Element Buffer Objects

Tags: opengl

Is a way to compactly represent a collection of points in less memory using vertex-buffer-objects.

Why?

For example a rectangle can be represented using 2 triangles:

float vertices[] = {
    // first triangle
     0.5f,  0.5f, 0.0f,  // top right
     0.5f, -0.5f, 0.0f,  // bottom right
    -0.5f,  0.5f, 0.0f,  // top left
    // second triangle
     0.5f, -0.5f, 0.0f,  // bottom right
    -0.5f, -0.5f, 0.0f,  // bottom left
    -0.5f,  0.5f, 0.0f   // top left
};

Notice how the same coordinates are repeated multiple times in this representation. This problem predictably scales to 1000s of triangles. EBOs allow you to keep the coordinates of a shape and where those coordinates are used in an object seperate.

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[] = {  // note that we start from 0!
    0, 1, 3,   // first triangle
    1, 2, 3    // second triangle
};

Creating a EBO

Create it.

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

bind it.

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

Draw it.

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

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