Question

我一直在阅读VAO如何与VBO合作。据我所知，当我们调用glVertexAttribPointer时，VAO只存储有关VBO的状态信息。我的问题是，它何时存储关于EBO的状态（对于索引绘图），当调用glVertexAttribPointer时它是否为两者保存状态？

//Vertices of the rectangle
std::vector<GLfloat> vertices
{
    0.5f,  0.5f, 0.0f,  // Top Right
    0.5f, -0.5f, 0.0f,  // Bottom Right
    -0.5f,  0.5f, 0.0f,  // Top Left 
    -0.5f, -0.5f, 0.0f,  // Bottom Left
};

//Indices of the triangle
std::vector<GLuint> indices
{
    0, 1, 3,
    0, 3, 2
};


GLuint VAO, VBO, EBO; 
glGenVertexArrays(1, &VAO);
glGenBuffers(1, &VBO);
glGenBuffers(1, &EBO);

glBindVertexArray(VAO); //Bind the VAO

//Bind the buffers
glBindBuffer(GL_ARRAY_BUFFER, VBO);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO);

glBufferData(GL_ARRAY_BUFFER, vertices.size() * sizeof(GLfloat), &vertices[0], GL_STATIC_DRAW);

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

glBindVertexArray(0); //Unbind the VAO

//Supply Index Buffer information
glBufferData(GL_ELEMENT_ARRAY_BUFFER, indices.size() * sizeof(GLuint), &indices[0], GL_STATIC_DRAW);

glBindBuffer(GL_ARRAY_BUFFER, 0);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);

//Custom shader class that stores the program to be used at render time
Shader shader("..path_to_shader\\shaders\\vertexshader.vert", "..path_to_shader\\shaders\\fragmentshader.frag");

while (!glfwWindowShouldClose(window))
{
    glUseProgram(shader.Program());
    glBindVertexArray(VAO); //Bind the VAO to draw.
    glClearBufferfv(GL_COLOR, 0, &color[0]);

    glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, nullptr);

    glfwPollEvents();
    glfwSwapBuffers(window);

    glBindVertexArray(0);
    glUseProgram(0);
}

Answer 1

当您将GL_ELEMENT_ARRAY_BUFFER绑定绑定到：

时，它将成为VAO状态的一部分

glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ...);

虽然VAO受到约束。

请注意，在没有VAO绑定的情况下绑定元素数组缓冲区也是合法的。这对于使用glBufferData()或类似调用填充数据非常有用。由于在Core Profile中你需要一个VAO绑定任何绘制调用，你需要绑定你想要用来渲染的元素数组缓冲区，同时绑定VAO。

关于您的代码，您使用的调用序列将无法提供所需的结果。以下是您的重要电话，其中的评论解释了结果：

glBindVertexArray(VAO);
// VAO is now bound. If this is the first time, it will have the default VAO state.

glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO);
// The element array buffer is bound, and this binding becomes part of the VAO state.

glBindVertexArray(0);
// The VAO is unbound. Since the element array buffer binding was part of the
// VAO state, the element array buffer is also unbound.

glBufferData(GL_ELEMENT_ARRAY_BUFFER, indices.size() * sizeof(GLuint),
             &indices[0], GL_STATIC_DRAW);
// This will not work, since you do not have an element array buffer bound.

为了实现这一目标，最简单的解决方案是更改最后两次调用的顺序：

glBufferData(GL_ELEMENT_ARRAY_BUFFER, indices.size() * sizeof(GLuint),
             &indices[0], GL_STATIC_DRAW);
// The currently bound element array buffer is filled with data.

glBindVertexArray(0);
// The VAO is unbound. Since the element array buffer binding was part of the
// VAO state, the element array buffer is also unbound.

为了进一步探讨这个问题，以下序列也可以起作用：

glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO);
// The element array buffer is bound, Since no VAO is bound, the binding becomes
// part of the global state (or more precisely, of the default VAO 0).
glBufferData(GL_ELEMENT_ARRAY_BUFFER, indices.size() * sizeof(GLuint),
             &indices[0], GL_STATIC_DRAW);
// The currently bound element array buffer is filled with data.

glBindVertexArray(VAO);
// VAO is now bound. If this is the first time, it will have the default VAO state.
// Since the element array binding is part of the VAO state, the previously bound
// element array buffer is not bound anymore.
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO);
// The element array buffer is bound, and this binding becomes part of the VAO state.
glBindVertexArray(0);
// The VAO is unbound. Since the element array buffer binding was part of the
// VAO state, the element array buffer binding is restored to the binding that
// was current before the VAO was bound.

请注意，在此序列中，您必须在绑定VAO后再次绑定元素数组缓冲区，因为一旦绑定VAO，VAO状态中的元素数组缓冲区绑定将覆盖当前绑定。

Answer 2

来自opengl.org：

顶点数组对象（VAO）是一个存储所有数据的OpenGL对象状态需要提供顶点数据（注意到一个小的例外下面）。它存储顶点数据的格式以及缓冲区提供顶点数据数组的对象（见下文）。

below, at Index Buffers

索引缓冲区绑定存储在VAO中。

对于GL_ARRAY_BUFFER，当您致电glVertexAttribPointer时，VAO会保存绑定。这主要是因为GL_ARRAY_BUFFER绑定不属于VAO的状态。因此，呼叫glBindBuffer(GL_ARRAY_BUFFER, vertexBufferHandle)不会对VAO的状态做任何事情。

对GL_ELEMENT_ARRAY_BUFFER而言并非如此：当你调用glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, indexBufferHandle)时，VAO（如果绑定）将保存索引缓冲区绑定到它的状态。

有关于OpenGL indexed buffering的精彩教程以及opengl-tutorial.org上的其他基本内容。

VAO和元素缓冲对象

2 个答案: