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View Full Version : SSS Pass and Compositing.Please help

ghostlake114
02-23-2008, 02:18 PM
I know many prof around here I break misss_fast_skin into pass, and we even could find a great movie tutorial here
http://www.sigillarium.com/index_e.html

However, the trouble is with compositing, I never make out with blending mode for these pass is. In this sample, I will disable specular (specular weight = 0) for easier comparasion

Let s make a misss_fast_skin shader, and then make weight = 0 for different parameter to get these pass

diffuse pass: with epi weight,sub weight, back weight = 0
http://i35.photobucket.com/albums/d152/ghostlake114/misss_passdiffuse.jpg
epi pass: with diffuse weight,sub weight, back weight = 0
http://i35.photobucket.com/albums/d152/ghostlake114/misss_passepi.jpg
sub pass: with diffuse weight,epi weight, back weight = 0
http://i35.photobucket.com/albums/d152/ghostlake114/misss_passsub.jpg
back pass: with diffuse weight,epi weight, sub weight = 0
http://i35.photobucket.com/albums/d152/ghostlake114/misss_passback.jpg

And this is original render of this shader
http://i35.photobucket.com/albums/d152/ghostlake114/skin_ori_screen.jpg

And composite result
http://i35.photobucket.com/albums/d152/ghostlake114/skin_mix.jpg

So you could see NONE of composite similar to original in both screen on and off case.

I have made search, many say use add/screen mode but as you could see no is right. I even read Zap say just use screen blending mode, but the fact is not right, or I am missing something here. ???

I have used mix_8_layer to test internally in Maya and also get this result
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So any one sucessfully in seperate SSS pass please help.

Thanks for your reading
ghostlake114
02-23-2008, 05:26 PM
The different seem come much from shadow of diffuse pass

Is there any trick here. Or break diffuse pass into passes, or need to blend sthing else here

Hope Zap is around here so he could give me some tips :love:
ghostlake114
02-23-2008, 08:51 PM
I decide to dig into .mi file to see the layer inside of misss_fast_skin and finally come to this formula

final_sss = overall_color * ( back_sss + subdemal + epidermal + diffuse_color)


where "+" mean: screen blending if screen composting ON and add if screen composite OFF
and "*" mean: multiply

And pass need to be floating point so multiply of overall_color dont clamp out color.
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This is what I come out after test and see the phenomenon. So could anyone confirm it.
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So to make pass for compositing, we need to to rebuilt the misss_ instead of using misss_fast_skin phen.
Ash-Man
02-26-2008, 04:19 PM
To be honest with you, I don’t think this can be done the way you are expecting

The SSS is a phenomena and I wouldn’t expect a “proper” pass out of it

but it will be intrestign to see what others has to say
ghostlake114
02-26-2008, 04:43 PM
no man, I am sure you are totally wrong. I have make a deep research with sss_fast_skin_phen and sure about what I say.
First, the way to canculate front and back sss is surely programing,but the way to layer them in shader is: add or screen.

Will split into 3 shader

1. First fast_sss_shader contain: back_sss and mid_sss, that blending mode is SCREEN or ADD (read document) and you will get this,or test by your own hand.

This shader contain: diffuse_color, that has been named to overall color in fast_skin. diffuse_color is MULTIPLY with shader. And we get result in first shader: overall*(mid + back + illum slot shader).
2. Illum slot shader above will contain another fast_sss_shader: front_sss, another is disable with weight = 0. Diffuse_color is is white so we get multiply of 1 (ONE) and could be ignored. Illum_slot here is misss_illum_gamma .So in this shader we have: front_sss + diffuse (mean result of misss_illum_gamma only).
3. So you could see the total result.

final_sss = overall_color * ( back_sss + subdemal + epidermal + diffuse_color)

so the different here is diffuse_color is result of misss_illum_gamma, so we will need to break fast_skin_shader into small shader to get the pass result.

This may be a long thread and not cost any penny in some way, but I just want to share my research and finalize how to MAKE SSS PASSES
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