absorption and scattering coefficients for liquids

General questions about Indigo, the scene format, rendering etc...
Phil
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Post by Phil » Mon Jan 29, 2007 11:42 am

Well it seems there is a problem in the file:

8BStudio wrote:
Here new pdf file! I hope there aren't any errors now!
http://www.8bstudio.com/bonus/scattering.pdf
All the value ar given for a certain concertration, in the case of a merlot the values are given for 1,5 liter of wine dissolved in 23 liter of water.
So if I understand I should first multiply Sigma and Beta with a concentration factor.
For example, if I want a 100% merlot concentration, I should multiply the value with 15,3, ist this right?

Regards, phil

ryjo
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Post by ryjo » Mon Jan 29, 2007 9:46 pm

So if I understand I should first multiply Sigma and Beta with a concentration factor.
For example, if I want a 100% merlot concentration, I should multiply the value with 15,3, ist this right?
I think that the concentration values are given as a reference (to repeat the experiment). The other values are for estimated 100% concentrations.

/J

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CoolColJ
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Post by CoolColJ » Sun Aug 05, 2007 4:51 pm

anyone have a copy of the 8bstudio PDF of the values converted to indigo format linked here?

can you send it my way or attach here? thanks

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Zom-B
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Post by Zom-B » Sun Aug 05, 2007 9:06 pm

CoolColJ wrote:anyone have a copy of the 8bstudio PDF of the values converted to indigo format linked here?

can you send it my way or attach here? thanks
jep!
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scattering-new.zip
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CoolColJ
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Post by CoolColJ » Sun Aug 05, 2007 11:28 pm

thanks!

currently Skindigo currently doesn't support entering RGB discrete values for absorbtion - it has a hybrid opacity value and RGB colour wheel combination so it makes it tough to replicate these values
I used a RGB value of 1,3,13 and then opacity value of 1 (out 100)

I must admit even just putting in the scattering RGB values for milk, with zero values for absorbtion makes SSS look amazingly like real milky substance!!!
Well the milk RGB values are very small anyway...

but if you use other colours then you get milky coloured material
who would have thunk 8)

still it would be nice if Skinidgo could do this...hint hint Whatt :wink:

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Whaat
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Post by Whaat » Mon Aug 06, 2007 6:00 am

CoolColJ wrote:currently Skindigo currently doesn't support entering RGB discrete values for absorbtion - it has a hybrid opacity value and RGB colour wheel combination so it makes it tough to replicate these values
Here is a technique to get the RGB absorbtion values into SkIndigo:
1) Create a Specular material with SSS and save it as an IGM file.
2) Open the IGM file and change the absoprtion and scattering values to the exact values that you want (i.e. from the PDF file)
3) Save the IGM file and then load it back into SkIndigo.
4) SkIndigo should convert the values into the proper color wheel and opacity settings so that they will get exported properly. (There could be a slight margin of error but the values should be exported nearly exactly.)
5) Obviously, you can repeat steps 2 and 3 for as many liquids as you want to create.

:?: Does anyone know if that PDF file is still valid? Ono changed the units for absortivity at some point. The values in the PDF seem quite low (DaveC's milk material had scattering of 500 580 800)
Do we need to convert the values in the table again??? :?:

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OnoSendai
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Post by OnoSendai » Mon Aug 06, 2007 6:30 am

It's a rather confusing paper.
As far as I can tell, the coefficients given are actually for the media arrived at from mixing the given volume V with 23-V liters of water.
So the fractional concentration is V/23-V.
So to get the coefficients for the 100% concentration, you need to multiply by (23-V)/V.
This seems to give a more reasonable scattering coefficient for milk of arround 1.9*10^4m^-1, or a mean path length of 0.05 mm.

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Whaat
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Post by Whaat » Mon Aug 06, 2007 10:50 am

so...you are saying that the values that have been corrected for Indigo 0.7test2 are still valid or not??? :?

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CoolColJ
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Post by CoolColJ » Mon Aug 06, 2007 12:30 pm

Whaat they seem to work just fine here :)

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CoolColJ
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Post by CoolColJ » Mon Aug 06, 2007 5:55 pm

Hmm I take that back - it would seem like there is way too much scattering in my small amount of tests with the the ocean water values

the difference between clear water and the SSS water with the provided values is like night and day!

perhaps multiplying by 10 is too much.....

pics to come

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CoolColJ
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Post by CoolColJ » Mon Aug 06, 2007 9:23 pm

this scene is sized accurate and I used the Mission Bay Surface Water values

the camera is moved slightly here and there, but you get the idea

the original scattering value makes the water go cloudy as you expect, but doesn't go overboard
Attachments
CCJ_SSS_water.JPG
CCJ_SSS_water.JPG (120.83 KiB) Viewed 2101 times
Last edited by CoolColJ on Tue Aug 07, 2007 9:04 am, edited 1 time in total.

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Kram1032
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Post by Kram1032 » Tue Aug 07, 2007 8:59 am

Scattering values x 10 looks ike flat snow^^
Nice pics :)

so, the values are correct...

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CoolColJ
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Post by CoolColJ » Mon Aug 13, 2007 4:34 pm

limiting ray bounces down to 3-5 will allow you to preview SSS and materials a lot faster!!!!!!!!

SSS material is set to Chocolate Milk settings
I actually used the x10 values from the PDF - so I guess they are correct after all :o
The model has correct size values. 20cm diameter for the bottom segment, 10cm for the 2nd, 5cm and then and 2.5 cm for the top one


left pic - ray bounces set to 5, on the right set to 10000, but even 100 bounces looks the same...
Both rendered to 50 samples, about 5 mins here on MLT, Max "large_mutation_prob" "0.4" "max_change" "0.03"

I know which looks closer to chocolate mlik already :D

BTW max_change value of 0.03 is far superior to the default value of 0.01 for getting SSS materials up fast rather than the smudgey splotch you normally get earlier on!
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SSS_5 vs 10000 ray bounces.JPG
SSS_5 vs 10000 ray bounces.JPG (66.57 KiB) Viewed 1925 times

mrCarnivore
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Post by mrCarnivore » Mon Aug 13, 2007 8:09 pm

Can it be that the sss and absorbtion value magnitudes have to be different for 0.8 than for 0.9?

Using my scenes from 0.8 I have to radically lower (maybe factor 10) both settings to achieve the same result in 0.9...

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CTZn
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Post by CTZn » Tue Aug 14, 2007 2:08 am

If I'm following the data flow correctly, it's a matter of gamma value in <rgb> for sss, should be 1 ?
obsolete asset

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