c++ - OpenGL:glDrawArrays 有效但 glDrawElements 无效

Question

我有一个创建正方形的类，但我使用的是 glDrawArrays，它工作得很好，但是当我尝试使用 glDrawElements 时，它不起作用。

Sprite .h

#pragma once
#include <GL/glew.h>
#include <windows.h>
#include <iostream>
#include "shaderloader.h";

//#define USING_INDEX_BUFFER 1
#ifdef USING_INDEX_BUFFER
    #define NUM_VERTICES 4
    #define NUM_INDICES 6
#else
    #define NUM_VERTICES 6
#endif

class Sprite
{
public:
    Sprite(float x, float y, float width, float height);
    ~Sprite();
    void init();
    void draw();

private:
    float _x, _y;
    float _width, _height;

    GLuint _vao, _vbo, _eao;

#ifdef USING_INDEX_BUFFER
    GLfloat _vertices[4][2] = {
        { 0.0f, 0.0f },
        { 0.0f, 0.0f },
        { 0.0f, 0.0f },
        { 0.0f, 0.0f }
    }; // A Quad
    GLfloat _color[4][4] = {
        { 0.0f, 0.0f, 0.0f, 0.0f },
        { 0.0f, 0.0f, 0.0f, 0.0f },
        { 0.0f, 0.0f, 0.0f, 0.0f },
        { 0.0f, 0.0f, 0.0f, 0.0f }
    }; 
    GLuint _indices[6] = { 0, 0, 0, 0, 0, 0 };
#else 
    GLfloat _vertices[6][2] = {
        { 0.0f, 0.0f },
        { 0.0f, 0.0f },
        { 0.0f, 0.0f },
        { 0.0f, 0.0f },
        { 0.0f, 0.0f },
        { 0.0f, 0.0f }
    }; // A Quad
    GLfloat _color[6][4] = {
        { 0.0f, 0.0f, 0.0f, 0.0f },
        { 0.0f, 0.0f, 0.0f, 0.0f },
        { 0.0f, 0.0f, 0.0f, 0.0f },
        { 0.0f, 0.0f, 0.0f, 0.0f },
        { 0.0f, 0.0f, 0.0f, 0.0f },
        { 0.0f, 0.0f, 0.0f, 0.0f }
    }; 
#endif
};

Sprite .cpp

#include "Sprite.h"

#define BUFFER_OFFSET( bytes ) ( (GLubyte*) NULL + ( bytes ) )

Sprite::Sprite(float x, float y, float width, float height)
{
    _x = x;
    _y = y;
    _width = width;
    _height = height;

#ifdef USING_INDEX_BUFFER
    _vertices[0][0] = x;
    _vertices[0][1] = y;
    _vertices[1][0] = x;
    _vertices[1][1] = y + height;
    _vertices[2][0] = x + width;
    _vertices[2][1] = y;

    _vertices[3][0] = x + width;
    _vertices[3][1] = y + height;

    for (int i = 0; i < 4; i++) {
        _color[i][0] = 1.0f;
        _color[i][3] = 1.0f;
    }

    GLuint _indices[6] = { 0, 1, 2, 3, 1, 2 };

#else
    _vertices[0][0] = x;
    _vertices[0][1] = y;
    _vertices[1][0] = x;
    _vertices[1][1] = y + height;
    _vertices[2][0] = x + width;
    _vertices[2][1] = y;

    _vertices[3][0] = x + width;
    _vertices[3][1] = y + height;
    _vertices[4][0] = x;
    _vertices[4][1] = y + height;
    _vertices[5][0] = x + width;
    _vertices[5][1] = y;

    for (int i = 0; i < 6; i++) {
        _color[i][0] = 1.0f;
        _color[i][3] = 1.0f;
    }
#endif
}


Sprite::~Sprite()
{
    glDeleteVertexArrays(1, &_vao);
    glDeleteBuffers(1, &_vbo);
}


void Sprite::init()
{
    ShaderLoader shader;
    GLuint programID = shader.getProgramID("basicShading");

    // Generate and bind the VAO
    glGenVertexArrays(1, &_vao);
    glBindVertexArray(_vao);

    // Generate, bind and update data
    glGenBuffers(1, &_vbo);
    glBindBuffer(GL_ARRAY_BUFFER, _vbo);
    glBufferData(GL_ARRAY_BUFFER, sizeof(_vertices) + sizeof(_color), NULL, GL_STATIC_DRAW);
    glBufferSubData(GL_ARRAY_BUFFER, 0, sizeof(_vertices), _vertices);
    glBufferSubData(GL_ARRAY_BUFFER, sizeof(_vertices), sizeof(_color), _color);

#ifdef USING_INDEX_BUFFER
    // Generate, bind and update data of the EAO
    glGenBuffers(1, &_eao);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, _eao);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, NUM_INDICES * sizeof(GLuint), _indices, GL_STATIC_DRAW);
#endif

    // Set up attributes: Vertices
    GLuint vPos = glGetAttribLocation(programID, "vertexPosition");
    glEnableVertexAttribArray(vPos);
    glVertexAttribPointer(vPos, 2, GL_FLOAT, GL_FALSE, 0, 0);
    // Set up attributes: Colors
    GLuint vCol = glGetAttribLocation(programID, "fragColor");
    glEnableVertexAttribArray(vCol);
    glVertexAttribPointer(vCol, 4, GL_FLOAT, GL_FALSE, 0, BUFFER_OFFSET(sizeof(_vertices)));

    // Unbind VAO, VBO and EAO 
    glBindVertexArray(0);
    glBindBuffer(GL_ARRAY_BUFFER, 0);
#ifdef USING_INDEX_BUFFER
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);
#endif

    glUseProgram(programID);
}

void Sprite::draw() {
    glBindVertexArray(_vao);

#ifdef USING_INDEX_BUFFER
    glDrawElements(GL_TRIANGLES, NUM_INDICES, GL_UNSIGNED_INT, NULL); // Doesn't work
#else
    glDrawArrays(GL_TRIANGLES, 0, NUM_VERTICES); // Works
#endif

    glBindVertexArray(0);
}

有人可以向我解释错误是什么吗？

Answer 1

看起来您没有绑定(bind)索引缓冲区。它应该作为 glDrawElements 的最后一个参数传递或与 glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, _eao); 调用绑定(bind)，就像您在 init 方法中所做的那样。

您的代码也没有错误处理您应该在每次 gl 调用后调用 glGetError。