By default? In counter-clockwise order.
Consider a triangle facing the camera:
A
|\
| \
| \
B---C
A->B->C would be front facing (counter-clockwise order), A->C->B would be rear-facing (clockwise order).
You can change which way OpenGL considers “front facing” via glFrontFace():
The projection of a polygon to window coordinates is said to have
clockwise winding if an imaginary object following the path from its
first vertex, its second vertex, and so on, to its last vertex, and
finally back to its first vertex, moves in a clockwise direction about
the interior of the polygon. The polygon’s winding is said to be
counterclockwise if the imaginary object following the same path moves
in a counterclockwise direction about the interior of the polygon.
glFrontFacespecifies whether polygons with clockwise winding in
window coordinates, or counterclockwise winding in window coordinates,
are taken to be front-facing. PassingGL_CCWtomodeselects
counterclockwise polygons as front-facing;GL_CWselects clockwise
polygons as front-facing.By default, counterclockwise polygons are taken to be front-facing.
For ordering your vertices, consider an ideal cube:
6---7
/"https://stackoverflow.com/"
2---3 |
| 4-|-5
"https://stackoverflow.com/"/
0---1
For each face mentally rotate it to face the camera (your mind’s eye):
Sides:
2---3 3---7 7---6 6---2
| | | | | | | |
| | | | | | | |
0---1 1---5 5---4 4---0
Bottom/Top:
0---1 6---7
| | | |
| | | |
4---5 2---3
Then you can just visually read off the quads or triangle pairs in the right counter-clockwise order:
2---3 3 2---3
| | becomes /| and "https://stackoverflow.com/" "https://stackoverflow.com/" |/
0---1 0---1 0
Triangles 0-1-3 and 0-3-2
Quad 0-1-3-2
It doesn’t matter which vertex you start the triangle/quad with, for example with the first triangle 0-1-3, 1-3-0, and 3-0-1 are all equally valid, front-facing triangles.