[Flightgear-devel] Random Buildings - memory consumption

[Stuart B. <stu...@gm...>]

Hi All,

Emilian and Vivian have pointed out a problem with the random
buildings - they gobble memory.  I'd like to get some advice on
whether there's any solution, and also to ask someone with more C++
knowledge than myself to take a look at the code and check I'm not
doing something stupid.

The specific problem scenario is starting a KLAX (Los Angeles) which
is in the middle of a massive urban area.

Using the default random building density,  the tiles that are loaded
initially when sitting on the runway generates ~ 340k random
buildings. Each building consists of between 9 and 12 quads (we have a
"basement" at the bottom to handle slopes, 4 walls, and a roof which
may be pitched).  In turn, each quad has 4 corners, each of which has
a position (vec3), a normal (vec3) and a texture coordinate (vec2).

So, the absolute minimum memory occupancy of the data for the random
buildings is 340k * 10 * 4 * 8 = 108MB.  On top of that will be some
OSG overhead and the building texture itself.

Once you start flying more tiles are loaded (presumably well before
any old tiles are unloaded?) so memory occupancy rapidly increases.
Flying east from KLAX is particularly bad.

Using a higher building density makes the problem much worse, as you
need 4X the number of buildings to get double the linear density.  I
ran out of memory on a 4GB system pretty quickly (and tile loading
takes an age - something I need to look at again).

So
- Does anyone have any bright ideas on what I can do to reduce the
base memory occupancy?  One option might be to not generate the
"basement" if the terrain is level.
- Could a fresh pair of eyes take a look at the obj.cxx, mat.cxx and
SGBuildingBin.[ch]xx code to see if I've missed something obvious.

Thanks,

-Stuart