GI troubleshooting

[mihai architect]

Hi people ! Yesterday I tried to render out something just to figure out different light setups for GI in Mantra and I found out that Distant and Environment Light does not work with PBR. Am I right or I'm doing something wrong?

I found this trying to match something that would be a Physical Sun & Sky, in few words "a realistic outdoor lighting setup". And for doing this I considered that a good way to do it was to use a distant light as acting the sun and a uniform background adding the Global illumination feel, and for that I used Mantra GI_light shop on a light template (this is the only way i know to simulate GI by now, beside HDRI). And so the directional does not work with PBR !!!

A second issue I want to ask: Is there a way to make the area light non square (eg. for placing them to fill a hole of a window or something) or maybe another way to put a light plane/geomtery in the windows ? I'm looking for the same GI look with soft shadows and light bounce. (in other softwares it is possible to make something an emiiter or something like "emitting geometry")

The third and the last question: How can I render something with just one light and to look realistic just like one light would do in real life (of course doing that with Physical Based Rendering) ? I mean something like a setup with GI and all that stuff per individual light. I attached some pictures to be more specific what I'm after. (both are rendered with just one spot light and the background is black not influencing the GI iat all, and tough they look very real like being lit with a just one light).

I have searched trough the documentation and I couldn't find the answer. It is not well documented at all.

Thank you !

Mihai

[stu]

A few things,

All of the lights (environment, etc.) do work with pbr, so, yes, you're doing something wrong. If I drop down an environment light, a grid (ground), a sphere, and do a pbr render, I get the expected result. Make sure that you turn the diffuse bounces up to something like 5 in the mantra ROP (under the pbr tab) or you won't get much inter-surface light energy distribution.

You shouldn't be using the GI_light shader in a pbr render - I couldn't quite tell if that's what you were getting at in your post, but I thought that I'd put it out there anyway.

With a single light, you should get something fairly realistic in a pbr render (with the diffuse bounces turned up), provided that your shaders are doing something meaningful. I'd recommend starting with one of the materials provided out of the box and working from there. If you're looking for something other that what might be considered typical or basic in the shading world, you might find yourself making adjustments or creating your own eventually.

As for your light shaping issue, and to the window example in particular, I'd place an area light outside of a geometric window frame and wall and let the pbr render take care of the rest knowing that you're going to get the appropriate result because you've created the appropriate set of circumstances. In my experience with pbr so far, the renderer behaves the best when everything meant to make a meaningful contribution to the scene actually affects the rendering circumstances in a real way.

If you're stuck, post a file.

[mihai architect]

Here is a test file. Just one distant and one environment and the Mantra ROP is set to simple PBR.

distant_env_blackrender.hipnc

[zasid]

Hey Stu,

Can you post some example hip file if you have any for that kind of setups in PBR?this PBR is still quite a mystery but you are right that it only works if things are placed in a meaningful way.

Mihai nice renders!!!

[mihai architect]

If there is someone guru like around here who knows the ways of working with the new pbr, enlighten us. I feel that it has a lot of potential, but...and I found that the documentation and examples are barely none, and the few tutorials and books around the web does not cover the rendering section. After all the rendering is all that matters at the end. It can ruin any superb scene.

[anamous]

Hi Mihai,

- Your environment light had a "physically correct" attenuation which makes it very, very dim. Set it to "half-distance", which is the default value.

- I'm not sure if distant lights work in PBR (I might be wrong about this), I'd rather use an environment map for that. Remember that you can render an environment map yourself, which gives you great flexibility. If you absolutely need a light, use an area light which you can set to grid, disc, sphere or tube shapes.

- No need to turn on shadows in the light shaders, as lights in PBR always have shadows.

- As Stu said, the shaders you use on your objects are extremely important.

- Also, you can turn off the environment light, leaving only one light and its bounces. This will take away from the overall softness of the image, but can still be useable in some cases.

- You will rarely need more than 3 or 4 diffuse bounces, unless you're dealing with some extreme indoor situation.

(both lights on, colored for viz sake)

(one light only)

Cheers,

Abdelkareem

distant_env_nonblack.hipnc

[mihai architect]

So I see the distant light is transformed into a big area disk whitch in this case does not look like a sun with somewhat sharp shadows. I'm still looking for a way to fake a distant paralel light such as the sun in PBR. I agree that in any of these cases the look is still GI-ish (but in cloudy day way) because of the nature of the lights and the PBR engine. As for the environment light I think I can live with the fact that physical corect atenuation does not work, since the halfway attenuation does the job. Looks like GI_light shop that I tried to use in the first place. The effects are similar. I agree with the posibility of using a map instead of a uniform lighting background, and I don't think it could replace the sharpness of the sun anyway. It would act more like a filling ambient & GI, by taking the color and irradiance information from it.

So about the sun faking issue in pbr does anyone know how to do it?

Thank you !

I attached a image that I rendered today while testing...it turned out quite VRay like, Very clean. I could afford the settings because of my 24ghz / 8gb workstation. Just 2 lights.

[anamous]

If you're looking for sharp shadows simply decrease the size of the area light. BTW, I also think that an environment map consisting of a mostly blue sky and a very, very bright little dot could do the trick.

(just made the area light smaller, no environment)

(added environment light to soften the shading and brighten the shadows)

(put a room with windows around the objects)

(added two ears and fur)

Cheers,

Abdelkareem

Edited July 18, 2008 by anamous

[mihai architect]

wow...it's getting better and better...I didn't tought of reducing the area size, and I think it's even better that way, since the sun shadows aren't entirely sharp, because the sun is in fact an big area light. It really does the trick. And when you added the environment the hole mood of the image really looks like sun burn around the lit areas. I guess now it's just a matter of getting the right settings and textures. Thank you!

I have a background of working with Unbiased Engines whitch are physical and that's why I was concearned about the PBR, but I wasn't used to think of lights that way. It's bit different than a unbiased engine, when I knew that everything I throw in the scene it was gonna look realistic anyway. It was some sort of the easiest way to render something realistic. But in Houdini, being more profesional with control about everything it needs a bit more attention and tweaking.

[edward]

(put a room with windows around the objects)

Damn it, why can't I make a teapot, cube, and sphere look interesting? That's all I do with Houdini all day anyways!

[jason_slab]

(added two ears and fur)

LOL thats funny!

i find it a little frustrating i must say at the lack of workflow information on mantra/PBR.

like:

scene scale, do i work in a physically correct scale and what is the unit size in Houdini?

is the "Physically correct" light falloff the correct falloff to use, and what are the "intensity" values of the light represent? i'd really like to use kelvin or watts

some info on tone mapping and gamma correction would be nice.

sorry for bit of a rant!!

jason

[mihai architect]

As far as I know from my experience with physical based engines the scale indeed matters and a lot. That's why we often see renders whitch seems very small (maquette size) or very large (a glass of water the size of a building). This has to do with the size of the geometry and for sure the PBR in Houdini has to work in the same way. The units can be changed in a personal manner in the Preferences (by default the units are in the International System whitch is Newtons, Meteres, if you want to change a units by your preferences have that in mind). If you change for example the meters to centimeteres, you will find that the DOPs and the gravity will act very different and thus will need different tweaks.

The Physical Corect fallofs I think it's the right way when seeking for realism, due to the more natural distribution of light. As you could see in the rest of the thred, Physical Correct Falloff does not work with some lights in PBR. The kelvin (temeprature) and the watts are not available as far as I know, altough this would be usefull. It would be very cool for any setup to have, for example a distant light with 6500k (the temperature of the sun in a clear day sky), and to know that it will look like a real sun in the render. Maybe in H10 the PBR will further evolve and support more things. Tonemapping would be nice to, but I don't know a way of doing this. Maybe there is a way of tweaking this in the compositor, but I don't know.

Cheers,

Mihai

[Mario Marengo]

The kelvin (temeprature) and the watts are not available as far as I know, altough this would be usefull. It would be very cool for any setup to have, for example a distant light with 6500k (the temperature of the sun in a clear day sky), and to know that it will look like a real sun in the render. Maybe in H10 the PBR will further evolve and support more things. Tonemapping would be nice to, but I don't know a way of doing this. Maybe there is a way of tweaking this in the compositor, but I don't know.

As I understand it, that 6500k (CIE-D65) that you mention could only be useful when interpreted as a color temperature -- i.e: the spectral distribution of black body radiation at that temperature. And all that verbiage reduces to one thing: a hue -- which is unrelated to intensity. The color temperature of "the real sun" varies greately depending on the conditions of the atmosphere and time of day (6500k is meant to represent an overcast noon sky).

I'm a little puzzled about the failure of the distant light though. If it was a matter of shaders which overwrite L failing, then I would expect the environment area light to fail as well (when used with a texture map), but you say it doesn't...hmm... strange. You could always duplicate the sun with a very powerful spherical area light source placed very far away (with a relative coverage of about 2 degrees iirc, and almost parallell rays) -- but I agree that wouldn't be very practical.

And I agree about the need for tonemapping tools.

[mihai architect]

Even and small rectangle area light or even a spherical area very far away are expenssive techniques...it acts just like a point light, most of the calculation will be in the areas that the camera donesn't see. It's just like using a point light insted of a spot. Just thinking of the wasted photon emission in the case of a sphere area or point...that's why I tought of the using the distant light in the first place, I tought it was better optimized by channeling the light in one direction, but apparently it does not work properly. Yeah something must be done regarding the distant light.

Cheers,

Mihai

[jason_slab]

As I understand it, that 6500k (CIE-D65) that you mention could only be useful when interpreted as a color temperature -- i.e: the spectral distribution of black body radiation at that temperature. And all that verbiage reduces to one thing: a hue -- which is unrelated to intensity. The color temperature of "the real sun" varies greately depending on the conditions of the atmosphere and time of day (6500k is meant to represent an overcast noon sky).

so if I'm using a area light (physically correct falloff) with a intensity of one hundred say, is it just a arbitrary value? or is it related somehow to Lumious power or Radiant power?

OT'ish: I've been playing with IES lights in Vray lately, it would be great to be able to use IES profiles in Houdini:)

jason

[Jason]

so if I'm using a area light (physically correct falloff) with a intensity of one hundred say, is it just a arbitrary value? or is it related somehow to Lumious power or Radiant power?

OT'ish: I've been playing with IES lights in Vray lately, it would be great to be able to use IES profiles in Houdini:)

jason

There'd be no falloff from the sun detectable at earth differences in distances - so I'm sure you can drop that bit, no? Just pick an intensity that looks right to you... the value itself has no special real-world meaning, especially that the camera doesn't simulate exposure and such.

It would seem like couple of day worth of an intern's time to implement some kind of IES support in Houdini, surely? It would probably be quite useful for architects and enthusiasts, do you think?

[jason_slab]

thx jason

i was talking however more about normal (fluorescent, Tungsten, etc) light falloff/intensity rather than the sun, guess this should be in it's own thread:)

i'm sure it would be easy for SESI to implement IES lights in Houdini, what about using IES profiles to match lights that are used on set? or maybe it's just still easier to shoot an HDR...

jason