Rectangular and Omni lights

First topic message reminder :

an excerpt from damien alomar (one of the developer of vrayfor sketchup). once you read it you will now be able to choose in unsing rectabgular and omni lighgts.


Everything when talking about speed almost always goes back to raytracing, so I'll explain both in those terms. Omni lights are the easiest to work with raytracing wise, and will typically result in the minimum number of rays that need to be traced. Everything gets traced from a single point, and there's no need to see if a given point should try and trace rays from a light (not so with a spot light, which is just an omni light with an angle check on it...outside that angle and don't trace it..).

Rectangular lights on the other hand have a physical area that is associated with its generation of light. Therefore, "theoretically" the entire area of the light must be sampled in order to by completely accurate. Of course this is impossible bc points are infinitesimally small, and there are an infinite number of them along the surface of any rectangular light (this is one of the "technicalities" that prevent any renderer from being truly "physically correct"...anyway). So what it comes down to is that we've got to sample a certain number of points to get an idea of the contribution of a rectangular light. Typically, these samples are generated through the concept of subdivisions, so however many subdivisions you have on your light just square that an you'll have the number of rays traced for that particular rec light. By default we have 8, so if you haven't changed a thing on your rec light settings, then you'll trace 64 rays every time you sample a rec light. Thats opposed to 1 ray for sampling a rec light and I think you'll know the answer.

Now, I went through all that explanation more to for proper use of rec lights as opposed to the rec light/omni light comparision. Omni lights are fast, but they tend to make your renders look fairly flat and sharp, and don't do much for your reflections (cause they won't show up). So I still recommend using rec lights in most situations. Anyway, back to the real story. Those 64 samples are regardless of the size of your light, so thats why the technique of using one large light above a ceiling or behind a wall tend to be faster than a light at every opening/light. However, there are some significant disadvantages with that, mainly that your actually taking some samples of that light that will never be used (the ones that are right behind geometry...not over openings), which is of course fairly inefficient. Remember you're going to take all 64 samples every time you sample that light, regardless of what parts of the light are truly visible. In the end, the "one large light" technique tends to require more sampling anyway, so say you bump it up to 16 subdivisions, which means 256 samples, more than half of those are probably thrown away.

So enter the "multiple lights where I need them" technique. Depending on the number of lights that are being used, this can be almost worse than the "one large light" technique. Say you're making a 4 X 6 grid of rectangle lights...that's going to be 24 lights. Each one of those with the same number of subdivisions will be 1536 (24*64) samples, which would be so much bigger than the 24 samples if they were omni lights, and 256 samples of a large highly sampled rec light. However, that's at the default 8 subdivisions, which may be overkill in this situation. If we manage those subdivision, we can bring our total number of samples down and get just as good quality. So lets say we take 4 subdivisions instead of 8, leading to 16 samples per light as opposed to 64. Looking at all the lights, this translates to 384 samples as opposed to 1536 with 8 subdivisions (with all of those 384 samples being used efficiently). Now this is starting to look much more manageable. The more rec lights you add, the more you have to start thinking about how their sampled. Sometimes, you just have to have them so you need to manage them, other times, the quality of the rendered image is night and day from omni lights to rec lights.

Lastly (and bringing this back to your original question), rec lights have an option called Store With Irradiance Map. Basically, what this does, rather than sample all of the lights when calculating a given pixel (which can be many individual samples depending on you're AA settings), it will sample your rec lights @ each IR sample as opposed to pixel sample. Generally (and I'm just guessing here) the number of irradiance map samples is about 25% of the number of pixel's you're rendering...not to mention that most pixels will require more than one sample. Needless to say, this can significantly decrease the amount of time that is require to sample rec lights. Just keep in mind that the quality of your rec light solution is tied into your IR quality (which isn't that much of an issue really).

So, take charge my friend, and don't resign yourself to the complacency of omni lights, simply for "speed's" sake. Speed is in your hands, regardless of your choice of light.

Nice sir, very informative. I usually use 30 subdivs on every rec lights i use. Kaya siguro mabagal mga rendering ko on interiors.

Thank you sir for sharing. Mat follow up question lang ako regarding pinlights...pano naman p ba mag generate ng caustics para sa pinlights? Tsaka, ano po ba mas ok gamitin...omni or rectangular?

Sir Nomer, am I correct subdivision pertains to noisiness of the shadow?... and so you mean we can use (subdivision=1) and (store with IR) to make render very much more faster ? since there is only very little difference between those images. thanks!!

Nice Sir Nomer, Big help po ito... Buti n lang nandyan kyo para s mga SU users to upgrade our knowledge. Appreciate your effort and help indeed. keep up the good work. Your the man.