There are many issues and tradeoffs with mesh simplification. There are
many algorithms and approaches, each with their own unique strengths and
weaknesses. Challenges include finding a strategy to hide the cracks
between adjacent tiles draw with different levels of details (and possibly
more or less details on the edges.) You also have issues of objects
floating or being partially (completely) burried if the underlying terrain
becomes coarser or more detailed (or you have to move the objects up or
down which can also be odd looking.) Of course there is the whole issue of
processing/managing the GIS data, and generating the tiles to support
whatever LOD scheme is chosen.
When the FlightGear scheme was originally devised, there was a leaning in
the flight sim world away from LOD schemes and back towards just drawing
all the tiles in full detail -- again, this is just another strategy on the
continuum of available options with it's own strengths and weaknesses.
Keep in mind that some of the popular strategies of the day are popular
because they are developed in response to the typical capabilities of
current gamer hardware.
After we committed to our strategy, continuous level of details algorithms
began to really catch steam and become popular -- thinks like ROAM. These
algorithms were designed to minimize how many vertices were sent to
the graphics hardware every frame. But again, they have their strengths
and weakness -- nothing that can't be solved or worked around, but only
through a large amount of effort to do what we need for a generic
flight simulator trying to model the entire world.
But then (maybe a bit like clothing styles), the capabilities of consumer
3d hardware improved and began to be able to store the actual geometry
right on the card. Shader language effects became popular, and there was
less need to send the geometry to the video card every frame. More and
more of the rendering was right on the card. That led to less popularity
with ROAM type algorithms because they are always sending the new geometry
over to the card each frame and you end up not utilizing the full
capability of the graphics hardware.
So there is always the universal principle that it's faster to do less
work. But styles and algorithm popularity is a moving target as hardware
is a moving target. If we were building a game that would go to market in
the next 6 months and be removed from the shelves a few months later, we
would probably use a different strategy compared to our development process
which started in 1996 and does follow the trends to some degree, but a
little less aggressively and a little more skeptically because we would
like to run on hardware from a few years ago, on hardware 6 months from
now, and on hardware 5-10 years from now.
Curt.
On Sun, Dec 16, 2012 at 10:39 AM, Mathias Fröhlich wrote:
>
> Hi Adrian,
>
> The same idea is behind the osg lod based whole world model.
> Where do you store the elevation data?
>
> Greetings
>
> Mathias
>
>
>
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--
Curtis Olson:
http://www.atiak.com - http://aem.umn.edu/~uav/
http://www.flightgear.org - http://gallinazo.flightgear.org
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