Pointbased Occulsion

在Renderman的rsl function列表中，
occlusion函式並沒有列出pointbased這個參數，
但是他確是使用pointcloud based occlusion不可或缺的設定。
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In Rendernman rsl function document,
you can't see nothing about pointbased description at occulsion part.
It's very important when using pointcloud based occlusion.


如果你已經將area資訊bake到ptc檔案中的__area channel，
可以使用occlusin(P, N, samples, "filename", "xx.ptc", "pointbased", 1, ....);
這樣就會用pointbased的方式算出occlusion，而不是用raytraced的方式。
這樣的方式在連續圖片的Occlusion較不會閃。
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If you have calculate area value and using bake3d to store it in a ptc "__area" channel.
When having a ptc file which stored area() information in __area channel(using bake3d function),
you can get pointcloud occlusion value by occlusion(P, N, samples, "filename", "xx.ptc", "pointbased", 1, ....).
This way will reduce image sequence flick.