[OpenGL] EBO

■ EBO

Element Buffer Objects는 VBO처럼 버퍼이며 필요한 정점을 따로 정의해두고, 정점에 인덱스를 붙여 그 인덱스를 호출하여 렌더링을 진행한다.
→ 중복되는 정점의 개수를 줄여 메모리를 절약할 수 있다.

● 인덱스 지정

삼각형 두개를 그려 사각형을 렌더링하려고 할 때,

float vertices[] = {
    // 첫 번째 삼각형
     0.5f,  0.5f, 0.0f, // 우측 상단
     0.5f, -0.5f, 0.0f, // 우측 하단
    -0.5f,  0.5f, 0.0f, // 좌측 상단 
    // 두 번째 삼각형
     0.5f, -0.5f, 0.0f, // 우측 하단
    -0.5f, -0.5f, 0.0f, // 좌측 하단
    -0.5f,  0.5f, 0.0f // 좌측 상단
};

→

float vertices[] = {
     0.5f,  0.5f, 0.0f,  // 우측 상단
     0.5f, -0.5f, 0.0f,  // 우측 하단
    -0.5f, -0.5f, 0.0f,  // 좌측 하단
    -0.5f,  0.5f, 0.0f   // 좌측 상단
};
unsigned int indices[] = {
    0, 1, 3, // 첫 번째 삼각형
    1, 2, 3 // 두 번째 삼각형
};

정점이 6개에서 4개로 줄었다.

● 생성 및 바인딩

unsigned int EBO;
glGenBuffers(1, &EBO);

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

● 그리기

glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO);
glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);

* glDrawArrays 함수 대신 glDrawElements 함수 사용

 

■ 소스 코드

#include <glad/glad.h>
#include <GLFW/glfw3.h>

void framebuffer_size_callback(GLFWwindow* window, int width, int height);

const char* vertexShaderSource = "#version 330 core\n"
"layout (location = 0) in vec3 aPos;\n"
"void main()\n"
"{\n"
" gl_Position = vec4(aPos.x, aPos.y, aPos.z, 1.0);\n"
"}\0";

const char* fragmentShaderSource = "#version 330 core\n"
"out vec4 FragColor;\n"
"void main()\n"
"{\n"
" FragColor = vec4(1.0f, 0.5f, 0.2f, 1.0f);\n"
"}\0";

int main(void)
{
    // 1. GLFW --------------------------------------------------

    // GLFW 초기화 및 설정
    glfwInit();
    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
    glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);

    // 2. Create Window --------------------------------------------------

    // GLFW 윈도우 생성
    GLFWwindow* window = glfwCreateWindow(800, 600, "LearnOpenGL", NULL, NULL);
    glfwMakeContextCurrent(window);
    glfwSetFramebufferSizeCallback(window, framebuffer_size_callback);

    // 3. GLAD --------------------------------------------------

    // GLAD 로드
    if (!gladLoadGLLoader((GLADloadproc)glfwGetProcAddress))
    {
        return -1;
    }

    // 4. Build and Compile Shader Program --------------------------------------------------

    // Vertex Shader 생성
    unsigned int vertexShader;
    vertexShader = glCreateShader(GL_VERTEX_SHADER);

    // Vertex Shader 컴파일
    glShaderSource(vertexShader, 1, &vertexShaderSource, NULL);
    glCompileShader(vertexShader);

    // Fragment Shader 생성
    unsigned int fragmentShader;
    fragmentShader = glCreateShader(GL_FRAGMENT_SHADER);

    // Fragment Shader 컴파일
    glShaderSource(fragmentShader, 1, &fragmentShaderSource, NULL);
    glCompileShader(fragmentShader);

    // Shader Program 생성
    unsigned int shaderProgram;
    shaderProgram = glCreateProgram();

    // Shader Program 연결
    glAttachShader(shaderProgram, vertexShader);
    glAttachShader(shaderProgram, fragmentShader);
    glLinkProgram(shaderProgram);

    // Shader 제거
    glDeleteShader(vertexShader);
    glDeleteShader(fragmentShader);

    // 5. Set Up Vertex Data --------------------------------------------------
    
    // Vertex 및 Index 선언
    float vertices[] = {
         0.5f,  0.5f, 0.0f,  // top right
         0.5f, -0.5f, 0.0f,  // bottom right
        -0.5f, -0.5f, 0.0f,  // bottom left
        -0.5f,  0.5f, 0.0f   // top left 
    };
    unsigned int indices[] = {
        0, 1, 3,  // first Triangle
        1, 2, 3   // second Triangle
    };

    // VBO, VAO, EBO 생성
    unsigned int VBO, VAO, EBO;
    glGenVertexArrays(1, &VAO);
    glGenBuffers(1, &VBO);
    glGenBuffers(1, &EBO);

    // VAO 바인딩
    glBindVertexArray(VAO);

    // VBO 바인딩 및 Vertex 버퍼 데이터 세팅
    glBindBuffer(GL_ARRAY_BUFFER, VBO);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);

    // EBO 바인딩 및 Index 버퍼 데이터 세팅
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);

    // Vertex 속성 포인터 설정
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(float), (void*)0);
    glEnableVertexAttribArray(0);

    // note that this is allowed, the call to glVertexAttribPointer registered VBO as the vertex attribute's bound vertex buffer object so afterwards we can safely unbind
    glBindBuffer(GL_ARRAY_BUFFER, 0);

    // You can unbind the VAO afterwards so other VAO calls won't accidentally modify this VAO, but this rarely happens. Modifying other
    // VAOs requires a call to glBindVertexArray anyways so we generally don't unbind VAOs (nor VBOs) when it's not directly necessary.
    glBindVertexArray(0);

    // 6. Render Loop --------------------------------------------------
    
    while (!glfwWindowShouldClose(window))
    {
        // esc 입력으로 종료
        if (glfwGetKey(window, GLFW_KEY_ESCAPE) == GLFW_PRESS)
        {
            glfwSetWindowShouldClose(window, true);
        }

        // 렌더 (컬러)
        glClearColor(0.2f, 0.3f, 0.3f, 1.0f);
        glClear(GL_COLOR_BUFFER_BIT);

        // 오브젝트 그리기
        glUseProgram(shaderProgram); // 프로그램 사용
        glBindVertexArray(VAO); // VAO 바인딩
        glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0); // 프리미티브를 그림 (삼각형 2개 → 사각형)

        glfwSwapBuffers(window); // 컬러 버퍼를 스왑 (더블 버퍼)
        glfwPollEvents(); // 입출력 이벤트 처리
    }

    // optional: de-allocate all resources
    glDeleteVertexArrays(1, &VAO);
    glDeleteBuffers(1, &VBO);
    glDeleteBuffers(1, &EBO);
    glDeleteProgram(shaderProgram);

    glfwTerminate();

    return 0;
}

void framebuffer_size_callback(GLFWwindow* window, int width, int height)
{
    glViewport(0, 0, width, height);
}