Notes

Random notes.

Geometry cropping

Negative performance impact

Currently, tiling dangerously multiplies polygon count:

Level #Tils Lines   Tris    Quads
None  1     2800740 187008  2084056
10    24    2824190 217803  2082132
11    84    2830148 251538  2074692
12    276   2840420 311439  2061616
13    1058  2864136 435651  2034552
14    4232  2913096 673050  1983936
15    16836 3012734 1153107 1884364

To counter this, support for generic convex polygon primitive is needed (rendered as triangle fan). This

• conserves vertex space for large polygons
• allows to get rid of quads
• helps cropping, as quad split in half now produces two fans (8 vertices) instead of four triangles (12 vertices).

Positive performance impact

On the other side, for tiler geometry tiling improves performance greatly.

Time required for rendering Moscow at z18 depending on tiling level:

Level Tiles/second (rendering only)
10    391.58
11    1215.67
12    3294.44
13    5477.83
14    6740.94 (!)
15    4881.76
16    2312.13
17    750.81

When higher tiling level (thus smaller tile size), rendering speed grows as it only renders polygons which are visible on this tile, however polygon overhead due to geometry splitting (see above) grows as well as number of state changes and general TileManager overhead. Obviously, optimal tiling level varies among rendering levels.

Table of optimal tiling for each tile level:

Level Tiling
  ...
11    11
12    12
13    12 
14    12
15    12
16    13
17    13
18    14

Geometry rendering

After moving geometry from separate arrays of triangles and quads which were rendered with glDrawArrays to a more generic array of convex polygons, following rendering methods became possible:

1. All polygons are rendered as triangle fans with glMultiDrawArrays
2. Polygons are converted to triangles and rendered with single glDrawArrays
3. Polygons are converted to triangles and quads and rendered with two glDrawArrays
4. Polygons are converted to vertex array with index and rendered with single glDrawElements

Performance results:

Method      #Vertices  FPS
Fans        3.38M      85.5
Tris        4.91M      77.2
Tris+quads  3.41M      95.4
Elements    3.38M      95.8