The two paragraphs under this image reads like this:
However, the union of the 12 blocks does not completely cover the ring. We fill the
small remaining gaps using a few additional 2D footprints, as explained next. Note that
in practice, these additional regions have a very small area compared to the regular m×m
blocks, as revealed in Figure 2-6. First, there is a gap of (n − 1) − ((m − 1) × 4) = 2
quads at the middle of each ring side. We patch these gaps using four m×3 fix-up regions
(shown in green in Figures 2-5 and 2-6). We encode these regions using one vertex
and index buffer, and we reuse these buffers across all levels. Second, there is a gap of one
quad on two sides of the interior ring perimeter, to accommodate the off-center finer
level. This L-shaped strip (shown in blue) can lie at any of four possible locations (topleft,
top-right, bottom-left, bottom-right), depending on the relative position of the fine
level inside the coarse level. We define four vertex and one index buffer for this interior
trim, and we reuse these across all levels.
Also, we render a string of degenerate triangles (shown in orange) on the outer perimeter.
These zero-area triangles are necessary to avoid mesh T-junctions. Finally, for the finest
level, we fill the ring interior with four additional blocks and one more L-shaped region.
I understand how each "ring" is broken into 12 square sections of m x m vertices, and then the holes are patched using a m x 3 block, and since the size is uneven (example above uses 15 x 15 vertices), we also need to fill top or bottom and left or right with an additional strip of (2m + 1) x 2 blocks. But, what I don't understand is the first sentence of the second paragraph, more precisely what is marked as orange in the image and called "Outer Degenerate Triangles".
- What are these used for?
- How are they drawn, is each long section of a orange line a stretched out quad? Does it use the same vertex resolution as the grid itself? Are they drawn using the same shader as the main geometry?
- Are they drawn using 0 size? And if so why?
Basically I'm looking for an explanation for the outer degenerate triangles, why they are there, how they are drawn and how their vertex layout is?
Edited by fjholmstrom, 24 September 2012 - 05:45 AM.