c++ - Understanding code used to draw quads in OpenGL 3.3+ core using triangles -

i'm trying draw quad background (2d) using opengl 3.x+. quads deprecated, goal use 2 triangles make rectangle fills screen. it's working, i'm not 100% clear on here.

1. setup

   gluint positionbufferobject; glfloat vertexpositions[] = {     -1.0f, -1.0f, 0.0f, 1.0f,     -1.0f,  1.0f, 0.0f, 1.0f,      1.0f,  1.0f, 0.0f, 1.0f,      1.0f, -1.0f, 0.0f, 1.0f,     -1.0f, -1.0f, 0.0f, 1.0f, }; glgenbuffers(1, &positionbufferobject); glbindbuffer(gl_array_buffer, positionbufferobject); glbufferdata(gl_array_buffer, sizeof(vertexpositions), vertexpositions, gl_static_draw); 

   • i understand vertexpositions, it's array of vertices.
   • glgenbuffers() saying, want 1 buffer , assign id &positionbufferobject?
   • glbufferdata() uploads vertexpositions gpu's memory; how know upload since didn't give id?

2. draw

   glenablevertexattribarray(0); glvertexattribpointer(0, 4, gl_float, gl_false, 0, 0); gldrawarrays(gl_triangle_strip, 0, 5); gldisablevertexattribarray(0); 

   • glenablevertexattribarray() says i'm going drawing array 0?
   • gldrawarrays() - if want draw 2 vertex arrays? how know ones render? knows above command?
   • not sure glvertexattribpointer() does?
   • gldrawarrays() clear.

3. clean up, think right?

   glbindbuffer(gl_array_buffer, 0); gldeletebuffers(1, &positionbufferobject); 

   i setup/cleanup once.

bonus points:

• is effective way render this? read i'm suppose submitting , rendering in "batches" [?] since 3.x+ doesn't immediate mode more. there 1 array, batches won't performance in case, if had "a large number" of vertxarrays draw, same process?
• in setup storing array id positionbufferobject, have hardcoded in rendering loop. seems confusing after dozen or arrays, why isn't practice use variable instead of hardcode it?

glgenbuffers(1, &positionbufferobject); says "make vertex buffer object, , positionbufferobject id."

glbindbuffer(gl_array_buffer, positionbufferobject); says "positionbufferobject current gl_array_buffer."

glbufferdata(gl_array_buffer, sizeof(vertexpositions), vertexpositions, gl_static_draw); says "upload vertexpositions id bound gl_array_buffer (which positionbufferobject)."

glenablevertexattribarray(0); says "vertex attribute array 0 available use."

glvertexattribpointer(0, 4, gl_float, gl_false, 0, 0); says "vertex attribute array 0 interpreted consisting of groups of 4 floats."

gldrawarrays(gl_triangle_strip, 0, 5); says "draw triangle strip 5 indices every enabled array."

gldisablevertexattribarray(0); says "we're done time being vertex attribute array 0."