[numerobis]

A realistic (and fast) leaf shader for single sided objects as combined front-back solution/model.
I still haven't found or seen any realistically behaving solution (under various conditions) using the current single sided approach, so i think it is not possible.

For me this is a very important point in architectural visualisation (just like a realistic AGS material, but this would be another wish).

[Mihai]

I think it's the different maps you need for normal front/back and translucency front/back that are going to play the biggest part in the realism. I've created several leaf materials I'm happy with but it wasn't so much about the "shader" as it was about having the proper maps. Can you be more specific about what is wrong with these for example:

http://www.maxwellrender.com/forum/view ... 48&t=44364

[numerobis]

Yes, i know your solution. I have bought your vine leaf (very good textures) and the Vizpark grass but i can't get a realistic result. Especially the contrast or different behavior between front and backside is not working for me.
The linked test images look very good, is this using a HDRI or emitter? I think the problem is more the scattering of the backside and how it reacts under various conditions.

Here is the result using your grape vine leaf (one of them, front and back) on a tree with physical sky (only sl12)



Here is a photo reference - not regarding the tree or leaf structure but only the behavior of the front and backside of the leafs. I'm not completely sure about it but i think the problem is mostly that the single sided scattering works more like a transparent material while a real leaf is catching light from all directions and sends it to the backside. The backside is almost always lighter than the front of the leaf even in front of a darker background with only ambient llight. And then there is this colored "glow" when the sunlight lits the leaf.




I normally use a similar approach but with three BSDFs instead of additive layers. I have to compensate the darkening a bit, but i think it renders faster. Is there any other drawback in using BSDFs instead of additive layers?
And i'm not sure about the r2 thing.

[Mihai]

There are several things to keep in mind if you compare with photo references:

- let the render go to at least SL 15 or so to better judge the light spread, especially with a tree with tens of thousands of leafs. The lighting will become a bit more "interesting" after SL 15 or so, when dealing with SSS/thinSSS.

- That particular leaf material I created is ment to mimic a very diffuse leaf, the front side reflections where very diffuse and weak. In your ref. image you can see the leafs are quit a bit more shiny on the front face. This can have a contrast enhancing effect of course. So try to decrease the roughness in this case, and maybe also raise the surface reflections.

- did you also add a ground plane to this render, because if not then the bottom hemisphere of the Physical Sky wil also send lighting from below and you lose more contrast.

- I did those test renders with an HDR, but testing under Physical Sky I didn't get any strange effects, you can try increasing the power of the sun, to get a stronger backside "glow". And also just decreasing the assymetry even more but I think they're already at around -0.7 or so.

Finally, the straight render from Maxwell will look very bland compared to a tone mapped image. Use one of the tonemapping presets to judge a bit better.

I normally use a similar approach but with three BSDFs instead of additive layers. I have to compensate the darkening a bit, but i think it renders faster. Is there any other drawback in using BSDFs instead of additive layers?
And i'm not sure about the r2 thing.

If you could post your material, and I suggest running a simple test to see if it's faster. Judge both the front speed and back speed. My approach renders the front side a LOT faster (it's basically clean by SL 12), compared to a material with simply one BSDF set to thinSSS, but the backside render at about the same time. Try it and post your findings.

[numerobis]

There is a groundplane. And i posted this without postprocessing because i don't think that it is needed to show the points i mentioned.
And as i said, this was not meant as a direct comparison with the reference (which should be obvious), but to show the result.

I will continue testing, but my wish persists... even if i don't think that anything like this will ever happen in the near future (looking the features for v4 i think that dirt and a better AGS is also an non-issue).

[eric nixon]

Bear in mind that dense leaves probably wont render well due to caustics of caustics issue.

Each leaf transmits caustic light, so maxwell will only see light coming through the first 2-3 leaves each ray hits, unless you render to a really high sl.

[Mihai]

The backside is almost always lighter than the front of the leaf even in front of a darker background with only ambient llight. And then there is this colored "glow" when the sunlight lits the leaf.

That glow when it's backlit is quite possible to get. And you can make the front of the leaf less translucent looking simply by turning down the influence of the thinSSS layer in the front side material. Or also increasing the min thickness parameter. For example (this is only SL 12):



I had to do some changes to the leaf material to get it closer to the references photos you posted because this particular leaf in reality looks very similar from the front and back (which I agree is not that common), and also has very similar translucency from front and back. It's quite a strong translucency because the leaf is pretty thin. You can see that by simply looking at the textures. So the changes I did where:

- (front mat only) increase the shiny layer to 40%, lowered the roughness from 65 to 55 and lowered quite a bit the brightness of the roughness map.
- (front mat only) decreased the thinSSS to 25-35%
- increased the Sun power from 1 (default) to 3. This can be interesting to do in other cases besides trees and leafs. I think it helps the realism when you really want to have a very bright looking day (our ozone layer ain't what it used to be...).

Of course the glow from the backside, you'll get when you view a leaf from the back, and depending on your model and camera angle, you may see mostly just the front of the leafs. Also, the model in my render only has completely flat leaf geometry and they are hanging down so I'm mostly seeing flat front faces. Finally of course using all 6 different materials will add more interesting variation. Here I've used only 2.

[numerobis]

Very nice. Thanks for your effort!
But the transparency problem is still there
Is there a way to increase the scattering? (without influencing the color)

[Mihai]

Transparency problem? If you could tell me in more detail what you're expecting to see...more translucency without affecting the color...that would depend on how it's lit - more translucent = more affected by backlighting.