a question about point clouds

[rohandalvi]

hi everyone,

I was wondering if anyone could explain why Houdini uses point clouds to calculate effects such as SSS and translucency and also what are the benefits of this method over the normal way in which it is done in say mental ray or vray or any other renderer.

Also could some one give a step by step procedure as to how to create and use Point clouds for SSS.

I have seen a lot of topics on odforce talking about point clouds, but I couldnt find anything about creating one.

Any help and explanation would be most appreciated

with regards

Rohan Dalvi

[Marc]

Hey Rohan

Have you read through this thread : http://forums.odforce.net/index.php?showtopic=2262 ? There's a ton of information pertaining to SSS and point clouds in there.

M

[rohandalvi]

Hi marc,

I have gone through the thread, but I must say a lot of it went way over my head. There is a lot of hardcore math and shader programming that they talk about and I am not really a programmer. I can do some basic math required for animation and expressions but nowhere near to the stuff they talk about.

My problem/request essentially is that if someone could just give a step by step procedure on how to create a pointcloud in houdini.

I even went through the help, but they talk about pcwrite and pcexport and other stuff, but they havent mentioned a procedure on how to use it except for a code, now the question arises where do I put that code.

I had tried once to just scatter some points over a sphere and do a ROP geometry export and save it with an extention of .pc It worked but when I put it into the SSS material nothing happened I was just taking a wild guess and hoping it would work, well it didnt.

What I need to know are what attributes need to be added to the points. If they need normal, colors etc. All they ever mention is that you use the scatter SOP, but then what next??

What i am asking for is basically a tut for creating a point cloud for SSS.

with regards

Rohan Dalvi

Edited August 6, 2008 by rohandalvi

[Alanw]

In order to render a point cloud for use with SSS in Houdini 9.5 you have to use the SSS Scatter SOP. Then, look in the Material Palette and you'll find the SSS Skin shader which will accept your point cloud. It's that easy. (Thanks to Mario Marengo for those nodes)

In other scenario's you can use regular .bgeo files as point clouds. I think SESI had some app notes on point clouds available for download at one time, but I can't seem to find them now.

In general, point clouds are "Arbitrary 3D data" stored in a file. They allow you to render complicated effects that would normally require raytracing without it. Although sometimes raytracing is still used. Good candidates are volumic renders, occlusion, SSS, and color bleeding. There are trade-offs to using them, but that's a fairly large subject of it's own.

Edited August 6, 2008 by Alanw

[rohandalvi]

thanks a lot Alanw

I never knew there was a SSS scatter sop.

I think this should work now. I havent tried it yet , but that for showing the way.

Sidefx should mention this in the help, because it's not mentioned in the 9.1 help. Havent checked the 9.5 help as yet if it's been updated.

You know I just feel that if sidefx were to redo the help files like the max or maya help, Houdini would get lot more easier to learn. I mean i remember the help in 8 and it was terrible, At least they have improved it a lot now but it still has a long way to go.

Anyways, Really grateful for you help, thanks once again.

with regards

Rohan Dalvi

[abvfx]

i was in the same situation as you, about a year ago.

Look through that thread Marc says again.

for my lack of knowledge on the subject and my persistence to gain some info on the topic Mario was patient enough to write this post.

http://forums.odforce.net/index.php?showto...ost&p=27338

It goes through what is going on with PC.

hope this helps

-andy

[berija]

There are trade-offs to using them, but that's a fairly large subject of it's own.

Could you be more specific about trade-offs of using point clouds, please?

[Jason]

Could you be more specific about trade-offs of using point clouds, please?

Using point clouds for shading (like SSS) requires you (1) generate them before using them (2) generate the appropriate distribution (spacing/density) of them (3) initialize them with accurate data.

(1) is related to (2) and (3). If you come up with a good method for (2) and (3) then all you have to do is quite manually manage the generation of point clouds. Or try to set up a system to try to do some of it automatically. And if you're using displacements in your render, it's an even more complex setup where you have use Mantra itself to generate the point-cloud and you have to forego the convenience of SOPs.

(2) and (3) depend heavily on your shader and at what frequency you want to store and access data from the point cloud. In the case of point-cloud SSS, you need a fair density of samples but this depends on the scattering distance and the frequency and shape of the features of the object. This is even complicated to think about, let alone implement. Thank goodness SSS is has such low frequency variations mixed in with diffuse and other high-frequency detail that it's fairly forgiving.

(3) can be a challenge because you might have to compute a shading spot size - in the case of SSS they compute pt_area for that purpose. Normals and other data might need to be attached for your shader to use and sometimes this info can be tricky to generate depending on the tools available in SOPs or in Mantra (in the case of displaced surfaces and such).

After all this, you must worry about the fidelity of that point-cloud when the surface deforms.

This sounds like a down-sell, but thank goodness much of SSS rendering happens to be applied on smooth animated characters with negligible displacement and so this process is as easy as writing out the results of the SSS SOP, and accessing it - and the speed benefits almost always significant.

[berija]

Jason,

thank you for your explanation.