in houdini confuses me. I came from maya and ok i have a light, shot photons out of it and make the settings right in the render area an voila.

i houidni i have the irradiance thing, the photons area and pbr. would be great if someone can explain this diffrent solutions?

also i have heard that i cannot use area lights for gi/pbr?

Whoever you are hearing things from needs to clean their ears.

Area lights are actually really fast in PBR - much faster to use area lights than any other light when in PBR. If you use PBR you dont need global illumination or ambient occlusion passes because PBR calculates all that and puts it into one image for you. You dont even need the photon stuff, just increase your diffuse limit to somewhere btwn 3 and 5, to get a nicely lit look. In Houdini, photons, afaik, are mostly just for caustics.

If you decide to use micropolygon rendering and want to have global illumination and ambient occlusion, you need to use a light template with a vex global illumination shader. Then you need to turn on irradiance cacheing in the mantra rop - this will speed up the irradiance renders as well as be more accurate. At this point, you'll need to render your layers and comp them.

The process really depends on what renderer you are going to use. Experiment with both processes and then decide which result you like better. If you have lots of RAM, go for PBR - does it all for you and renders everything beautifully!

thank you for explaination. this was for the most part very helpfull. so, the global illumination rendering in houdini is just with a global light so to say?

i really stuck in the maya world right now. there you using lights in the scene to generate the gi effect. and what abotu final gather? is there an equivalent in houdini? i ask just to understand it better in which houdini will work.

thanks

I'm not a big maya person so not real sure what Final Gather is but from what I understand of it, its just another way of saying Global Illumination, at least in the sense of Houdini, but I'm not real sure. I think PBR is Houdini's version of Final Gather since it does all the color bleeding onto other objects.

Yeah, in Houdini you shoot irradiance from the shader instead of photons from the light. So 256 is the default but if its too slow you can lower it down for testing. So yes, it's like a global light.

moonlightkiss
I'm not a big maya person so not real sure what Final Gather is but from what I understand of it, its just another way of saying Global Illumination.

While I haven't worked on Mantra, so Andrew and Mark will know more about this than I do, I know that in many other packages “Final Gather” means “shoot secondary GI rays from the primary camera ray hit”. Meaning that without final gather, photons are only gathered around the primary ray hit location. With it, a number of secondary rays are sent out into the scene from that location, photos gathered around the hit point of each, and then averaged to get a sample of the light flux at the primary ray intersection point.

Essentially, to me, with Final Gather off GI isn't really proper GI, but rather a quick approximation. With Final Gather it becomes a proper GI algorithm, with colour bleeding and all.

I think the help files say that using photon maps (generated from View-Dependant Photon Maps render-mode in mantra) for the secondary GI cache is sort of the equivalent to final gather.

i really stuck in the maya world right now. there you using lights in the scene to generate the gi effect. and what abotu final gather? is there an equivalent in houdini? i ask just to understand it better in which houdini will work.

From what I know, in your light template GI shader, you just need to choose “Full irradiance” instead of “ambient occlusion”, than you will get color bleeding just like in final gather…

Hi everyone, i know it's kind of long for a first post but…

so following is an explanation i provided once as an answer on another forum
that hopefully will clarify things about GI and FG

Hi everyone

as far as everything in CG goes, there is many methods to tackle every task,same goes for indirect lighting.
here you can find GI, FG, PT, radiosity … etc
for this time i'll try to explain FG and GI as simple as possible (trying not to be app specific)

so let us begin with GI :
GI short for global illumination is an indirect lighting technique that is based on photons, what does that mean.
A photon in physics is the elementary particle responsible for ‘light energy’(wikipedia),
you can think of each photon as a very small particle holding a certain amount of energy and when put together with maaaany other photons they form light.
now what does that have to do with GI, well GI isn't more than a bunch of those photons shot into your scene, by a photon emitting light, transfering their
energy to the objects in your scene, depending on the depth settings of your photons, a photon may do multiple bounces and with each bounce it looses energy until it dies off or it reaches the max depth. after that photon shooting process something called a photon map is usually created, this map holds all the necessary infos concerning the photons just shot, and will help in further calculations. so now comes the raytracing, rays are shot from the camera towards your scene (i'll not elaborate the raytracing but i'll talk only about its relation to the GI) so whenever an object on which there are photons (from your shooting process) a pass that will average the energies takes place, to understand that, imagine your photons as small spheres distributed on a plane for example, those spheres can be big enough and near enough to touch each other, as well as they can be small and far apart and not be able to touch each other in both cases and depending especailly on your radius settings the energies and colors of neghbouring photons are averaged to give the GI energy and color corresponding to that hit point (N.B. of course this is a part of the final result which contains also the direct lighting, the FG …).
so as you can see to obtain a very good result with GI the averaging pass should have enough data for each point, which implies a veeeery big number of photons with smaller radiuses, then a small averaging radius, and eventually higher depth.


now to the FG :
FG short for final gather is an indirect lighting technique that is based on raytracing algorithm, the general idea is that eye rays are sent into the scene,
the number of course depends on the sampling and the FG density, so when a ray of those eye rays hits an object in your scene, gathering rays are then sent out from that point (the number is dertermined by the final gather rays parameter) into a hemisphere, now in their turn those gathering rays will intersect with geometry and will gather information from their hitting points such as color and brightness; All these information about the lighting of the scene is finally stored in a 3D Gathering map.
for the determination of the local radiation intensity neighbouring points are consulted (interpolation process), The MAX radius value determines the maximum distance apart that FG samples can be from each other. The MIN radius value determines the closest they'll be taken from each other.
here also depending on the depth settings of your FG, an FG ray may do multiple bounces. and it dies whenever it reaches a certain energy threashold depending on your falloff settigns or when it reaches the max trace depth.
then your FG results contribute to the overall render.
to obtain good results with FG of course density has to be high enough, interpolation should be the lowest possible, max and min radius should be small, multiple bounces have to be used … etc which leads to very high render times.


so now comes the best part how to use those 2 techniques to complement each other, in a way to profit from the advantages of each, while keeping low on render times. the best way to do so is to use the GI to get a global light distribution in the scene without fine details (means lesser photons with bigger radiuses), and to use the FG to get the details in fine parts of your scene this way reducing the number of bounces for example.
also it's to note that FG has problems in tight indirect light sources, such for example a room in which the light comes indirectly from a window, if you use FG and only FG inside that room you will get a very dark scene unless you increase the number of bounces really high. so to solve that prob you shoot photons into the window, which gives an overall lighting for the scene without much details ( of coures with low big radius photons number) then you use the FG to catch the details. (in newer versions of mental ray you may use also what's called portal light)

that was long ..

hope that was clear enough for you to understand the difference between those 2 techniques, and to be able to use them as efficiently as possible in your projects.

Regards

Georges

Hope that helps

Regards

Geo