View Full Version : Vulumetric Light Rendering Bug?

10-22-2003, 02:30 AM
I have found what I believe is a bug or at least a very curious issue in Lightwave's rendering pipeline.

I am using multiple lights in a scene to render a number of volumetric effects (See image). To start I used a single light to get the effect I wanted, then I duplicated them and placed each light where I wanted it. These light are not supposed to affect any surface whatsoever so I turned off Affect Diffuse and Specular items for all of these lights (See Fig. 1). To my surprise my render time for my objects went through the roof (22 Seconds Vs 80+ seconds) when compared to having only one vulumetric light present! :eek:

To try to get around this I then set all of the volumetric lights to exclude all other items in the scenes (See Fig. 2), so in theory they should have NO effect on ANY polygons, I got the same result! :mad:

I expected when I tell a light not to affect Diffuse and Specular or exclude other items it would not even be considered in rendering any polygons. It seems to me that the renderer is obviously taking these new lights into account when rendering these polygons and wasting CPU cycles in the render pipeline somewhere, even though they don't effect the final appearance of the polygons. Is this just me being dumb here?

Again I tried another test, I proceeded to turn the lights off in the scene editor (See Fig. 3) so they aren't actually there right? I found to my horror that turning them off in the scene editor did not actually remove them from the renderer, at least for volumetric lighting calculations, as can be seen by the resulting render (See Fig. 4). :confused:

Anyone have any thoughts?

10-22-2003, 05:06 AM
I'm confused, are you saying you expected to be able to get a lot more volumetric effects with no increase in rendering time?
Creating volumetric effects - 'faking' dealing with an infinite number of points in 3D space - is always vastly more computationally expensive than simply calculating the a light's effects on whatever's at the the midpoint of each pixel.
You have dramaticaly incresed the amount if the frame in which volumetric lights are being calculated, so of course the render time goes up. The effect on the render time of asking the lights to light surfaces or cast shadows is tiny in comparison.

Tip 1: using shadow mapping with lowish sizes can sometimes speed up volumetric calculations a lot.

Tip 2: most people use separate lights for volumetrics and actual lighting. It is NORMAL to have all the 'lighting' attributes for a light switched off while just using it for volumetrics, it is also normal to switch off the volumetrics for an individual light if you don't want to see them. Casting light and computing volumetrics are entirely separate calculations (although they can share shadow maps).

10-22-2003, 10:48 AM
Hi Adrian,

I do expect the volumetric rendering pass at the end of the render to take more time, but as I watch the rendering in progress, the rendering of the object polygons is MUCH slower with these volumetric lights activated, I'm talking an order of magnatude here. What I'm saying is that since these lights are not supposed to have any effect on these polygons, why is lightwave taking so much longer to render them when I add these lights which are supposedly only supposed to render volumetrics?

Secondly why does lightwave still render the volumetrics for those lights when those lights are supposedly "turned off" as you can see in Figure 3... in my original post...

So I'm still a bit confused about the two issues..


10-22-2003, 12:49 PM
Well, activating volumetrics will cause LW to emit more rays whilst rendering. It assumes it needs to trace some transparency. This will change the way the polys are sorted by the renderer and is independent of all other settings. The other problem remains a mystery however. I never ran into it, 'cos I always use spreadsheet or the item panels to toggle things on/ off. Mabe it's a (known) bug?


10-22-2003, 12:54 PM
If you have raytraced reflection on. The objects might take longer to render if they are reflective. This could be the reason that the objects take so long to render. The object may be trying to reflect the volumetric lights. Just a thought.

10-22-2003, 01:02 PM
That would be a good oen to know...
However, volumetric lights are known to have several bugs... shadow leaks, weird behavior... But turning them off should turn off the volumetric effect at least!
Well, that really must be a bug... I just teste dit in my lightwave, and they wouldnt turn off either! That really sucks. Well I did see that turning off volumetrics did not erase the settings of the volumetrics, so it could be sued as a substitute... still annoying tho...

10-23-2003, 11:31 PM
As far as I can tell, the checkmark in the scene editor is only a shortcut and doesn't turn off everything. Objects will still be seen by rays for example.

It would be nice to have a toggle for effects though, if you turn off volumetric lights, dof, motion blur, some others, the settings don't save with the scene - that's bitten me a few times already

Oh yea, there is a toggle for Volumetric Lights under 'Lights/Enable' -

10-24-2003, 04:06 AM
Originally posted by toby

Oh yea, there is a toggle for Volumetric Lights under 'Lights/Enable' - yep, in the global illumination panel

10-24-2003, 04:26 AM
Volumetric Lights are unfortunately reflected also if you disable the object using the exclusion list! :(

There's no way, as far as I know, to bypass this behaviour!


10-24-2003, 04:46 AM
So is the consensus that the original drastic increase in render time was probably due to the raytracing of the reflections of the volumetric beams?

It rings a bell with my past experiences, Calamari, if you try your original render with 'raytrace reflections' turned off, what is the render time like?

10-24-2003, 04:57 AM
Originally posted by [email protected]
So is the consensus that the original drastic increase in render time was probably due to the raytracing of the reflections of the volumetric beams?Yep.. I Think!

This bad behaviour is stopping me in finishing a scene.
I need RayTrace Reflection but I don't want Volumuteric Lights reflected... so there's no way to go on... :(

Maybe... starting a Thread in "Feature Requests"? ;)


10-24-2003, 07:24 AM
Started a new Thread in "LW - Feature Requests".

Take a look:
Volumetric Lights bug!!!

A simple example:

10-24-2003, 09:47 AM
Thanks for all your help guys...

I will be exploring the Raytrace situation and seeing if that is the issue, thanks for the ideas guys! I'll post my findings here...

One other thing I just found out, when you "Turn off" a light in the scene editor the lens flares for that light are still renderded...

LOL :p