Why can VAO use VBO data whthout bind it? - c++

Question

Why VAO can use VBO data without binding it?
https://learnopengl.com/Advanced-OpenGL/Instancing
i find it from this webpage.
the code page:
https://learnopengl.com/code_viewer_gh.php?code=src/4.advanced_opengl/10.3.asteroids_instanced/asteroids_instanced.cpp
unsigned int buffer;
glGenBuffers(1, &buffer);
glBindBuffer(GL_ARRAY_BUFFER, buffer);
glBufferData(GL_ARRAY_BUFFER, amount * sizeof(glm::mat4), &modelMatrices[0], GL_STATIC_DRAW);
for (unsigned int i = 0; i < rock.meshes.size(); i++)
{
unsigned int VAO = rock.meshes[i].VAO;
glBindVertexArray(VAO);
// set attribute pointers for matrix (4 times vec4)
glEnableVertexAttribArray(3);
glVertexAttribPointer(3, 4, GL_FLOAT, GL_FALSE, sizeof(glm::mat4), (void*)0);
glEnableVertexAttribArray(4);
glVertexAttribPointer(4, 4, GL_FLOAT, GL_FALSE, sizeof(glm::mat4), (void*)(sizeof(glm::vec4)));
glEnableVertexAttribArray(5);
glVertexAttribPointer(5, 4, GL_FLOAT, GL_FALSE, sizeof(glm::mat4), (void*)(2 * sizeof(glm::vec4)));
glEnableVertexAttribArray(6);
glVertexAttribPointer(6, 4, GL_FLOAT, GL_FALSE, sizeof(glm::mat4), (void*)(3 * sizeof(glm::vec4)));
glVertexAttribDivisor(3, 1);
glVertexAttribDivisor(4, 1);
glVertexAttribDivisor(5, 1);
glVertexAttribDivisor(6, 1);
glBindVertexArray(0);
}
the rock.meshes[i].VAO use the buffer data without bind it?
i think it should be this：
for (unsigned int i = 0; i < rock.meshes.size(); i++)
{
unsigned int VAO = rock.meshes[i].VAO;
GLuint buffer;
glBindVertexArray(VAO);
glGenBuffers(1, &buffer);
glBindBuffer(GL_ARRAY_BUFFER, buffer);
glBufferData(GL_ARRAY_BUFFER, amount * sizeof(glm::mat4), &modelMatrices[0], GL_STATIC_DRAW);
......
}

Answer 1

No, buffer is still bound to the target GL_ARRAY_BUFFER.
OpenGL is a state engine, and the last instruction which binds a buffer object to the the target GL_ARRAY_BUFFER was:
glBindBuffer(GL_ARRAY_BUFFER, buffer);
So buffer is still bound when you do the definition of generic vertex attribute data.
When the array of generic vertex attribute data is specified (glVertexAttribPointer), then the vertex specification (index, tuple size, format, stride, offset, ...) and the "name" of the current buffer object, which is bound to the target GL_ARRAY_BUFFER, is set to the Vertex Array Objects state vector.
See Vertex Buffer Object.
Note, an Index buffer would be stated in the Vertex Array Object directly. So the Vertex Array Object has to be bound, before a buffer is bound to the target GL_ELEMENT_ARRAY_BUFFER.
Binding a buffer object to the target GL_ARRAY_BUFFER does not change the the current Vertex Array Object's states, but binding a buffer object to the target GL_ELEMENT_ARRAY_BUFFER modifies the VAOs states.
A Vertex Array Object can refer to only one index buffer, but it can refer to a separate array buffer for each attribute.