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How to render a model with shadows in alpha channel (HDR Lighting)

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Simon HuangHow to render a model with shadows in alpha channel (HDR Lighting)
by on Jun 21, 2014 at 3:46:41 am

hey guys, I am having massive problems trying to render a model with shadows in alpha channel (HDR Lighting). So, basically, I am using HDR Light Studio Live to create the reflection maps.

I understand how you can use a "shadow-catcher" with a light source to render the shadows. However, in my case, since there is no light ( but fake HDRI lighting), how do I create the shadows and render it out properly? Any advice/ tips. Much appreciated! :)


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Adam TrachtenbergRe: How to render a model with shadows in alpha channel (HDR Lighting)
by on Jun 21, 2014 at 1:41:58 pm

Unfortunately it isn't possible to catch GI shadows because they are just a part of the GI solution, i.e., the shadow areas are darker because they receive fewer GI samples than brighter parts of the scene. The render engine has no way of identifying them as shadows.


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Simon HuangRe: How to render a model with shadows in alpha channel (HDR Lighting)
by on Jun 22, 2014 at 1:04:20 am

Thanks for your reply! Is there any other ways to do it then? I'm sure there must be a way. Otherwise what I have bought would be useless if it did not allow shadows to be rendered.

Cheers :)


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Adam TrachtenbergRe: How to render a model with shadows in alpha channel (HDR Lighting)
by on Jun 22, 2014 at 4:03:33 am

There is no way to extract a shadow pass from a GI render.


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Simon HuangRe: How to render a model with shadows in alpha channel (HDR Lighting)
by on Jun 22, 2014 at 7:54:17 am

If that's the case, I guess I have to use a normal light.. I would then have to create 2 project files, with both models in the same position. However, for the first one, I would have to use a normal light and then render that shadow ( since it is not gl illumination, it would work)But I have to do everything twice :((.. How does that sound? :)


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Adam TrachtenbergRe: How to render a model with shadows in alpha channel (HDR Lighting)
by on Jun 22, 2014 at 5:07:15 pm

Sounds like a pain. :-) Why do you need the separate shadow pass?


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Simon HuangRe: How to render a model with shadows in alpha channel (HDR Lighting)
by on Jun 25, 2014 at 1:01:53 pm

Sorry, had been busy for a while. Why do i need a separate shadow pass? To create a shadow...lol.. What do you mean?


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Adam TrachtenbergRe: How to render a model with shadows in alpha channel (HDR Lighting)
by on Jun 25, 2014 at 2:03:59 pm
Last Edited By Adam Trachtenberg on Jun 25, 2014 at 2:12:09 pm

I mean you already have shadows in the render ... why do you need it to be a separate pass?

Anyway ... it occurs to me that you may be able get light shadows without having to render twice. You just have to go into your lights' Details tab and check the "shadow caster" box. The lights will then produce shadows but they won't add any illumination to the scene.


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Simon HuangRe: How to render a model with shadows in alpha channel (HDR Lighting)
by on Jun 26, 2014 at 6:11:08 am

I'm at a loss as to which one you are talking about. HDR Lighting or normal lighting? Anways, i have uploaded two images to clarify this situation.

http://postimg.org/image/l10ny85of/

http://images.creativecow.net/274255/alphachannel.png

I hope now you know what i'm talking lol.


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Peter ZubekRe: How to render a model with shadows in alpha channel (HDR Lighting)
by on Mar 11, 2015 at 7:01:33 pm

I had exactly the same problem and here's how I solved it:
Besides your regular passes also export Multi-Pass for Illumination and Global Illumination. You don't need to export Multi-Pass for Shadow because you'll get empty channel anyway. Now throw everything in After Effects and set your Illumination layer mode to "Overlay" and put your Global Illumination layer underneath your Illumination layer. Then just turn TrackMate to "Luma Inverted" on your Global Illumination layer and bam! you got yourself transparent shadows from HDR light source ;) Now of course add your RGB pass which you'll need to mask-out using an Object Buffer but that you probably know already. Here's a link to a pic of my AE timeline where those 2 important layers/passes are marked green: https://drive.google.com/file/d/0B5JGbWZ-yPheSmgzMkRGZEhzY1U/view?usp=shari...


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