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Old 11-27-2013, 01:39 AM   #1
vfxTD
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Bernhard Eiser
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Question V-Ray matte / VRayMtlWrapper problem with alpha channel

Hi, in the following simple scene I have a "table_top" matte object matchmoved.

In this scene I am trying to integrate the blue box onto the backplate.
Including shadow, reflection and "secondary bounce" light spill.

Here is the setting of my v-ray full dome light, with a spherical HDRI map and the "table_top" excluded from illumination.


This is my VRayMtlWrapper setup, with a simple white reflective material as Base.

Matte surface is ticked and Alpha contribution is set to -1.0


The result I get from the render in rgb is perfect, exactly what I am looking for.

The table_top renders black (invisible) I get reflection and the light spill.

Here is the problem...
In my alpha channel only the reflection is masked in dark grey under the box.

The light spill and potential shadow is ignored by the alpha channel.

Could you please tell me what I am doing wrong?

I am stuck with this problem for a while now and I could not find any information online that provided a working solution for me.

Here is a mock-up of the alpha channel I am looking for. (Made in PS)


Thank you very much in advance.
 
Old 11-27-2013, 04:22 AM   #2
TheOnlyAaron
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You will likely want to break your render up into passes, render elements would be the easiest for this task. Take a look at this tutorial on how to break this stuff up and use it.

http://www.ruffstuffcg.com/journal/...er-effects.html
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Old 11-27-2013, 08:30 AM   #3
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Quote:
Originally Posted by TheOnlyAaron
You will likely want to break your render up into passes, render elements would be the easiest for this task. Take a look at this tutorial on how to break this stuff up and use it.

http://www.ruffstuffcg.com/journal/...er-effects.html


Hello TheOnlyAaron.

Unfortunatly your link does not answer my question at all. The guy in the tutorial uses the original alpha channel of the beauty to comp over his passes.
As you can see the beauty alpha is really what's broken here.

I agree that Render Elements is the way to go, eventually a VRay MultiMatte Element, in any case I think I would have to make the beauty work before I can break it down.
If someone here knows how I can get the correct alpha channel out of this scene with an element I am happy to comp it back together. Thanks
 
Old 11-27-2013, 11:42 AM   #4
vfxTD
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more render_tests

First of all thank you for your inputs so far.

Quote:
Wie schaut die shadow matte aus? Der sollte dem alpha recht ähnlich sein. Hatte auf die Weise zwar schon Probleme (sooo viiiiel noooise) aber dann müsstest den DMC Sampler raufdrehen und könntest mit dem die Schatten reincompen. Ansonsten statt über den material wrapper könntest du auch probieren vray object properties zu benutzen, die haben das selbe wie der Wrapper eingebaut für hold outs in den matte properties. Würd ich als erstes probieren weil das Ergebnis über den Alpha in der Regel besser ist wie über die shadow matte (vom sampling her). Warum das so ist, ist mir nach wie vor ein Rätsel.


Thomas I did use the parameter in the v-ray properties rollout and i get 100% identical results as the VRayMtlWrapper.

And! Yes indeed the grain in the alpha is bad, but i guess i can blur/ fix it in post !XD?



I am testing some more matte_results and they are interesting but not perfect.

Here i exclude the matte object from the dome light, and i get shadow/lightbounce in rgb but not in alpha.
rgb:

alpha:


If i include the matte in the dome light, and render the matte with no reflection on the base material, i get a really good result, but no reflection.
alpha:

btw. in this alpha it appears that there is also a red channel included. could this be the color spill?

But now of course with this setting if i have a reflective base material, it goes all wrong:
rgb:

alpha:


Cheers

Last edited by vfxTD : 11-27-2013 at 11:48 AM.
 
Old 11-27-2013, 02:02 PM   #5
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Sorry if I miss something, didn't read the entire thread. But if your diffuse pass looks good, could you save that as a PNG with an alpha channel built in? Then you don't even really need the actual "alpha" channel at all?

I just finished a project doing exactly this, but it was my first time so I don't remember exactly the steps I took to get there. But I remember that no passes were needed, just the object with the Matte behind it, and my beauty pass saved as a png with alpha (in the png, not separate channel) was all I needed.
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Old 11-27-2013, 02:21 PM   #6
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I'd render out shadow, reflection and color bleeding as three separate passes, each with its own settings for correct alpha for this element, and then merge them back with cube in composing program.
 
Old 11-27-2013, 02:37 PM   #7
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Quote:
Originally Posted by Decency
Sorry if I miss something, didn't read the entire thread. But if your diffuse pass looks good, could you save that as a PNG with an alpha channel built in? Then you don't even really need the actual "alpha" channel at all?

I just finished a project doing exactly this, but it was my first time so I don't remember exactly the steps I took to get there. But I remember that no passes were needed, just the object with the Matte behind it, and my beauty pass saved as a png with alpha (in the png, not separate channel) was all I needed.


Hello Decency!

Thank you for replying...

If my matte has no reflection in the base material, it generates the correct alpha and i can save it as a .png for example.

If there is reflection in the base of the matte, the alpha channel ignores shadow and light spill and if save it as a .png the alpha channel is multiplied, leaving me without any shadow or light spill.

I did a test saving the render as a .exr opened it in PS, see the following images.




The alpha ignores all shadow or light spill.
 
Old 11-27-2013, 03:32 PM   #8
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Quote:
Originally Posted by Darth-Biomech
I'd render out shadow, reflection and color bleeding as three separate passes, each with its own settings for correct alpha for this element, and then merge them back with cube in composing program.


Hi Darth-Biomech,

I am now rendering matte_shadow and color bleed, without reflection in one session and the reflection in a new session with the reflection of the base material applied.

Could you recommend a render element for color-bleed?

With your technique i could achieve the desired result after a lot of shuffling around channels.


(the box is now white because i applied reflection to it =D)

In my opinion i am using the wrong passes, to get to this result. also my alpha is seriously grainy. Tom said earlier the grain can be removed with DMC settings, but i had no luck.

Would be nice to know, what the exact passes and settings for those are.
For example my VRay-reflection pass renders black, altho there is loads of reflection going on.

There must be a golden setup with a system that is universal and works with most shots :O
 
Old 11-28-2013, 06:56 PM   #9
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First, I would use passes, as others suggested. This is the most flexible.

But if you enable illumination on your table object (and it has a diffuse component) you should be able to get a single image with shadows and reflections with a proper alpha with those VRayMtlWrapper settings. Camera map the BG plate on to the table for proper gi interaction with the cube.

That said, you really want the passes anyway, as shadows are typically not neutral in color, and you can easily modulate the amount in the post as you have seen.
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Old 11-28-2013, 07:03 PM   #10
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For the shadow grain try upping the subdivision in your light(s).

Much of the time it is easier to just make some layers with duplicates of your catcher objects (the table). Set each one up with its own material wrapper and configure one for shadow, one for reflection. Then render those in separate renders (you can try state sets if you want). Then you have another one for proper gi interaction with the CG object (box). You set the table as invisible to camera for the box render, and set the box as invisible to camera for the table renders, etc.

While it is nice when you can, don't feel compelled to get everything into a single render by using elements. Sometimes it is easier to hit the render button more than once.
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Last edited by PelDaddy : 11-28-2013 at 07:07 PM.
 
Old 11-30-2013, 04:52 AM   #11
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Hello PelDaddy,

Thank you very much for your valuable advice. I think you are right and you gave me confidence to not worry about pressing the render button twice if the situation requires it.

Also big ups for the light subdivision comment, this really does the trick.
Got superb smooth alpha channels.

In the following video I did only press it once but the result is quiet pleasing.
This shot was really R+D and now I am ready to apply the learned aspects to the ShowReel shot I am working on.




Thank You all for engaging and helping out.
 
Old 11-30-2013, 04:52 AM   #12
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