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Chaos Corona for 3ds Max => [Max] General Discussion => Topic started by: cecofuli on 2013-05-14, 15:51:37

Title: Thin plastic chair - Ghost = How?
Post by: cecofuli on 2013-05-14, 15:51:37: Hi,

as in the title, I'm finding difficult, near to impossible, to obtain the standard shader for a Starck - Ghost chair.
Especially contrast between the dark edge and the internal color.
In attachment the 3ds max model and the photo reference. Do you have some suggestion?
Title: Re: Thin plastic chair - Ghost = How?
Post by: Ondra on 2013-05-14, 15:57:06: I would try refraction + attenuation color ;)
Title: Re: Thin plastic chair - Ghost = How?
Post by: maru on 2013-05-14, 16:26:02: Just a quickie. I bet it could also be faked by AO or falloff map.

What comes to my mind in this situation is that a "bias" parameter for absorption would be nice so that you could change "thickness" of the dark areas. Then it would basically work like fog.
Title: Re: Thin plastic chair - Ghost = How?
Post by: cecofuli on 2013-05-14, 17:09:31: Thanks. This is my test ( I need it for exterior project)
Yes, bias, like in VRay, will be very useful!

But.. why is my shadows solid and not transparent?
The light comes from a daylight system with CoronaSun.
Title: Re: Thin plastic chair - Ghost = How?
Post by: Ondra on 2013-05-14, 18:04:19: You would have to switch to bidir here to get correct caustics from using solid glass
Title: Re: Thin plastic chair - Ghost = How?
Post by: cecofuli on 2013-05-14, 18:16:01: Mmm... I have strange result... :/
Just change Progressive to Bidir/VCM
BTW, why can I have a transparent shadows with BF/IM and/or LC, and here we have to change Renderer?
Or do you have to solve this problem (transparent shadow)
Title: Re: Thin plastic chair - Ghost = How?
Post by: cecofuli on 2013-05-14, 18:32:53: And... how can be possible render glasses (Onesided) of water or tables in glass with this problem and without these unrealistic, solid shadows?

Polymax: I see in your render a table in glass. Why don't you have a solid shadow near the objects in glass?
http://forum.corona-renderer.com/index.php/topic,745.0.html

Also in this famous render:

(http://jakubcech.net/wp-content/uploads/tineKhome_teaser_JakubCech.jpg)
Title: Re: Thin plastic chair - Ghost = How?
Post by: Polymax on 2013-05-14, 18:40:28: Quote from: cecofuli on 2013-05-14, 18:32:53
And... how can be possible render glasses (Onesided) of water or tables in glass with this problem and without these unrealistic, solid shadows?

Polymax: I see in your render a table in glass. Why don't you have a solid shadow near the objects in glass?

Because light with soft shadows(sky light, big size emitters or coronaLights make a great caustic in PT+HD or PT+PT mode.

fast test yours chairs. Th scene have a 2 Coronalights + HDRI.
Title: Re: Thin plastic chair - Ghost = How?
Post by: cecofuli on 2013-05-14, 18:49:42: Thanks Polymax. Here there isn't solid shadows cause a lot of lights and softshadows ;-)

Keymaster wrote in this (http://forum.corona-renderer.com/index.php/topic,798.msg6177.html#msg6177) thread: ad 1) Solid is darker sometimes, because there is bias added to prevent any fireflies by default. You can set Max sample Intensity to 0, eliminating the bias, but at cost of fireflies (which will eventually converge to the correct solution). Since there is no efficient way to render sun behind solid glass with path tracing, you would have to use for example bidir to get correct and fast results.

This remember me the old maxwell version with the famous "Sun shadow+Glasses" problem :(

And Solid is darker not only with sun, but with also normal Corona lights :(
Title: Re: Thin plastic chair - Ghost = How?
Post by: Polymax on 2013-05-14, 18:56:58: bidir/vcm not support absorbtion.
Title: Re: Thin plastic chair - Ghost = How?
Post by: cecofuli on 2013-05-14, 19:01:36: Yes, I saw...
But, transparent shadows for glass are so important!
Why there isn't a good, robust solution in Corona? Do we have to wait some new version?
Title: Re: Thin plastic chair - Ghost = How?
Post by: Polymax on 2013-05-14, 19:10:46: At this point the solution of this problem does not exist in the Corona! ONLY fake(
And Vray will give also not correct shadows. There will help only caustic.
I hope bidir will be completed in the future!
Title: Re: Thin plastic chair - Ghost = How?
Post by: cecofuli on 2013-05-14, 19:14:29: And, how can we fake a transparent shadows!! XD Impossible!
Keymaster has to do something for this problem ;-)
I add a new thread in the request section, ok?

EDIT: yes, I know. But I can do a transparent shadow with every method (BF, IM, LC, PPT. VRay-RT) . And for the 99% of our works, this is enough ;-)
Title: Re: Thin plastic chair - Ghost = How?
Post by: patoaltaco on 2013-05-14, 19:28:50: some test
Title: Re: Thin plastic chair - Ghost = How?
Post by: Polymax on 2013-05-14, 19:29:49: Quote from: cecofuli on 2013-05-14, 19:14:29
And, how can we fake a transparent shadows!! XD Impossible!
hard fake: image with ground and 2sided shadows + image with chairs only (with alpha) in photoshop ... like this ;)
Title: Re: Thin plastic chair - Ghost = How?
Post by: cecofuli on 2013-05-14, 19:31:09: Thanks: patoaltaco. But, again, soft, SOLID, shadows =(
They make the rendering so unrealistic...
Title: Re: Thin plastic chair - Ghost = How?
Post by: cecofuli on 2013-05-14, 19:32:23: Quote from: Polymax on 2013-05-14, 19:29:49
Quote from: cecofuli on 2013-05-14, 19:14:29
And, how can we fake a transparent shadows!! XD Impossible!
hard fake: image with ground and 2sided shadows + image with chairs only (with alpha) in photoshop ... like this ;)

Ehh.. ok, this is a very easy scene. But... try to think with a complex interior... Brr... :/
Title: Re: Thin plastic chair - Ghost = How?
Post by: Polymax on 2013-05-14, 19:37:24: I think a solution would be, well, later. While for these tasks using a another renderer(vray, mental ect) (.
Title: Re: Thin plastic chair - Ghost = How?
Post by: cecofuli on 2013-05-14, 19:51:41: =(
Title: Re: Thin plastic chair - Ghost = How?
Post by: Polymax on 2013-05-14, 20:35:35: Or ask Ondra to make a option for choice type of shadow(thin mode shadow) for solid glass ;), but this is fake, so us need to very ask :)
Title: Re: Thin plastic chair - Ghost = How?
Post by: cecofuli on 2013-05-14, 20:39:19: Mmm.. not bad idea )))
Title: Re: Thin plastic chair - Ghost = How?
Post by: Chakib on 2013-05-14, 23:44:30: here is my test :
(http://imageshack.us/a/img199/2108/testchairs.jpg)
Title: Re: Thin plastic chair - Ghost = How?
Post by: Polymax on 2013-05-15, 12:06:20: Other caustic from bottle, 2 CoronaLight + HDRI. In interiors this is working in most cases.
Title: Re: Thin plastic chair - Ghost = How?
Post by: cecofuli on 2013-05-15, 12:15:12: How is the model? With thickness? And the shader? Try with only one little rectangular light, without and HDRI or environment GI and see what happen.
Title: Re: Thin plastic chair - Ghost = How?
Post by: Ondra on 2013-05-15, 13:48:14: The shadows get more opaque the more sharp the lighting is. This is done automatically to reduce fireflies, you can increase the max sample intensity parameter to get more opaque shadows (although with more fireflies). Or you can use bidir, or fake it ;)
Title: Re: Thin plastic chair - Ghost = How?
Post by: cecofuli on 2013-05-15, 13:51:41: Keymaster, as Polymax wrote, can you make a option for choice type of shadow(thin mode shadow) for solid glass?
Or do you have some idea for the next update in progressive or Bidif method?
I don't want to switch to VRay for this project...
Title: Re: Thin plastic chair - Ghost = How?
Post by: Polymax on 2013-05-15, 18:02:25: Quote
Try with only one little rectangular light, without and HDRI or environment GI and see what happen.
I try(MSI=200):
Title: Re: Thin plastic chair - Ghost = How?
Post by: cecofuli on 2013-05-15, 18:28:24: Not bad, but in my chair scene, with 200 MSI, I have again solid shadows... =(
Title: Re: Thin plastic chair - Ghost = How?
Post by: Polymax on 2013-05-15, 18:33:45: With Corona Sun this is not working ((
Title: Re: Thin plastic chair - Ghost = How?
Post by: cecofuli on 2013-05-15, 18:35:46: I saw. Anyway, in your test, the shadows are too solid, dark and unrealistic for a glass objects =(
Title: Re: Thin plastic chair - Ghost = How?
Post by: Polymax on 2013-05-15, 18:46:00: Compare Corona vs Vray. Glass more realistic in the Corona, than in the Vray!
Title: Re: Thin plastic chair - Ghost = How?
Post by: cecofuli on 2013-05-15, 18:53:17: Mmm... in Corona the glass, on the bottom, is too dark,. Maybe absorption low?
But this is not the problem)) The question is that, with CoronaSun or CoronaLight, it isn't possible to obtain a transparent shadows. =)
We need transparent shadows!!! For design and architectural projects.
Title: Re: Thin plastic chair - Ghost = How?
Post by: maru on 2013-05-15, 19:44:58: Quote from: Polymax on 2013-05-15, 18:46:00
Compare Corona vs Vray. Glass more realistic in the Corona, than in the Vray!

I think it's a matter of proper shaders an you can achieve similar results with different renderers. But it may be true it's easier and faster to set up a nice glass shader in corona.
Title: Re: Thin plastic chair - Ghost = How?
Post by: Polymax on 2013-05-15, 20:16:01: Quote
I think it's a matter of proper shaders an you can achieve similar results with different renderers.
try compare vray Glass shader vs Corona glass shader ;)!
Title: Re: Thin plastic chair - Ghost = How?
Post by: cecofuli on 2013-05-16, 09:11:46: Keymaster? What do you say about this thread? =) We need "Affect shadow" parameter like in VRay ;-)
Title: Re: Thin plastic chair - Ghost = How?
Post by: Polymax on 2013-05-16, 09:28:49: Unbiased caustic
PT+PT, MSI=0, PT sampl=32
Title: Re: Thin plastic chair - Ghost = How?
Post by: cecofuli on 2013-05-16, 09:31:31: Great, great test O_O ! But... 11 hours =)
Title: Re: Thin plastic chair - Ghost = How?
Post by: pionier on 2013-05-16, 10:59:38: Did you use Bidir/VCM or Progressive method for that render?
Title: Re: Thin plastic chair - Ghost = How?
Post by: Polymax on 2013-05-16, 11:05:58: Quote from: pionier on 2013-05-16, 10:59:38
Did you use Bidir/VCM or Progressive method for that render?
Progressive
Title: Re: Thin plastic chair - Ghost = How?
Post by: Ondra on 2013-05-16, 11:21:49: I'll get to it...
Title: Re: Thin plastic chair - Ghost = How?
Post by: cecofuli on 2013-05-16, 11:35:20: Great!!! =)
Title: Re: Thin plastic chair - Ghost = How?
Post by: maru on 2013-05-16, 12:53:52: Quote from: Polymax on 2013-05-15, 20:16:01
Quote
I think it's a matter of proper shaders an you can achieve similar results with different renderers.
try compare vray Glass shader vs Corona glass shader ;)!

Ok. Here are my tests. I can't see any significant difference. You can achieve same result with both renderers.
I mean, if a renderer supports: refraction, fresnel reflections, glossiness and fog or absorption - what other parameters would cause differences between goo glass shaders?

Models and setup are crappy but it's not important in this comparison.

Noticed a bad thing: low MSI makes reflections on backsides invisible (red circles).
Title: Re: Thin plastic chair - Ghost = How?
Post by: cecofuli on 2013-05-16, 12:58:45: Maru, look the image in attachment:
In the first test, the material is transparent, but the shadow no :(
In VRay, in this case, we use "affect shadows" in VRayMat.
The correct result is like in Bidir, but Bidir doesn't support absorption and, i think, is less robust and fast
Title: Re: Thin plastic chair - Ghost = How?
Post by: maru on 2013-05-16, 13:01:54: I know. I'm refering to this Polymax's post only:

Quote from: Polymax on 2013-05-15, 18:46:00
Compare Corona vs Vray. Glass more realistic in the Corona, than in the Vray!
Title: Re: Thin plastic chair - Ghost = How?
Post by: Polymax on 2013-05-16, 13:44:50: Quote from: maru on 2013-05-16, 13:01:54
I know. I'm refering to this Polymax's post only:

Quote from: Polymax on 2013-05-15, 18:46:00
Compare Corona vs Vray. Glass more realistic in the Corona, than in the Vray!
ok, and in this case?:
Can you set up a glass-shader in the Vray so it look like in the Corona?
Share scene: http://www.cgvision.ru/temp/Glass_shader.rar
Title: Re: Thin plastic chair - Ghost = How?
Post by: maru on 2013-05-16, 16:09:24: Here. Not sure what specific differences you meant but I think my vray render looks more similar to your corona render.
Title: Re: Thin plastic chair - Ghost = How?
Post by: Ondra on 2013-05-16, 16:24:05: This is because you dont have a sharp light source in the scene.
Title: Re: Thin plastic chair - Ghost = How?
Post by: cecofuli on 2013-05-16, 16:27:36: Yes! GI and light coming from Global illumination (HDRI?) "hide" the solid shadows.
In fact, the base of the bottle, in Corona, is dark.
Title: Re: Thin plastic chair - Ghost = How?
Post by: maru on 2013-05-16, 16:50:10: I'm lost. What are we talking about now?
Title: Re: Thin plastic chair - Ghost = How?
Post by: cecofuli on 2013-05-16, 16:57:59: Maru, the glass is ok. I'm talking, again, on the shadow, as Keymaster did.
You and Polymax talk about glass shader =)
In the VRay version, did you turn on the parameter "Reflect on the back side"?
Or change the "Max depth" from 5 to, I don't know, 10.
Title: Re: Thin plastic chair - Ghost = How?
Post by: maru on 2013-05-16, 17:36:26: You are right, we are having two discussions in one thread here. Sorry.

To Polymax: to achieve Corona's level of detain on glass in Vray I used these settgins:

-reflect on backside for glass shader
-cutoff = 0 for glass shader
-increased max depth in glass shader and in render settings

And generally removed all biases and fakes that could be removed. This makes Vray's glass renders look as good as Corona's but again - I agree it's much easier and faster to set up a realistic looking glass in Corona.
Title: Re: Thin plastic chair - Ghost = How?
Post by: cecofuli on 2013-05-16, 17:42:03: No problem ;-) Glass and shadows are a strong argument in my opinion and both need to be in the same thread
Title: Re: Thin plastic chair - Ghost = How?
Post by: Polymax on 2013-05-16, 18:12:38: Sorry gays :)
2 Maru:
I understood in what was my error when I create Glass in Vray. Thank you!
Title: Re: Thin plastic chair - Ghost = How?
Post by: cecofuli on 2013-05-18, 15:51:23: After some test, I understand that in Progessive, Corona glass (Onesided) permits for the GI (HDRI or CoronaSky) to pass through the glass. This is not true for direct lights rays, like VRaySun or VRayLights
Title: Re: Thin plastic chair - Ghost = How?
Post by: patoaltaco on 2013-05-18, 16:13:23: Quote
After some test, I understand that in Progessive, Corona glass (Onesided) permits for the GI (HDRI or CoronaSky) to pass through the glass. This is not true for direct lights rays, like VRaySun or VRayLights

I had that problem, anyway i used solid material with untilded cast shadow to get sun inside with absorption on the thikness of the grlass.

Cheers,
Title: Re: Thin plastic chair - Ghost = How?
Post by: cecofuli on 2013-05-18, 16:37:17: Good idea and work around! I'll try =)
Title: Re: Thin plastic chair - Ghost = How?
Post by: Alessandro on 2013-05-20, 11:57:32: Yes, good idea. I often use this trick in my renders, also with MR, and not just for the glass.
One more solution for Corona is to decrease the opacity for the glass, for ex. 0.5, so you can adjust the effect of the direct light.
Title: Re: Thin plastic chair - Ghost = How?
Post by: cecofuli on 2013-05-20, 12:01:29: In VRay you don't need this ;-) And, if it's possible, it's better to avoid this tricks, cause the rendering look "fake".
Title: Re: Thin plastic chair - Ghost = How?
Post by: emieuserr on 2013-05-20, 16:55:18: Hi folks, somethig easy:
1 plane,1 mirror, 1 tiny plane emitter & 1 t.knot (glass)
no sun, no environment...
what do think about this?
Title: Re: Thin plastic chair - Ghost = How?
Post by: cecofuli on 2013-05-20, 17:27:36: Nice, but Bidir doesn't support absorption, it's slow and he creates some splotches.
Title: Re: Thin plastic chair - Ghost = How?
Post by: Ondra on 2013-05-20, 19:42:57: I can easily add the absorption support. I would love to see the scene where it creates splotches (unless you use it in VCM mode, in that case just tweak the photon search radius parameter). The speed could be certainly improved too, but it will probably never be as fast as path tracing for the typical scenes you create. Although I had some surprising results where BDPT was actually faster than PT (which screwed us over big time in the last paper)
Title: Re: Thin plastic chair - Ghost = How?
Post by: Ondra on 2013-05-20, 19:44:42: Quote from: emieuserr on 2013-05-20, 16:55:18
Hi folks, somethig easy:
1 plane,1 mirror, 1 tiny plane emitter & 1 t.knot (glass)
no sun, no environment...
what do think about this?

The thing on the right is mirror, right? If so, you are definitely using VCM, right? (Iliyan, feel free to boast! ;))

edit: nevermind, I see it in the config ;)
Title: Re: Thin plastic chair - Ghost = How?
Post by: Polymax on 2013-05-20, 20:34:00: Quote
I can easily add the absorption support.
Keymaster, and you can add support for anything that supports progressive mode?
Title: Re: Thin plastic chair - Ghost = How?
Post by: emieuserr on 2013-05-21, 23:35:25: Ok Keymaster, the thing on the right is mirror & i'm using VCM.
Even in my slow pc & legacy version, i think the caustics are quite accurate...
bye.
Title: Re: Thin plastic chair - Ghost = How?
Post by: Ondra on 2013-05-21, 23:40:42: yes, because VCM is consistent and not memory-bound, doesnt matter what PC or version you have ;)
Title: Re: Thin plastic chair - Ghost = How?
Post by: Alessandro on 2013-05-28, 10:48:41: After reading this very interesting tread, I've prepared a set for some glasses, I've started corona render and I've forgot to stop it during the weekend...!!! So it's worked for 62 H
This is the result, I've reduced it half size, I like glass effect, what about it?
Title: Re: Thin plastic chair - Ghost = How?
Post by: maru on 2013-05-28, 11:56:54: Hah, new world record. Prepare to pay your bills. But the glass is nice.
Title: Re: Thin plastic chair - Ghost = How?
Post by: Ondra on 2013-05-29, 01:28:25: ok, I've added absorption support to Bidir
Title: Re: Thin plastic chair - Ghost = How?
Post by: Alessandro on 2013-05-29, 09:13:52: Quote from: maru on 2013-05-28, 11:56:54
Hah, new world record. Prepare to pay your bills. But the glass is nice.
Eheh, yes, I hope that the bill will not be a world record too!!! ;)

Btw, great news Ondra!
Title: Re: Thin plastic chair - Ghost = How?
Post by: cecofuli on 2013-06-10, 18:26:39: Hi Ondra,
finally I found time to test the new tools... I have to say that the best result is, obviously, Bidir, cause is physically correct.
Unfortunately "PT + Red Colored ABS + transparent white shadow" is visually wrong and ugly.
If you see, in my test n°9 or n°12, the shadows is a "punch in the stomach".
Ok, if I have PT + White transparent + white ABS, it will ok, but with colore ABS.. Mmm... I don't think it's a good solution.

So, my question is: it will be possible to have a PT + ABS + colored, transparent shadows? Like with Bidir, but without caustic. (n°13)

(http://www.francescolegrenzi.com/Temp/149_Test_Shadow.png)
Title: Re: Thin plastic chair - Ghost = How?
Post by: Alessandro on 2013-06-11, 09:35:48: Hi Cecofuli,
thanks for this great test, I haven't had the time to do the same, we've got a very busy period!!
I'll take some ideas from your test to share my one too as soon as I can.

Rgds,
Alessandro
Title: Re: Thin plastic chair - Ghost = How?
Post by: Tylox on 2013-06-15, 02:21:46: Hi Cecofuli,

did you test the PPM in the Bidir Mode ? it's quite efficient and i guess faster than BDPT.

Regards.
Title: Re: Thin plastic chair - Ghost = How?
Post by: Ondra on 2013-06-15, 02:41:11: Quote from: Tylox on 2013-06-15, 02:21:46
Hi Cecofuli,

did you test the PPM in the Bidir Mode ? it's quite efficient and i guess faster than BDPT.

Regards.
only for SDS paths. VCM is the optial combination of both.
Title: Re: Thin plastic chair - Ghost = How?
Post by: ecximer on 2013-06-19, 13:12:35: One does not want to produce topics, as a problem in general and the same.

So we have:
- 1 sphere with a radius of 10 cm and 0,5 cm thick
- material: dark pink or not saturated red glass (RGB 250 30 30) with Onesided or Hybrid refraction.
- in the sphere there is a light source (the emitter or a lamp).

Problem:
Being appeared through by a light source gains lilac color (RGB 255 0 244), that is appears very BLUE color! From where it undertakes?

(http://s6.hostingkartinok.com/uploads/images/2013/06/08b688d23c04ba189341073918dc5209.jpg) (http://hostingkartinok.com)

In principle the light source has to change color from white to dark pink, depending on level of brightness of the source.
Title: Re: Thin plastic chair - Ghost = How?
Post by: Ondra on 2013-06-19, 13:40:21: interesting ;) post the scene
Title: Re: Thin plastic chair - Ghost = How?
Post by: ecximer on 2013-06-19, 13:45:57: scene
Title: Re: Thin plastic chair - Ghost = How?
Post by: Ondra on 2013-06-19, 14:29:39: ok, the scene is fine, no bug there. There is slight color shift after the colored refraction, because Corona uses internally different color space than RGB, so you get something like 0.59 -0.01 0.01. Multiply it by 100 (because the light is very strong), you get 59, -1, 1. Clamp it, and you get 1, 0, 1.
Title: Re: Thin plastic chair - Ghost = How?
Post by: ecximer on 2013-06-19, 14:35:29: And what You Ondra suggest to do in such cases?
And it will be used very often - a color plafond of lamps.
How to set up?

vray: no problem.....

(http://s6.hostingkartinok.com/uploads/images/2013/06/a54944a9ec723324f44bd0874bd35c32.jpg) (http://hostingkartinok.com)
Title: Re: Thin plastic chair - Ghost = How?
Post by: Ondra on 2013-06-19, 14:55:05: best would be to use better colormapping/lower intensity of the light. If that is not possible, try tweaking the shade color, smallest changes could make significant differences
Title: Re: Thin plastic chair - Ghost = How?
Post by: ecximer on 2013-06-19, 14:57:34: It is called "crutches"
It isn't convenient for work - to guess in 0,000001 without knowing will help or not... It is a pity. :(
Title: Re: Thin plastic chair - Ghost = How?
Post by: vicnaum on 2013-06-19, 15:31:05: Why is it negative? The -0.01

The source glass colour is 255 30 30. There is green there. Why there is no green when it's lit up? (there's even negative green)
Title: Re: Thin plastic chair - Ghost = How?
Post by: Ondra on 2013-06-19, 15:43:44: Because Corona internally does not work in RGB. See here: http://forum.corona-renderer.com/index.php/topic,60.msg207.html#msg207
Title: Re: Thin plastic chair - Ghost = How?
Post by: ecximer on 2013-06-19, 16:35:16: I ask me to forgive, for my stupidity, but it isn't normal:
(http://s2.hostingkartinok.com/uploads/images/2013/06/c6c1dd511f333338229816cddf25fb72.jpg) (http://hostingkartinok.com)
Title: Re: Thin plastic chair - Ghost = How?
Post by: Ondra on 2013-06-19, 16:41:47: I dont exactly know what are the two setups, but I would suggest starting by turning diffuse color off (unless you actually want it there)
Title: Re: Thin plastic chair - Ghost = How?
Post by: ecximer on 2013-06-19, 16:46:28: At the left from below - diffusion color 0 0 0 - you see, as there is any imbalance?
Why I shall set up color at random?
In an example what to receive RED, I shall set color of refraction by the ORANGE. ?
Title: Re: Thin plastic chair - Ghost = How?
Post by: vicnaum on 2013-06-20, 13:18:01: The setup is simple:
1) A plane Emitting light (Multiplier: 1500) with a gradient (from RGB(1 0 1) to RGB(254 254 253)
2) A thin red film (made with a Plane), and a thin red box (made with a Box), with Material like this one: http://www.naumik.com/temp/scr4/20130620-552-65kb.jpg (Red color is: 172 2 1)
3) Several Refraction modes (onesided, twosided, hybrid), and different Exposure compensation.

Here is what is expected in reality:

And here's what Corona renders:

Plane, Onesided (solid):
+1EV: http://www.naumik.com/temp/scr4/20130620-1k4-164kb.jpg
+8EV: http://www.naumik.com/temp/scr4/20130620-haw-162kb.jpg

Plane, Twosided (thin):
+1EV: http://www.naumik.com/temp/scr4/20130620-7ul-320kb.jpg
+8EV: http://www.naumik.com/temp/scr4/20130620-t02-138kb.jpg

Plane, Hybrid (Solid w/o shadows):
+1EV: http://www.naumik.com/temp/scr4/20130620-h80-313kb.jpg
+8EV: http://www.naumik.com/temp/scr4/20130620-3as-151kb.jpg

---

Box, Onesided (solid):
+1EV: http://www.naumik.com/temp/scr4/20130620-41a-288kb.jpg
+ManyEV: http://www.naumik.com/temp/scr4/20130620-7u0-168kb.jpg

Box, Twosided (thin):
+1EV: http://www.naumik.com/temp/scr4/20130620-pml-307kb.jpg
+ManyEV: http://www.naumik.com/temp/scr4/20130620-5f3-195kb.jpg

Box, Hybrid (Solid w/o shadows):
+1EV: http://www.naumik.com/temp/scr4/20130620-w97-316kb.jpg
+ManyEV: http://www.naumik.com/temp/scr4/20130620-h91-180kb.jpg

As you can see - when using a simple one-sided Plane - everything works like in reality.

But if you use something solid, like a box - then Green is running deeply in negative numbers, and you NEVER get white, even if you make a 1000000 multiplier and +1000000 Exposure compensation, cause when you make it brighter - green channel becomes even darker (?!).

To me - this is absolutely WRONG, no matter what color space is used internally - XYZ, Lab, HSB, etc. It shouldn't act like that.
I think there is some tricky little error somewhere in volume refractions calculations...

--------

Also repeated the test using ordinary CoronaLights, and a simple Plane, Onesided (solid):

6500K: http://www.naumik.com/temp/scr4/20130620-264-115kb.jpg
5000K: http://www.naumik.com/temp/scr4/20130620-172-312kb.jpg
4500K: http://www.naumik.com/temp/scr4/20130620-x8q-131kb.jpg

As you can see - if the light temperature is 5000K or lower - it also cannot give white overexposure no matter what, even with a Plane.

Tricky...

Btw, in the video test above there are all kinds of lamps (in order of appearance):
1) Circular = 6800K
2) Piramydal = 4000K
3) Simple tubeal on a wire = 2600K
4) Spherical = 4000K
5) Flashlight = 6000K

But each and every one of them gave me Red/Purple/Yellow/White transition on over-exposure through Red film.

If I make the red film less red (by rising the G/B colors to 10-15, and lowering the red to 150) - then transitions work to white with Plane, but not with a Box.
To get white overexposure on Box - I need to make it something like (100, 39, 25), but this is no longer a Red...

------

Taking this a bit further:

Set the Refraction map like this:
(http://www.naumik.com/temp/scr4/20130620-hr0-17kb.jpg)

The gradient goes from 255 full-Red on the left, to 0 Red on the right.
Also G/B channels are added from 0/0 in the top to 255/255 G/B in the bottom.

And no matter of the Light color temp, we have a clear line, where green goes negative:
(http://www.naumik.com/temp/scr4/20130620-220-57kb.jpg)
(http://www.naumik.com/temp/scr4/20130620-5er-39kb.jpg)

Taking this to Photoshop, we can see where the "non-overexposure" border exactly goes:
(http://www.naumik.com/temp/scr4/20130620-5i4-46kb.jpg)

This test is made with a Box, Onesided (solid). The color temp of CoronaLight doesn't shift this border anyhow, so it doesn't matter.

So, if you make the Refraction color above the line - it will give you violet when overexposure. If below - then it will give you normal overexposure.
So the most red glass you can have passing the light normally is (255, 75, 75). Making G/B lower will lead to violet.

Here's the (255, 75, 75) Box:
(http://www.naumik.com/temp/scr4/20130620-7tk-163kb.jpg)

As you can see - there is still negative green on the sides (resulting in violet color). So the problem is not completely solved.

Also, two boxes one on top of another give the same violet, so color works only if the object is one:
(http://www.naumik.com/temp/scr4/20130620-sk0-156kb.jpg)
Title: Re: Thin plastic chair - Ghost = How?
Post by: Polymax on 2013-07-30, 20:02:38: Keymaster, what do you think about this research?