Question

I want a vertex array object in OpenGL ES 2.0 to hold two attributes from different buffers, the second buffer being read from client memory (glBindBuffer(GL_ARRAY_BUFFER, 0)) But I get a runtime error:

GLuint my_vao;
GLuint my_buffer_attrib0;
GLfloat attrib0_data[] = { 0, 0, 0, 0 };
GLfloat attrib1_data[] =  { 1, 1, 1, 1 };

void init()
{
    // setup vao
    glGenVertexArraysOES(1, &my_vao);
    glBindVertexArrayOES(my_vao);

    // setup attrib0 as a vbo
    glGenBuffers( 1, &my_buffer_attrib0 );
    glBindBuffer(GL_ARRAY_BUFFER, my_buffer_attrib0);
    glBufferData( GL_ARRAY_BUFFER, sizeof(attrib0_data), attrib0_data, GL_STATIC_DRAW );
    glVertexAttribPointer(0, 4, GL_FLOAT, GL_FALSE, 0, 0);

    glEnableVertexAttribArray( 0 );
    glEnableVertexAttribArray( 1 );

    // "end" vao
    glBindVertexArrayOES( 0 );

}

void draw()
{

    glBindVertexArrayOES(my_vao);
    // (now I assume attrib0 is bound to my_buffer_attrib0, 
    //  and attrib1 is not bound. but is this assumption true?)

    // setup attrib1
    glBindBuffer( GL_ARRAY_BUFFER, 0 );
    glVertexAttribPointer(1, 4, GL_FLOAT, GL_FALSE, 0, attrib1_data);

    // draw using attrib0 and attrib1
    glDrawArrays( GL_POINTS, 0, 1 );  // runtime error: Thread1: EXC_BAD_ACCESS (code=2, address=0x0)

}

What I want to achieve is to wrap the binding of two attributes as a vertex array buffer:

void draw_ok()
{
    glBindVertexArrayOES( 0 );

    // setup attrib0
    glBindBuffer( GL_ARRAY_BUFFER, my_buffer_attrib0 );
    glVertexAttribPointer( 0, 4, GL_FLOAT, GL_FALSE, 0, 0);

    // setup attrib1
    glBindBuffer( GL_ARRAY_BUFFER, 0 );
    glVertexAttribPointer(1, 4, GL_FLOAT, GL_FALSE, 0, attrib1_data);

    glEnableVertexAttribArray( 0 );
    glEnableVertexAttribArray( 1 );

    // draw using attrib0 and attrib1
    glDrawArrays( GL_POINTS, 0, 1);  // ok
}

Is it possible to bind two different buffers in a vertex array object? Are OES_vertex_array_object’s different from (plain) OpenGL vertex array objects? Also note that I get this error in XCode running the iOS simulator. These are related links:

• Use of VAO around VBO in Open ES iPhone app Causes EXC_BAD_ACCESS When Call to glDrawElements
• OES_vertex_array_object

Answer 1

Well, a quote from the extension specifications explains it quite simply:

Should a vertex array object be allowed to encapsulate client vertex arrays?

RESOLVED: No. The OpenGL ES working group agreed that compatibility with OpenGL and the ability to to guide developers to more performant drawing by enforcing VBO usage were more important than the possibility of hurting adoption of VAOs.

So you can indeed bind two different buffers in a VAO, (well, the buffer binding isn’t stored in the VAO, anyway, only the source buffers of the individual attributes, set through glVertexAttribPointer) but you cannot use client space memory in a VAO, only VBOs. This is the same for desktop GL.

So I would advise you to store all your vertex data in VBOs. If you want to use client memory because the data is updated dynamically and you think VBOs won’t buy you anything there, that’s still the wrong approach. Just use a VBO with a dynamic usage (GL_DYNAMIC_DRAW or even GL_STREAM_DRAW) and update it using glBuffer(Sub)Data or glMapBuffer (or the good old glBufferData(..., NULL); glMapBuffer(GL_WRITE_ONLY) combination).