需要多次调用 glVertexAttribPointer 吗？

Question

while following the lighting chapter in the learnopengl series，作者在创建多个VAOs(Vertex Array Objects)时提供了这种代码：

    unsigned int VBO, cubeVAO;
    glGenVertexArrays(1, &cubeVAO);
    glGenBuffers(1, &VBO);

    glBindBuffer(GL_ARRAY_BUFFER, VBO);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);

    glBindVertexArray(cubeVAO);

    // position attribute
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(float), (void*)0); //#this
    glEnableVertexAttribArray(0);

    // second, configure the light's VAO (VBO stays the same; the vertices are the same for the light object which is also a 3D cube)
    unsigned int lightVAO;
    glGenVertexArrays(1, &lightVAO);
    glBindVertexArray(lightVAO);

    // we only need to bind to the VBO (to link it with glVertexAttribPointer), no need to fill it; the VBO's data already contains all we need (it's already bound, but we do it again for educational purposes)
    glBindBuffer(GL_ARRAY_BUFFER, VBO);

    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(float), (void*)0); //#this
    glEnableVertexAttribArray(0);

这里我们使用相同的VBO 和多个VAO，但对glVertexAttribPointer 的调用使用相同的参数完成了两次。 Earlier in this lesson他提到：

每个顶点属性从由 VBO 管理的内存中获取其数据，从哪个 VBO 获取数据（您可以有多个 VBO）由调用 glVertexAttribPointer 时当前绑定到 GL_ARRAY_BUFFER 的 VBO 确定。由于之前定义的 VBO 在调用 glVertexAttribPointer 之前仍被绑定，因此顶点属性 0 现在与其顶点数据相关联。

那么，这是否意味着这两个调用是多余的，或者这是必要的，如果不这样做会导致问题？

Answer 1

那么，这是否意味着这两个调用是多余的，或者这是必要的，如果不这样做会导致问题？

没有。看看这里的操作顺序。

// the buffer bound to GL_ARRAY_BUFFER is VBO, 
// from here, until the end of the code in this block
glBindBuffer(GL_ARRAY_BUFFER, VBO);

// start modifying the cubeVAO
// cubeVAO currently knows NOTHING about which attributes are needed.
glBindVertexArray(cubeVAO);

// set up info about vertex attribute 0, within cubeVAO. 
// cubeVAO now knows about 1 attribute, index == 0
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(float), (void*)0); //#this
glEnableVertexAttribArray(0);


// now start setting up the lightVAO. 
// note that at this point, VBO is still bound to GL_ARRAY_BUFFER
glBindVertexArray(lightVAO);

// set up info about vertex attribute 0, within lightVAO. 
// lightVAO now knows about 1 attribute, index == 0
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(float), (void*)0); 
glEnableVertexAttribArray(0);

如果您省略了对 glVertexAttribPointer 和 glEnableVertexAttribArray 的第二次调用，lightVAO 将包含精确为零的顶点属性的绑定。这意味着您的顶点着色器中的任何属性都不会接收任何数据。

在您的原始代码中，不需要第二次调用 glBindBuffer。也可以这么说，因为 cubeVAO 和 lightVAO 只有一个属性，从同一个缓冲区读取；在这里可以简单地使用单个 VAO。

\编辑

从插槽而不是实例的角度来考虑它可能会更好。您的 GPU 将支持固定数量的顶点属性（在 GL_MAX_VERTEX_ATTRIBS 上执行 glGet 以找出数量）。所以唯一支持的索引是：0 -> (GL_MAX_VERTEX_ATTRIBS-1)，所以说“创建了一个新实例”并不准确（因为这意味着动态分配）。这种行为更类似于：

// struct to store info about a vertex attribute
struct VertexAttribute
{
  bool enabled = false; //< on or off?
  int size;
  GLenum dataType;
  bool normalise;
  int stride;
  size_t offset;
  GLuint buffer; //< which buffer was bound to GL_ARRAY_BUFFER
};

// the VAO just stores the current state of the vertex bindings 
struct VAO
{
  VertexAttribute vertexAttribs[GL_MAX_VERTEX_ATTRIBS];

  void glVertexAttribPointer(
     int index, int size, GLenum type, 
     bool normalise, int stride, size_t offset)
  {
    vertexAttribs[index].size = size;
    vertexAttribs[index].dataType = type;
    vertexAttribs[index].normalise = normalise;
    vertexAttribs[index].stride = stride;
    vertexAttribs[index].offset = offset;

    // grab buffer
    vertexAttribs[index].buffer = glGet(GL_ARRAY_BUFFER_BINDING);
  }

  void glDisableVertexAttribArray(uint32_t index)
  {
    vertexAttribs[index].enabled = false;
  }

  void glEnableVertexAttribArray(uint32_t index)
  {
    vertexAttribs[index].enabled = true;
  }
};