Notes

A buffer is raw GPU memory with no type. The target (slot) it is bound to decides its role at that moment. A buffer of vertex data is called a VBO; a buffer of indices an IBO / EBO. How VBOs get wired to shader inputs is in VAO and VBO.

Targets

Target	Holds	Read by
GL_ARRAY_BUFFER	Per-vertex data: position, color, normal, UV	glVertexAttribPointer
GL_ELEMENT_ARRAY_BUFFER	Triangle indices	glDrawElements
GL_UNIFORM_BUFFER	A block of uniforms	Shader uniform blocks (later)

Rule: “data per vertex” → ARRAY. “Which vertices form a triangle” → ELEMENT_ARRAY.

Other objects use the same slot idea with their own bind call: glBindTexture(GL_TEXTURE_2D), glBindFramebuffer(GL_FRAMEBUFFER).

Positions and indices

pos and idx are ordinary CPU vectors. Each index is a position in pos. glDrawElements(GL_TRIANGLES) reads indices in groups of 3. Counter-clockwise order is the front face.

3 ───── 2
│    ╱  │     pos = {v0, v1, v2, v3}          4 vertices
│  ╱    │     idx = {0,1,2,  0,2,3}            2 triangles
0 ───── 1

Why indices: vertices are shared between triangles. Without them each shared vertex is stored again (6 here instead of 4), and real vertices also carry normals, colors and UVs.

Fan triangulation of an N-gon: {0,i,i+1} for i = 1..N-2 → N-2 triangles, 3(N-2) indices.

Upload

glGenBuffers(1, &bufferPosition);
glBindBuffer(GL_ARRAY_BUFFER, bufferPosition);
glBufferData(GL_ARRAY_BUFFER, pos.size() * sizeof(glm::vec3), pos.data(), GL_STATIC_DRAW);

• Size is in bytes: element count × bytes per element. Take it from the data being uploaded.
• pos.data() returns a pointer to the vector’s first element. Works because std::vector storage is contiguous.
• GL_STATIC_DRAW: upload once, draw many times. GL_DYNAMIC_DRAW if the data changes every frame.
• After upload the CPU vector can be destroyed. The GPU keeps its own copy.
• Number of buffers tracks number of attribute kinds, not number of vertices.

Related

OpenGL Hub · OpenGL handles and state machine · std::vector · Back-face culling