1, 1, 0}, {1, 0, 1, 1}, {1,1}},
    {{1, 0, 0}, {1, 0, 1, 1}, {1,0}}

};

// Code removed for clarity - sets up geometry and colours    

GLuint vertexBuffer;
glGenBuffers(1, &vertexBuffer);
glBindBuffer(GL_ARRAY_BUFFER, vertexBuffer);
glBufferData(GL_ARRAY_BUFFER, sizeof(Vertices), Vertices, GL_STATIC_DRAW);

return vertexBuffer;
}

I’ve tried various permutations of this and have sprinkled the code with glGetError() to see if that helps point to where the problem arises. Alas I get no errors, other than the BAD_ACCESS crash on the drawElements call.

EDIT: As suggested, this unfortunately also doesn’t work

objectToDraw.vertexBufferObject = [objectToDraw createVBO];

glGenVertexArraysOES(1,&_vao);
glBindVertexArrayOES(_vao);
glBindBuffer(GL_ARRAY_BUFFER, objectToDraw.vertexBufferObject);

glVertexAttribPointer(_positionSlot, 3, GL_FLOAT, GL_FALSE, stride, 0);
glVertexAttribPointer(_colorSlot, 4, GL_FLOAT, GL_FALSE, stride, colourOffset);
glVertexAttribPointer(_texCoordSlot, 2, GL_FLOAT, GL_FALSE,stride, textureOffset);    

objectToDraw.vertexArrayObject = _vao;
glBindVertexArrayOES(0);

I must be doing something dumb with the vertex array object … but can someone figure out what the problem is?

Answer 1

The vertex array enabled flags are part of the VAO state, so you need to enable the vertex attribute arrays using glEnableVertexAttribArray while the VAO is bound.

From: http://www.khronos.org/registry/gles/extensions/OES/OES_vertex_array_object.txt

The resulting
vertex array object is a new state vector, comprising all the state values (listed in Table 6.2, except ARRAY_BUFFER_BINDING):

• VERTEX_ATTRIB_ARRAY_ENABLED
• VERTEX_ATTRIB_ARRAY_SIZE,
• VERTEX_ATTRIB_ARRAY_STRIDE,
• VERTEX_ATTRIB_ARRAY_TYPE,
• VERTEX_ATTRIB_ARRAY_NORMALIZED,
• VERTEX_ATTRIB_ARRAY_POINTER,
• ELEMENT_ARRAY_BUFFER_BINDING,
• VERTEX_ATTRIB_ARRAY_BUFFER_BINDING.