当 glVertexAttribPointer() 和 glEnableVertexAttribArray() 调用被移动到另一个函数时没有绘制？

Question

完整代码（工作正常）：

#include "shapes.h"

// ...

GLuint vao;
GLuint ebo;

// Creates and binds the EBO and the VAO buffers
// Should call this function before any drawing
void PrepareShapes()
{
    glGenVertexArrays(1, &vao);

    glGenBuffers(1, &ebo);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ebo);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), &indices, GL_STATIC_DRAW);
}

Paddle::Paddle(float x, float y, float width, float height)
{
    const float paddleVertices[] =
    {
        0, 0,
        width, 0,
        width, height,
        0, height
    };
    glGenBuffers(1, &vbo);
    glBindBuffer(GL_ARRAY_BUFFER, vbo);
    glBufferData(GL_ARRAY_BUFFER, sizeof(paddleVertices), &paddleVertices, GL_STATIC_DRAW);
    glm::mat4 proj = glm::ortho(0.0, 640.0, 360.0, 0.0, 0.0, 1.0);
    model = glm::mat4(1.0);
    model *= glm::translate(model, glm::vec3(x, y, 0));
    model = proj * model;
}

void Paddle::Draw()
{
    glBindVertexArray(vao);
    glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, sizeof(float) * 2, (void*)0);
    glEnableVertexAttribArray(0);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ebo);

    glBindBuffer(GL_ARRAY_BUFFER, vbo);
    glUniformMatrix4fv(glGetUniformLocation(GetShader(), "model"), 1, GL_FALSE, &model[0][0]);
    glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);
}

这个程序画了一个“桨”。在绘制之前，您应该调用函数

PrepareShapes

来创建和绑定所需的缓冲区（EBO 和 VAO）。它的用法是这样的：

PrepareShapes();
Paddle pad(0, 0, 8, 80);
pad.Draw();

到目前为止一切正常。当我尝试将这两个调用从

Draw

函数移动到

PrepareShapes

函数时，没有绘制任何内容。为什么？这之后的

PrepareShapes

函数：

void PrepareShapes()
{
    glGenVertexArrays(1, &vao);
    glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, sizeof(float) * 2, (void*)0);
    glEnableVertexAttribArray(0);
    glGenBuffers(1, &ebo);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ebo);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), &indices, GL_STATIC_DRAW);
}

还有一个问题。我已经在

glBind*

函数中调用了

Draw

函数，为什么还要在

PrepareShapes

函数中再次调用它们？

Answer 1

当

glVertexAttribPointer

被调用时，当前绑定到

ARRAY_BUFFER

目标的缓冲区，关联到指定的属性索引，对象的ID存储在当前绑定的VAO的状态向量中。因此，必须在指定属性之前绑定缓冲对象和VAO：

glBindVertexArray(vao);
glBindBuffer(GL_ARRAY_BUFFER, vbo);
glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, sizeof(float) * 2, (void*)0);