http://www.terathon.com/lengyel/Lengyel-VoxelTerrain.pdf
but i got really stuck at page 34/35, it's about voxel sharing
(Fig. 3.8a) -> i understand that only on the locations 0,1,2,3 a new vertex is allowed, but what i don't understand is the following 3 Bit direction code!
If you didn't already, read the paper
it's very intresting - but not very easy, at least for me, to understandIn the event that the sample value at a corner is exactly zero, the vertex lying on any
active edge adjacent to that corner is placed precisely at the corner location. The only
corner at which a new vertex is allowed to be created is corner 7, so vertices for any other
corners must be reused from preceding cells. A 3-bit direction code leading to the proper
cell can easily be obtained by inverting the 3-bit corner index (bitwise, by exclusive
ORing with the number 7), where the bit values 1, 2, and 4 indicate that we must subtract
one from the the x, y, and/or z coordinate, respectively.
For cells occurring along the minimal boundaries of a block, the preceding cells
needed for vertex reuse may not exist. In these cases, we allow new vertex creation on
additional edges of a cell. While iterating through the cells in a block, a 3-bit mask is
maintained whose bits indicate whether corresponding bits in a direction code are valid.
When a direction code is used to locate a preceding cell, it is first ANDed with the
validity mask to determine whether the preceding cell exists, and if not, the creation of a
new vertex in the current cell is permitted. (This test always fails for 4-bit direction codes
having the bit with value 8 set. This is the motivation for assigning the codes 0x81,
0x82, and 0x83 to the maximal edges instead of 0x01, 0x02, and 0x03.)
For each cell, we must have space to store four vertex indexes corresponding to the
locations shown in Figure 3.8(a) so that they can be reused by cells triangulated at a later
time. It is never the case that all four vertex slots are used at once (because a vertex lying
on the interior of an edge implies no vertex at the corner), but we must be able to identify
the vertices using a fixed indexing scheme. The vertices used by a cell are always owned
by the cell itself, the preceding cell in the same row, one of two adjacent cells in the
preceding row, or one of four cells in the preceding deck. We can therefore limit the
36
vertex history that we store while processing a block to two decks containing 16?16 cells
each and ping-pong between them as the z coordinate is incremented.
