Raytrace Settings

[quizzy]

In the help file from MAX is the true story behind quadtree depth... READ IT!!!

okay okay... here it is:

quote:"
Quadtree
A quadtree is a data structure used to calculate ray-traced shadows.

The quadtree represents the scene from the point of view of the light. The root node of the quadtree lists all objects that are visible in that view. If too many objects are visible, the node generates four other nodes, each representing a quarter of the view, each with a list of objects in that portion. This process continues adaptively, until each node has only a small number of objects, or the quadtree's depth limit (which can be set for each light) is reached.

Each shadow-casting light ray needs to test intersection with the objects in only one of the leaf nodes of the quadtree. This helps speed up the ray-tracing process. In general, increasing the maximum quadtree depth can speed up ray-tracing at a cost of memory.

The maximum size of a quadtree is the square of two to the power of the maximum quadtree depth. At a depth of 7, the largest quadtree has 128 x 128 leaf nodes; at a depth of 10, the largest quadtree has a size of 1028 x 1028 leaf nodes, and so on. (On the other hand, because each successive node contains fewer objects, the size of a node's record decreases the deeper it is in the tree.)

Note: An omni light can generate up to six quadtrees, so omni lights that cast ray-traced shadows use more memory at render time than spotlights do.

[ June 20, 2002, 02:21 PM: Message edited by: quizzy ]

Nisus needs to crack his MAX manual every once in awhile.

I have been adjusting the quad tree depth for awhile and it seems to increase the speed of my frame times considerably.

[sandpiper]

Nisus: How much RAM do you have on your machine? I think the way it works is that the lower the value the longer time it takes to render but it uses less RAM, whereas the higher the value, the faster it renders and the more RAM it uses.

On my machine I have 1 gig of RAM and that is probably helping my situation out a lot. That may have a lot to do with it.

I tested out a scene with about thirty spheres on a plane with raytraced shadows and set the depth to 1 and it took 5 minutes and then set it to 10 and it took less than 30 seconds.

[nisus]

Hi all,

Mmm... very strange I have to admit.
For the record, I am using max3.1 under NT and have only 1Gig ram in my machine. Btw, I found the previous quote in the online reference of max3.1.

My testscene was a residential building of about 150.000 polygons. I didn't use any omni lights in the scene, but I did use a 122 shadow-mapped spotlights from a lightdome (fakeiosity). So this could be a fund if it appears out to be ram-problem.

I'll check the seetings again on a simpler testscene this weekend.

rgds

nisus

[quizzy]

---------------------------

In general:

Shadow maps use memory.

Raytrace use processor.

---------------------------

[nisus]

Hi all,

I got some more results on my test.

quadtree value:
6 --> long long long...
7 --> 6:16
8 --> 3:10
9 --> 1:53
10 --> too long (more than 30min)

memory usage: about 380Mb Out of 1Gb
this did not change much with the different settings, so ram is not an issue here.
Anyone has an idea for the last result that does not seems to be in line with the rest?

rgds

nisus

[quizzy]

well it depends on the scene, and what the light touches, quadtree=10 should not be 30 minutes..

[sandpiper]

Yeah, I agree with Quizzy. That's pretty freaky if it was getting faster and faster and then at 10 it crapped out. Maybe you should just use 9 instead of 10 eh? I'm wondering if this quadtree feature was brand new with 3.1 and if release 4.2(6) has fixed some bugs with it?? I started out with 3.1 so I wouldn't know if it was part of 2.5 or anything earlier on...

[quizzy]

well nisus,

maybe you can post your scene here and i'll be glad to test it out in MAX 4.26

if its not a secret company thing or anything...

[img]Graemlin/biggrin2.gif[/img]

[nisus]

Hi all,

Unfortunately I can't post the scene for some reasons (first, it's a commercial project, so these stay in-house and second, it's not finished yet)

Anyway, I'm using a quadtree depth of 9 now and I will definately try it in the newer models that will be finished next week.

rgds

nisus