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.

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