Faces in 3D

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I think I have my brain wrapped around meshes and materials as they relate to Gladius. Tonight, I aim to mess about with them to be sure.

The examples that come with Gladius are cubes:



If I understand things well, I ought to be able to convert that to a pyramid. A pyramid has a square base. In Cartesian coordinates, that could be (x, y) of (1, 0), (0, 1), (-1, 0), and (0, -1). The last point would be the tip of the pyramid, which is right in the middle (0,0), but with a z coordinate: (0, 0, 1).

So I convert the mesh proc definition to reflect that:

function proc( options ) {

  options = options || {};
  options.size = options.size || 1.0;
  var point = options.size / 2.0;

  var mesh =
  {
    points: [
      [ point, 0,      0],
      [ 0,     point,  0],
      [-point, 0,      0],
      [ 0,    -point,  0],
      [ 0,     0,      2.5*point]
    ],
    faces: [
      [0, 1, 2, 3],
      [0, 1, 4],
      [1, 2, 4],
      [2, 3, 4],
      [3, 0, 4]
    ],
    // ...
  };

  return mesh;

}

The faces connect the points to form one square and four triangles. Loading the demo up in my browser, I find that I have, in fact, created a pyramid:



Interestingly, it has no bottom:



I reexamine my definition to make sure that I have not missed a point. I try re-ordering the faces and the points. To no avail.

What fixes this in the end is reversing the face definition for the square. Instead of [0, 1, 2, 3], I use [3, 2, 1, 0]:

  var mesh =
  {
    // ...
    faces: [
      [3, 2, 1, 0],
      [0, 1, 4],
      [1, 2, 4],
      [2, 3, 4],
      [3, 0, 4]
    ],
    // ...
  };

Yikes! That would seem to indicate that the rotation that the points in a faces definition decides the side that is the "face". That is, the face is the side from which the points are defined in a clockwise order. In the first case, I went from 0 through 3 which is a clockwise rotation from above:



In the second case, when I went from 3 through 0, the clockwise rotation would have been seen from below, defining the face:



That seems hard to remember.


Day #408