Alain wrote:
> Ray Gardener nous illumina en ce 2009-06-05 16:10 -->
>> Christian Froeschlin wrote:
>>> Nice. I suppose only the trees are billboards?
>>
>> Thank you; yes.
>>
>>
>>> Can this technique produce shadows as well?
>>
>> Yes, but the shadows look increasingly wrong when the lightsource is 
>> at right angles to the viewing direction. The light "sees" the 
>> billboards from the side instead of face-on. Ground-level renders fare 
>> better because shadows get thin anyway, but overall I prefer to leave 
>> shadows off.
>>
>> Shadows could probably be rendered in a separate pass by facing the 
>> billboards at the light and then compositing them back, but there's a 
>> point where the complexity of setting that up weighs against the 
>> simplicity of just using actual geometry (and it defeats the point of 
>> a raytracer to employ scanline-style workarounds). You could use cross 
>> billboards too.
>>
>> The savings in memory and bounding-tree traversal are probably not 
>> high enough (or perhaps don't matter in these days of powerful 
>> hardware) to use billboards primarily, but I find them very helpful 
>> when prototyping because it's usually easier to get a picture than a 
>> model, and pictures are more portable between different renderers, 
>> gallery building is easy, etc. It's nice to have the option of 
>> dropping in billboards for any models that I don't have available yet 
>> -- I can quickly storyboard a scene, play with ideas, etc. before 
>> committing to making or finding the real geometry.
>>
>> Joerg Schrammel's (Genesis Toolkit) work with textured cones is a neat 
>> in-between approach, which requires less (or no) camera-facing of 
>> trees and can cast reasonable shadows. The downside is that the 
>> textures are a little trickier to set up.
>>
>> Mixing geometry with billboards is done to good effect for some trees. 
>> Trunk and branches are 3D but leaf clusters are billboards. They are 
>> rotated fully to camera-face (not just around the Y axis), and it's 
>> easy to sway them in animations to simulate wind. They also light better.
>>
>>
> You can have the shadows and the bilboards in one pass.
> Just double the billboards. One set always faces the camera and is set 
> to no_shadow.
> The second set is set to face the light and is set to no_image.

Hey, yeah, thanks, I'll try that.
Ray

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