[osg-users] automatically merge close-by geometries to reduce cull/draw overhead?

[Christian Buchner]

My building's aren't (yet) textured, so that removes the need for a texture
atlas. However they are individually shaped (being created from an ESRI
shape file essentially), so instancing is ruled out.

I might try to group the buildings into tiles of equal size, trying to run
the osgUtil::Optimizer with MERGE_GEOMETRY and MERGE_DRAWABLES individually
on each tile. Let's see how that goes. If it is too slow, I will have to
refactor my code that generates buildings.

Also I am wondering what the SPATIALIZE_GROUPS feature of the optimizer
does.

Christian


2015-09-07 12:32 GMT+02:00 Robert Osfield <robert.osfield at gmail.com>:

> Hi Christian,
>
> Since you are creating the building yourself I would recommend that you
> build them grouped to start off with rather than post process them.
>
> The first step I'd tack would be to create a texture atlas from the wall
> and roof textures and then just create a single osg::Geometry and
> associated osg::StateSet.   This will half the number of Drawables and
> state changes.
>
> If you have a set of roof and wall textures then see if you create a
> single texture atlas from them so that you can then reuse the same
> osg::StateSet between separate Drawables.
>
> Then final step would be to merge groups of Drawables that are in
> geographical location.  The osgforest example does grouping of randomly
> placed trees so have a look at ways of doing this.
>
> Another approach you could take would be to create a single osg::Geometry
> and then use instances to repeat the building geometry and provide a size
> for a shader to scale the geometry and place it in it's final position.
> Again the osgforest has a code path that does this so have a look at this.
>
> Robert.
>
>
>
> On 7 September 2015 at 10:50, Christian Buchner <
> christian.buchner at gmail.com> wrote:
>
>> Hi,
>>
>> we're using code loading some buildings (outline and height), creating a
>> Geode with a two drawables per individual building - one drawable for the
>> walls, one for the roof polygon. This has served us well to display a few
>> hundred to a few thousand buildings.
>>
>> Fast forward to current date. Our client has sent us a new geo data set
>> containing 55000 building polygons. Once you zoom out the camera to show
>> most of these buildings, frame rates drop into the low single digit, mostly
>> due to all the culling effort done by the CPU (maybe also from the large
>> number of draw calls). D'oh!
>>
>> Are there any specific features within OSG to group close by geodes, and
>> to merge their drawables?
>>
>> I know the osgUtil::Optimize has flags for merging geodes and drawables,
>> but I guess it would not automatically merge only very close objects.
>>
>> What path should I try to take for tackling this problem, if possible
>> using built-in OSG features?
>>
>> Christian
>>
>>
>>
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