View Full Version : Rendering only Shadows (shadow pass) using Shadow Catcher, HDR

10-21-2012, 01:05 PM

I'm wondering if there is a way to render out just what the shadow catcher catches as a shadow pass using HDR lighting in Image World/Radiosity

I'm needing to import the passes into After Effects for some compositing.



10-21-2012, 01:21 PM
GI that is in LW doesn't cast shadows.

Only real lights can be caught by Shadow Catcher node.

So the answer is no, Shadow Catcher won't catch what you want.

10-21-2012, 01:51 PM
It appears that HDR shadows must be a different animal all together.
I have a ground plane that when I use Radiosity as the only means of lighting my scene, a shadow appears on the ground plane.
However, I can't create an Alpha for that shadow or capture it as far as I can tell.

Is this true, or is there a way to capture a shadow or render a 'shadow pass' using HDR/Radiosity lighting?



10-21-2012, 05:51 PM
You could possibly use Light B*tch (http://www.hdrlabs.com/lightbitch/index.html) to add lights based on the HDR to use to cast shadows.

10-22-2012, 03:05 AM
Use compositing buffer export, and check the radiosity buffer. This is basically the shading detail from rad, much like its shadow pass equiv.

10-23-2012, 02:26 AM
Internally, shadows from cg lights are calculated separately from the shading / color render, then multiplied together. With radiosity, a shadow is simply an area that recieved no light. RebelHill's suggestion should work with flat ground, but if there's any shape to the ground, that shading will show up in the buffer too. You could also make a separate, fake GI shadow pass using a dome light, especially if you use the DP Light kit dome, which accepts your HDR.

10-23-2012, 06:06 AM
The old-school way of solving this was by using the differential rendering technique described here (http://www.free3dtutorials.com/how-to-compose-shadows.php). Tutorial is old but principle still applies. A more easier way of getting GI shadowing only (commonly known as radiance occlusion pass) is by modifying a bit the LW's radiosity buffer. This can be done in any compositing package, but here (http://lightwiki.net/wiki/Multipass_Rendering_with_Filter_Node_Editors#Radia nce_Occlusion_.28RO.29_Pass)'s is a way of obtaining this pass ready and straight from LW through the DP Image Filter Node Editor (http://pagesperso-orange.fr/dpont/main_en.htm) (updated section).


10-23-2012, 08:54 AM
Thanks guys, looking forward to trying this all out. Been away from Lightwave for a few years, things have gotten a lot nicer since I've been gone.