opengl - 为什么图元是三角形时显示的图形，而图元是四边形时却消失了？

Question

我是 OpenGL 的新手。我想渲染一个简单的矩形，当图元为 时显示矩形GL_TRIANGLES，但是当我使用 时GL_QUADS，矩形消失了。我不知道为什么。以下是一些核心代码：

// set vertices
float vertices[] = {
    -0.5f, -0.5f, 0.0f,
    0.5f, -0.5f, 0.0f,
    0.5f, 0.5f, 0.0f,
    -0.5f, 0.5f, 0.0f
};
unsigned int indices[] = {
    0, 1, 2, // first triangle
    2, 3, 0  // second triangle
};
unsigned int VBO, VAO, EBO;
glGenVertexArrays(1, &VAO);
glGenBuffers(1, &VBO);
glGenBuffers(1, &EBO);

glBindVertexArray(VAO);

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

glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);

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

// set transform mat
shader.use();
glm::vec3 Eye = glm::vec3(0.0f, 0.0f, 0.0f);
glm::vec3 Target = glm::vec3(0.0f, 0.0f, -2.0f);
glm::vec3 Up = glm::vec3(0.0f, 1.0f, 0.0f);
glm::mat4 model = glm::mat4(1.0f);
model = glm::translate(model, glm::vec3(0.0f, 0.0f, -2.0f));
glm::mat4 view = glm::lookAt(Eye, Eye + Target, Up);
glm::mat4 projection = glm::perspective(glm::radians(45.0f), (float)SCR_WIDTH / (float)SCR_HEIGHT, 0.1f, 100.0f);
shader.setMat4("model", model);
shader.setMat4("view", view);
shader.setMat4("projection", projection);

// draw rectangle
glBindVertexArray(VAO);
glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);

但是当我如下设置矩形时indices，draw矩形消失了：

unsigned int indices[] = {
    0, 1, 2, 3 // rectangle
};

glDrawElements(GL_QUADS, 4, GL_UNSIGNED_INT, 0);

矩阵是一样的，只是图元不同，为什么不能画矩形呢？我应该如何改变它？ 接下来是主要功能：

int main(){
    glfwInit();
    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 4);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 0);
    glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);

    GLFWwindow *window = glfwCreateWindow(SCR_WIDTH, SCR_HEIGHT, "DrawRectangle", NULL, NULL);
    if (window == NULL){
        std::cout << "Failed to create GLFW window" << std::endl;
        glfwTerminate();
        return -1;
    }
    glfwMakeContextCurrent(window);
    glfwSetFramebufferSizeCallback(window, framebuffer_size_callback);

    if (!gladLoadGLLoader((GLADloadproc)glfwGetProcAddress)){
        std::cout << "Failed to initialize GLAD" << std::endl;
        return -1;
    }

    // shader
    Shader shader("vertex.glsl", "fragment.glsl");

    // VAO
    float vertices[] = {
        -0.5f, -0.5f, 0.0f,
        0.5f, -0.5f, 0.0f,
        0.5f, 0.5f, 0.0f,
        -0.5f, 0.5f, 0.0f
    };
    unsigned int indices[] = {
        0, 1, 2, // first triangle
        2, 3, 0  // second triangle
        //0, 1, 2, 3
    };
    unsigned int VBO, VAO, EBO;
    glGenVertexArrays(1, &VAO);
    glGenBuffers(1, &VBO);
    glGenBuffers(1, &EBO);

    glBindVertexArray(VAO);

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

    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);

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

    glBindBuffer(GL_ARRAY_BUFFER, 0); // unbind VBO
    glBindVertexArray(0); // unbind VAO

    //glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);
    while (!glfwWindowShouldClose(window)){
        processInput(window);

        glClearColor(0.2f, 0.3f, 0.3f, 1.0f);
        glClear(GL_COLOR_BUFFER_BIT);

        // set matrix
        shader.use();
        glm::vec3 Eye = glm::vec3(0.0f, 0.0f, 0.0f);
        glm::vec3 Target = glm::vec3(0.0f, 0.0f, -2.0f);
        glm::vec3 Up = glm::vec3(0.0f, 1.0f, 0.0f);
        glm::mat4 model = glm::mat4(1.0f);
        model = glm::translate(model, glm::vec3(0.0f, 0.0f, -2.0f));
        glm::mat4 view = glm::lookAt(Eye, Eye + Target, Up);
        glm::mat4 projection = glm::perspective(glm::radians(45.0f), (float)SCR_WIDTH / (float)SCR_HEIGHT, 0.1f, 100.0f);
        shader.setMat4("model", model);
        shader.setMat4("view", view);
        shader.setMat4("projection", projection);

        glBindVertexArray(VAO);
        glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);
        //glDrawArrays(GL_TRIANGLES, 0, 6);

        glfwSwapBuffers(window);
        glfwPollEvents();
    }
    glDeleteVertexArrays(1, &VAO);
    glDeleteBuffers(1, &VBO);
    glDeleteBuffers(1, &EBO);
    glfwTerminate();
    return 0;
}

标签： openglglfwprimitiveglad