Very strange and annoying problem

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Old 10 October 2004   #1
Very strange and annoying problem

I've recently added hundreds of 2D trees to my scene and the render time growed without justification (4 times slower).

The trees are the typical composed of 2 double sided polygons. One parented to the other one. One is visible to camera and the other one not. Both cast shadows. The parent are targeted to the camera. There are hundreds of this object all around my scene. I can't understand why so simple geometry can multiply by 4 my render times.

The strange fact about it, is that occurs even when the trees are out of view. You can't see the trees but the render is slow!!!

Anyone knows how is possible that no more than 300 polys could produce a so significant impact on render times??? .

I'm sure it's much faster using HD_Instances or sprites.

Any ideas? Please!! It's no logical !!

By the way, i'm using LW8 on Windows XP, 1Gb.
 
Old 10 October 2004   #2
If you are using transparency on the polys it will dramatically increase render times. I would use clip maps instead.
 
Old 10 October 2004   #3
seconded on the clip maps. That render hit is perfectly normal.
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Old 10 October 2004   #4
I use clip maps.

The strange thing is that the render hit occurs even when there's no 2D Trees in the view.
 
Old 10 October 2004   #5
Clip maps are the way to go.

But if you DO use transparency (to get colors or controlled edges), make sure to turn ON the "Trace Transparency" flag. LW's normal transparency routine is extremely slow when you have overlapping layers, like a tree in front of a tree in front of a tree. Forcing LW to trace the transparency can boost speed in those bad cases 10X.

That won't affect clip maps though.
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Old 10 October 2004   #6
Thanks for your efforts. But the problem has nothing to do with clip maps or transparency.

The Trees don't even show in the render, but render time is 4X
 
Old 10 October 2004   #7
Apparently LW calculates for polygons not within the cameras view.

For example, create two polygons, one subdivided into a million polys and the other just one poly and put them in separate layers. (I actually subpatched the single poly and froze it a few times to get 810,000 poly object.) Then in Layout organize them so the single poly is completely in front of the subdivided one and make sure no render options are selected (like trace transparency or anything else.

Finally, go into Object Properties for the subdivided poly and make it unseen by camera.
Hit F9.
My computer rendered the image of a single gray polygon in 0.7 seconds.

When I unchecked the subdivided polygon's "Unseen by Camera" property, and eventhough it's still hidden from the camera by the single polygon, it took 5.6 seconds to render the exact same image.

Why? Who knows...
 
Old 10 October 2004   #8
My scene has various differences:

- The scene had more than 1000000 polys even before adding the trees.
- The trees are under 300 polys (all of them).
- The trees are out of view, not behind any object.
- I don't know if extra raytrace optimization (In render panel. it takes time before rendering when hi polys exists) occurs only when the object is seen by camera.
 
Old 10 October 2004   #9
Originally Posted by kangonto: My scene has various differences:

- The scene had more than 1000000 polys even before adding the trees.
- The trees are under 300 polys (all of them).
- The trees are out of view, not behind any object.
- I don't know if extra raytrace optimization (In render panel. it takes time before rendering when hi polys exists) occurs only when the object is seen by camera.

Ok I take my guess...

you have 2 "no-nos"

1. "no" : double sided polygons they render slower than single sided polys
2. "no" : you have targeted many objects to the camera. Lw will calculate this for each frame.
that gives you a severe rendertime hit!

good luck and happy lightwavin'
 
Old 10 October 2004   #10
- Yes, double polys probably render slower, BUT render times are unbearable even when don't appear in view.

- Yes, i thought of that. The first thing i did trying to fix the problem was untargeting all the trees. Result: Nothing.

My scene: Until now it consists of 1,528,738 'typical' polys plus more than 10,000,000 polys via HD_instance. Ambient occlusion is applied to some surfaces (Thx to skidbladnir). My render times at this point are around 15 min. Now i added some hundreds of easy rendering polys and render times are suddenly unbearable (> 1h.) . I mean that those arguments of transparency versus clips and things of the like seems irrelevant. I think that a nasty bug is playing a important role here.

I'll try to isolate the trees and create an scene to show you the problem.

CU Soon.
 
Old 10 October 2004   #11
I surrender

I isolated the trees and the problem has dissapeared. But the scene is too complex to keep making experiments to detect the fatal combination.

I'll use HD_instances instead.

THX
 
Old 10 October 2004   #12
That's wierd - I was going to say raytracing occurs outside the camera view if there are reflections, etc involved... Also, raytraced shadows will cause render times to increase, specially if you are casting shadows onto a ground plane, or object (even when the shadow casting objects are outside the camera) - That's why I often purge any unseen geometry from scenes before doing any major rendering. The time saving is usually quite impressive.

I'm gonna take a guess at another possible reason - Do you still have one of the scenes that caused a lot of render time hits saved on disk? If so, load it and press 'o' for options panel, then press 'f' and see if it says 'Full Scene Parameter Evaluation Disabled'.
If it does, that was your problem - If it instead says its 'enabled', then just press 'f' again to disable it. FSPE can be a killer on LW performance...

This is just a shot-in-the-dark... Since you said the problem went away after isolating the trees.

Last edited by kevman3d : 10 October 2004 at 09:27 AM.
 
Old 10 October 2004   #13
Shot missed

I'm using Hd_instances of 2D Trees instead. Problem solved
 
Old 10 October 2004   #14
pp-tiiinnnggggg! (Sound of idea ricocheting off LightWave interface)
 
Old 01 January 2006   #15
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