Using VAO with glDrawElements

Question

I'm trying (for the first time) to use OpenGL 3.2 to draw some sprites to the screen. I'm trying to set up a VAO, but it's not working. I'm getting an EXC_BAD_ACCESS on the call to glDrawElements.

VAO setup:

// setting up VAO

glGenVertexArrays(1, &vao_);

glBindVertexArray(vao_);

glGenBuffers(1, &indices_id_);

glGenBuffers(1, &attributes_id_);

glBindBuffer(GL_ARRAY_BUFFER, attributes_id_);

constexpr GLfloat* ptr = 0;

::glVertexAttribPointer(attribute_position_, 2, GL_FLOAT, false, STRIDE, ptr);
::glVertexAttribPointer(attribute_region_, 2, GL_FLOAT, false, STRIDE, ptr + 2);
::glVertexAttribPointer(attribute_color_, 4, GL_FLOAT, false, STRIDE, ptr + 4);

::glEnableVertexAttribArray(attribute_position_);
::glEnableVertexAttribArray(attribute_region_);
::glEnableVertexAttribArray(attribute_color_);

glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, indices_id_);

{
    auto data = std::make_unique<short[]>(BATCH_SIZE * 6);

    short j = 0;

    for (std::size_t i = 0; i < BATCH_SIZE * 6; i += 6, j += 4)
    {
        data[i] = j;
        data[i + 1] = (short)(j + 1);
        data[i + 2] = (short)(j + 2);
        data[i + 3] = (short)(j + 2);
        data[i + 4] = (short)(j + 3);
        data[i + 5] = j;
    }

    glBufferData(GL_ELEMENT_ARRAY_BUFFER, static_cast<GLsizeiptr>(BATCH_SIZE * 6 * sizeof(short)), data.get(), GL_STATIC_DRAW);
}

glBindVertexArray(0);

Then elsewhere in the drawing loop:

// drawing

glBindVertexArray(vao_);

glBufferData(GL_ARRAY_BUFFER, static_cast<GLsizeiptr>(sizeof(float) * buffer_index_), attributes_.data(), GL_DYNAMIC_DRAW);

glDrawElements(GL_TRIANGLES, static_cast<GLsizei>(6 * number_of_sprites_), GL_UNSIGNED_SHORT, 0);

glBindVertexArray(0);

Can anyone see what I'm doing wrong?

UPDATE

I've added the suggested changes (very grateful for those). However, I'm now finding that I need to add a call to glBindBuffer(GL_ARRAY_BUFFER, attributes_id_);before glBindVertexArray(vao_); when drawing to get rid of the EXC_BAD_ACCESS error.

score 1 · Answer 1 · answered Apr 20 '18 at 19:29

glBufferData() expects as first argument the target buffer. The naming is a bit confusing here, but what that means is that you tell OpenGL which bound buffer you mean, not the name of the buffer. Here is an example:

glBindBuffer(GL_ARRAY_BUFFER, foo); // Binds the buffer named foo to the GL_ARRAY_BUFFER target

glBufferData(GL_ARRAY_BUFFER, ...); // Tells GL you mean the buffer bound to the GL_ARRAY_BUFFER target

Rabbid76 · Accepted Answer · 2018-04-20T19:46:11.663

1

OpenGL 4.6 API Core Profile Specification; 10.3. VERTEX ARRAYS; page 347

A vertex array object is created by binding a name returned by GenVertexArrays with the command
void BindVertexArray( uint array );
array is the vertex array object name. The resulting vertex array object is a new state vector, comprising all the state and with the same initial values listed in tables 23.4- 23.7.

Table 23.4 contains ELEMENT_ARRAY_BUFFER_BINDING

This means, that the state of the bound ELEMENT_ARRAY_BUFFER is stored in the Vertex Array Object.

In your case indices_id_ is stored in the state vector of vao_, because of

glBindVertexArray(vao_);

.....

glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, indices_id_);

When you call

glBindVertexArray(0);

then indices_id_ is not longer in the current state vector.

You have to change the order of the instructions:

glBindVertexArray(vao_);

.....

glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, indices_id_);
{
    .....
    glBufferData(indices_id_, static_cast<GLsizeiptr>(BATCH_SIZE * 6), data.get(), GL_STATIC_DRAW);
}
glBindVertexArray(0);

edited Apr 20 '18 at 19:46

answered Apr 20 '18 at 19:39

Rabbid76

202,892
27
131
174

Thank you for your helpful suggestions. I've updated my code however I'm still getting an odd problem that I have to call `glBindBuffer(GL_ARRAY_BUFFER, attributes_id_);` before `glBindVertexArray(vao_);` in the drawing code. Is that normal? – keith Apr 20 '18 at 20:24
The reason is that `glBindBuffer(GL_ARRAY_BUFFER, attributes_id_);` has to be called befor `glBufferData(GL_ARRAY_BUFFER, ..)` – Rabbid76 Apr 20 '18 at 20:29
Yes thanks just found a post about it (very grateful for your help): https://stackoverflow.com/questions/17227031/vertex-array-objects-confusion-regarding-exactly-what-state-information-is-sav?utm_medium=organic&utm_source=google_rich_qa&utm_campaign=google_rich_qa – keith Apr 20 '18 at 20:30

Using VAO with glDrawElements

2 Answers2