The key weakness of osgSim::Impostor is that it doesn't do load balancing. It'll update impostors when the error metrics say they need updating, not when there is time in the frame to accomodate the extra draw.
There is also no capping on how much resources that all the impostors combined will take up so if there are too many impostors you can hit memory limits on the graphics card.
osgSim::Impsotor os pretty heavy on full rate as the impostor sprites are just quads, not trimmed down to just fit the region in question. The textures are all power of two, making them overly large. Using a texture atlas/FBO would be one around this.
Finally using parallax mapping perhaps one could reduce the need to updates. This wasn't possible in the original implementation as this was done before the days of programmable hardware.
Robert.
On 11/14/06, Wojciech Lewandowski <[EMAIL PROTECTED]> wrote:
Thanks, we will see what we can do. I looked at osgSim::Impostor and
osgSim::ImpostorSprite code. I think these classes are generally okay for
our purpose. We will try to derive our cloud impostor classes from them but
maybe modify computation Impostor error and valididty checking and use
bounding box instead of bounding sphere to minimize texture size. Our cloud
groups will be generally much wider than tall.
Thanks again,
Wojtek
----- Original Message -----
From: "Robert Osfield" <[EMAIL PROTECTED]>
To: "osg users" < [email protected]>
Sent: Monday, November 13, 2006 12:57 PM
Subject: Re: [osg-users] Quad tree like hierarchy of impostors ? Is
itpossible ?
> Hi Wojtek,
>
> osgSim::Impostor supports nesting so it full supports quad tree
> layout, or any other layout you can do with the scene graph.
>
> Impostor have lots of problems in general though, and the
> osgSim::Impostor adds a few of its own caveates. I wouldn't generally
> recommend impostors, but so very specific purposes a custom impostor
> implementation might work well. A cloud system might just be one such
> example. I wouldn't reuse a the general purpose osgSim::Impostor
> though.
>
> Robert.
>
> On 11/13/06, Wojciech Lewandowski <[EMAIL PROTECTED]> wrote:
>>
>>
>> Hi,
>>
>> Like everybody at some point of Engine development we are thinking about
>> implementing cloud system. We thought about 2D array of boxes each one
>> containing few cloud sprites. Of course displaying all this at onece
>> would
>> be too taxing for the rendereres so we would like to use impostors.
>>
>> We thought that it would be nice if we could group each neighbouring four
>> cloud boxes under one impostor and each four neighbouring impostor groups
>> under parent impostor and so on building impostor quad tree hierarchy.
>>
>> We attempted to modify osgimpostor example to do this but results are
>> discouraging either no dispaly or very bad framerate. Is it possible ? If
>> so
>> how ? Was impostor class developed taking into consideration that its
>> childrem may be another impostor ? Will they properly depth sorting
>> during
>> impostor generation ?
>>
>> Regards,
>> Wojtek Lewandowski
>>
>> _______________________________________________
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>> http://www.openscenegraph.org/
>>
>>
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