opengl - Vertex Array Objects - 关于保存当前绑定(bind)的顶点缓冲区的确切状态信息的混淆

Question

我正在学习 arcsynthesis 上的优秀教程在构建图形引擎时发现我并不像我想象的那样了解 VAO。

来自教程Chapter 5. Objects In Depth

Buffer Binding and Attribute Association

You may notice that glBindBuffer(GL_ARRAY_BUFFER) is not on that list, even though it is part of the attribute setup for rendering. The binding to GL_ARRAY_BUFFER is not part of a VAO because the association between a buffer object and a vertex attribute does not happen when you call glBindBuffer(GL_ARRAY_BUFFER). This association happens when you call glVertexAttribPointer.

When you call glVertexAttribPointer, OpenGL takes whatever buffer is at the moment of this call bound to GL_ARRAY_BUFFER and associates it with the given vertex attribute. Think of the GL_ARRAY_BUFFER binding as a global pointer that glVertexAttribPointer reads. So you are free to bind whatever you want or nothing at all to GL_ARRAY_BUFFER after making a glVertexAttribPointer call; it will affect nothing in the final rendering. So VAOs do store which buffer objects are associated with which attributes; but they do not store the GL_ARRAY_BUFFER binding itself.

我最初错过了最后一行“......但他们不存储 GL_ARRAY_BUFFER 绑定(bind)本身”。在我注意到这一行之前，我认为一旦调用 glVertexAttribPointer 就保存了当前绑定(bind)的缓冲区。由于缺少这些知识，我构建了一个网格类，并且能够获得一个具有多个网格正确渲染的场景。

下面列出了该代码的一部分。请注意，我没有在绘图函数中调用 glBindBuffer。

// MESH RENDERING

/* ...            */
/* SETUP FUNCTION */
/* ...            */

// Setup vertex array object
glGenVertexArrays(1, &_vertex_array_object_id);
glBindVertexArray(_vertex_array_object_id);

// Setup vertex buffers  
glGenBuffers(1, &_vertex_buffer_object_id);
glBindBuffer(GL_ARRAY_BUFFER, _vertex_buffer_object_id);
glBufferData(GL_ARRAY_BUFFER, _vertices.size() * sizeof(vertex), &_vertices[0], GL_STATIC_DRAW);

// Setup vertex attributes
glEnableVertexAttribArray(e_aid_position);
glEnableVertexAttribArray(e_aid_normal);
glEnableVertexAttribArray(e_aid_color);
glEnableVertexAttribArray(e_aid_tex);

glVertexAttribPointer(e_aid_position, 3, GL_FLOAT, GL_FALSE, sizeof(vertex), (GLvoid*)offsetof(vertex, pos));
glVertexAttribPointer(e_aid_normal,   3, GL_FLOAT, GL_FALSE, sizeof(vertex), (GLvoid*)offsetof(vertex, norm));
glVertexAttribPointer(e_aid_color,    4, GL_FLOAT, GL_FALSE, sizeof(vertex), (GLvoid*)offsetof(vertex, col));
glVertexAttribPointer(e_aid_tex,      2, GL_FLOAT, GL_FALSE, sizeof(vertex), (GLvoid*)offsetof(vertex, tex));  

/* ...           */
/* DRAW FUNCTION */
/* ...           */

glBindVertexArray(_vertex_array_object_id);  
glDrawArrays(GL_TRIANGLES, 0, _vertices.size());

现在我要开始照明了，我想绘制一些调试图，这样我就可以验证我所有的法线是否正确。目前，我只是将要为一个帧渲染的所有行存储在一个向量中。由于这些数据可能会在每一帧都发生变化，因此我使用 GL_DYNAMIC_DRAW 并在渲染之前指定数据。

最初，当我这样做时，我会得到指向无穷远的垃圾线。违规代码如下:

// DEBUG DRAW LINE RENDERING 

/* ...            */
/* SETUP FUNCTION */
/* ...            */

// Setup vertex array object
glGenVertexArrays(1, &_vertex_array_object_id);
glBindVertexArray(_vertex_array_object_id);

// Setup vertex buffers  
glGenBuffers(1, &_vertex_buffer_object_id);
glBindBuffer(GL_ARRAY_BUFFER, _vertex_buffer_object_id);
  // Note: no buffer data supplied here!!!

// Setup vertex attributes
glEnableVertexAttribArray(e_aid_position);
glEnableVertexAttribArray(e_aid_color);

glVertexAttribPointer(e_aid_position, 3, GL_FLOAT, GL_FALSE, sizeof(line_vertex), (GLvoid*)offsetof(line_vertex, pos));
glVertexAttribPointer(e_aid_color,    4, GL_FLOAT, GL_FALSE, sizeof(line_vertex), (GLvoid*)offsetof(line_vertex, col));

/* ...           */
/* DRAW FUNCTION */
/* ...           */

glBindVertexArray(_vertex_array_object_id);
  // Specifying buffer data here instead!!!
glBufferData(GL_ARRAY_BUFFER, _line_vertices.size() * sizeof(line_vertex), &_line_vertices[0], GL_DYNAMIC_DRAW);
glDrawArrays(GL_LINES, 0, _line_vertices.size());

经过一番寻找，以及找到我上面遗漏的细节，我发现如果我在绘制函数中调用 glBufferData 之前调用 glBindBuffer，一切正常。

考虑到这一点，我对为什么我的网格渲染首先起作用感到困惑。如果我更改缓冲区中的数据，是否只需要再次调用 glBindBuffer ？或者如果您不绑定(bind)缓冲区并且我只是不走运并且让它工作，那么行为是否未定义？

请注意，我的目标是 OpenGL 3.0 版。

Answer 1

Do I only need to call glBindBuffer again if I change the data in the buffer?

是的，每次调用 glVertexAttribPointer 时，VAO 对象都会记住绑定(bind)了哪些缓冲区。虽然该 VAO 已绑定(bind)，所以您通常不需要调用 glBindBuffer再次。但是，如果您想更改缓冲区中的数据，OpenGL 需要知道您正在更改哪个缓冲区，因此您需要调用 glBindBuffer在调用 glBufferData 之前.此时绑定(bind)哪个 VAO 对象无关紧要。