IOR values for blended materials

[haluperido]

Hi Guys!

For some time unsuccesfully I've been trying to create realistic grass material for my scenes. I mean realistic in terms of enviroment reflections. Every IOR value in normal range (1-2.5) I've been been setting for the material was resulting mostly in lack of reflections from enviroment, so I've started using higher IOR (6-12) values to match my reference photos.

I couldn't figure out why is that happening because simple materials like wood with just Phong shader, texture and bump were looking good without tuning IOR too much.

Just yesterday I've decided to spend some time to check how simple material reflection will match blended and double blended material reflection - It didn't lol

So I suppose blending materials together results in reduction of reflectiveness by the factor of blending and to achieve same strength of reflection with blended materials we should adjust IOR value accordingly.

After couple minutes I've figured those adjustments with random easy to calculate values.

Blended_Material_IOR = Desired_IOR_Value / Blending_factor

So like on the attached picture we need to set 2.64/0.33 = IOR 8 for our blended material to match simple phong material with IOR 2.64 in terms of reflection strength.

Is it correct to use similar calculations to adjust Phong IOR in double blended material? Does it makes sense? How is actually material blending solved in indigo?



I've tested IOR values calculated this way on a double blended grass material and it looks quite close to reference photos I had so it seems like such adjustment might be actually correct. Is it?

http://img214.imageshack.us/img214/6093 ... enders.jpg

[CTZn]

I have only a simplistic answer for you, I'm hoping that someone else will be more specific regarding your questions:

The amount of light that is transmitted is not taking part to the reflections. It is expected that those transmitted rays will weaken the overall reflections because by definition they are not contributing to them.

Did you try with a somewhat higher exponent to compensate (x2 or so)? The amount of light reflected will be the same for a constant IOR, but the grass blades will be shinier (less glossy). My personal feeling is that the highlights are spreading too wide and therefore are dimming the impression from reflections.

Most of your questions were not answered still...

[CTZn]

Found this post, might help a bit:

http://blenderartists.org/forum/showthr ... ost1425863

Wood, also phong, IOR 1.34 -1.36 depending on the finish of the wood exponent between 250 - 400 higher values more varnished wood. Grass, similar IOR very low exponent 6 - 8.

I don't think that IOR for grass should go above 1.5 really. That's more reflections than a typical dense glass already.

[|MM|]

I'm not sure what kind of an answer you're expecting, but I think you're taking a wrong approach.
In real world there's all kinds of grass with different properties in terms of reflectivity etc. also obviously it depends on things like weather, humidity so no one will give a totally correct value.
I can't say much about physical corectness of your materials, but you won't create a realistic grass with just applying one material to all the blades.
Create a couple of blades with different textures to have some variation no grass in real world is just one color.

[lycium]

Currently in Indigo you can't blend between two specular materials; this condition may be relaxed in the future when both materials use the same medium.

[CTZn]

this condition may be relaxed in the future when both materials use the same medium.

That is very cool !

Otherwise you have been catched OT hand in the basket

Who said I was trolling ? For once !?

[haluperido]

The amount of light that is transmitted is not taking part to the reflections. It is expected that those transmitted rays will weaken the overall reflections because by definition they are not contributing to them.Did you try with a somewhat higher exponent to compensate (x2 or so)? The amount of light reflected will be the same for a constant IOR, but the grass blades will be shinier (less glossy). My personal feeling is that the highlights are spreading too wide and therefore are dimming the impression from reflections.

Hi and thanks for reply!

I've been trying to fiddle with exponent at the beginning but it doesn't compensate for the loss of reflection with blended material that well. I mean when using small value for index of refraction (1.2-1.6) on actual grass material increased exponent value causes change of the amount of those really shiny reflections. Lower exp value was giving slightly higher number (like a soft reflection but spread to the particular strands) of "blinks" on the grass and higher values were decreasing number (and size slightly) of those blinks on a particular grass surface. Anyways it didn't behave realistically, although it looked like it sometimes and under certain camera/sun angles.

I'm looking to figure out approximately accurate grass material which behaves good under most commonly used lighting condition, I don't mind increased rendering time at the moment just realistic behavior matters for me.

I don't think that IOR for grass should go above 1.5 really. That's more reflections than a typical dense glass already.

This value - I'm actually really suspicious about it. I would consider a grass material to be constructed from two different layers: Resin and Pigment - this is much simplified but I think it quite matches for the Indigo material setup:

Pigment Layer - the inner layer of the grass strand created with a blend of Diffuse Transmitter (to get semi sss effect) and Phong shader (to get nice soft (exp ~150) environmental reflections from the sky or other surrounding objects)

ResinLayer - this will be surface of the grass strand (most likely in nature grass strand will have some kind of really thin amount of resin on it's surface) and this is to catch reflections from stronger emitters (sun/lamp blinks etc.). It would consist of Phong shader (exp ~750) mixed with our Pigment Layer.

And how about those IOR values? I've found this really informative article, take a look: http://www.naturalpigments.com/educatio ... rticleID=8

Considering fact "that a substance which has a higher refractive index is a substance that impedes the velocity of light or offers more resistance to it, and therefore a larger proportion of the light will be reflected" - we will have to find green opaque pigment to match our material (pretty natural approach), and the most "grassy" looking one was the Volkonskoite pigment with IOR value of 2.5, which is extracted from pea (http://kremer-pigmente.de/bilder/Erbse.jpg) what makes it even more natural lol

With ResinLayer IOR I would just go for something around 1.3-1.4 (I haven't change it with the material yet to test) as it would be something around ice IOR I reckon.

I'm not sure what kind of an answer you're expecting, but I think you're taking a wrong approach.
In real world there's all kinds of grass with different properties in terms of reflectivity etc. also obviously it depends on things like weather, humidity so no one will give a totally correct value.
I can't say much about physical corectness of your materials, but you won't create a realistic grass with just applying one material to all the blades.
Create a couple of blades with different textures to have some variation no grass in real world is just one color.

Of course I realize realistic grass can't be achieved with only one material and one grass patch. This is just for testing purposes I'll post more complex scene when I finish it.

I would use texture and blending factors for the layers in material created this way solely to leverage atmospheric conditions of specific scene and to differentiate between different types of organic texture. And actually heres big question mark about blending mode for double blended material. Can we treat those as layers? Are they stacked as a layers one on top of each other? Could we actually simulate double refraction/reflection effect withing one double blended material as this:



It would be very nice feature in double blended materials to have second layer (PigmentLayer in this case) receiving only refracted light from first layer (ResinLayer), to compute further refraction and reflection. I think such feature would greatly increase material realism and material creating workflow.

Now I have to sort out in my head what to think about adjusting IOR values for those materials, and maybe there could be a way to actually calculate this and simulate double refraction (but still don't know how indigo blend layers so I guess it's good as is and maybe overcomplicated already )

Anyway I'm waiting for your insight guys, please feel free to share your thoughts!

[CTZn]

First off I think that the comparison of IOR between painting pigments and for instance chlorophylle in grass does not apply:

All inorganic pigments have high refractive indices, and hence, when used to color paint give high opacity.

This is specific to minerals, grass is somewhat different. Probably inorganic pigments also are of an higher density than a grass blade (for the same volume). Also these pigments imply a variation in IOR provided that they are bound by a ligand if I'm correct.

And actually heres big question mark about blending mode for double blended material. Can we treat those as layers? Are they stacked as a layers one on top of each other? Could we actually simulate double refraction/reflection effect withing one double blended material as this:

No, coated materials were discussed just a couple days ago in the forum for Indigo hasn't got them at this time. Blends are much like alloys, there is no notion of depth. I can see why you blended blends this way, and you can see now why it is a bit of an overkill.

Since time is less of a concern for you, you can model these layers and apply delta materials (materials using a medium, plus the null material if I'm correct) to the different parts of the blades. You would use precedence in place of blends in order to preserve the inner medium (bind it to a diffuse transmitter) and perhaps use a glossy transparent for the resin. It makes now sense to underline lycium's statement on blending two delta materials together, wich is not allowed. However you can create an "air" medium (ior 1.0) to do boolean substractions on media.

Keep volumes sealed then.

You may or may not experience scale related issues with such thin geometry. Anything below 0.2mm would cause issues by default, probably.

Having said that just use what fits you (or your reference photos) best

[CTZn]

we will have to find green opaque pigment to match our material (pretty natural approach), and the most "grassy" looking one was the Volkonskoite pigment with IOR value of 2.5

That's a major confusion.

You have read that IOR was affecting the amount of reflections over refractions. Colour do not relate with IOR, unless we are talking about specific chemical components as in your link.

Changing the IOR of a Phong material will affect the perceived brightness of its albedo layer (for the matter), NOT its color !

A green gas would have an ior much like air. So we can tell that beside the optical properties of each specific component involved, density is playing a role too.

[Zom-B]

Currently in Indigo you can't blend between two specular materials; this condition may be relaxed in the future when both materials use the same medium.

Isn't blending of two specular mats with same medium (not similar, but same?!) possible since quite some time already???
Only issue here is that the exporters create a own medium for each material and you need to edit the igs by hand...

[CTZn]

Thanks for the heads up Zom-B:

- specular sharing the same medium can be blended indeed.
- there was a bug in MtI's blend code (3.2.2.2).

mti allows multiple materials to use a single medium !

To clarify the images below: one specular has a constant absorbtion layer, the other has no absorbtion layer.

[haluperido]

First off I think that the comparison of IOR between painting pigments and for instance chlorophylle in grass does not apply:

All inorganic pigments have high refractive indices, and hence, when used to color paint give high opacity.

This is specific to minerals, grass is somewhat different. Probably inorganic pigments also are of an higher density than a grass blade (for the same volume). Also these pigments imply a variation in IOR provided that they are bound by a ligand if I'm correct.
[...]
Blends are much like alloys, there is no notion of depth. I can see why you blended blends this way, and you can see now why it is a bit of an overkill.

Yes indeed! btw I didn't noticed Volkonskoite is actually mineral pigment But anyway I had lot of fun playing with Phong shader and double blended material and not including physical correctness my grass material improved quite a bit during last few days and I've learned a bit about refraction

Since time is less of a concern for you, you can model these layers and apply delta materials (materials using a medium, plus the null material if I'm correct) to the different parts of the blades. You would use precedence in place of blends in order to preserve the inner medium (bind it to a diffuse transmitter) and perhaps use a glossy transparent for the resin. It makes now sense to underline lycium's statement on blending two delta materials together, wich is not allowed. However you can create an "air" medium (ior 1.0) to do boolean substractions on media.

Thanks a lot for you clarification! Will try that approach when I'll finish an animation I'm currently working on, but seems like this technique will meet my requirements just fine. Hopefully I'll post some renders in couple days.