Rectangular and Omni lights

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.

Math calculation on number of samples
8^2 = 8*8 = 64
16^2 = 16*16 = 256
4 * 6 = 24
24 * 64 = 1536
4^2 = 4*4 = 16
24 * 16 = 384

test renders

I set up an isolated test; a box with an opening where the sun is coming from the back so it's quite dark inside the box by default.

Then I inserted a rectangular light covering 90% of the ceiling where I rendered the same scene with various subdivs.

2 subdivs: 26.4s
20 subdivs: 27.3s
200 subdivs: 1m21.2s
2000 subdivs: (still rendering after watching one Mythbusters episode) {1h33m4.8s - about two Mythbusters episodes}

What I also notice with the various test renders is that I can't really see any difference in them. Though this is with a completely empty room.
But I see that I can reduce the subdivs in the scene I mentioned in the OP drasticly. Knowing that the number of ray is the square, I see how much impact even 1 subdiv makes. Now I can place them more smartly.

thomthom's experiment

I'm posting some of the test renders I did while experimenting with LC. I prefer images over text, guessing more people do as well.
Rendered with physical camera, light source is sky with one rectangular light inside the box.
First of is a series of four renders with different subdivision. You can see that there's hardly any difference in the resulting image except render time. (first image is missing sky as I forgot to remove the alpha in VFB)
IR (low quality) + LC




Next is a render with same settings as the third image, except this time the rectangular light is stored with IR. Notice the render time difference.

conclusion... use lower subdivision when using rectangular lights

sir, MAraming salamat dito... marahil un nga cgro problema ko minsan kung bakit antagal magrender.. eniwei.. hmm cgro sir requet nlang eh example nman ng omni light

kc diba we use omni for pinlights.. is it posible to use rec lights for our pinlights?

pwede mas maganda nga eh. kasi yung omni flat yung light nya. pero madaling controlin ang shadow.

Sir, nomeradona

Thanks for sharing ulit wala na akong masabi SU Guru ka tlga.
dami akong nalalaman na techniques and knowledge.

Salamat ng marami!

basta ba kapag nalaman mo at may nagtanong at alam mo i share mo rin.

Sir,

Noted yan salamat ulit!

galing galing mo talaga hehehe... andi2 n lahat ng kelangan ko para thesis salamat yahoooo..

sir newbie lng po ako and naencounter ko po this forum by shear luck from fnding tutorials and tips regarding vray for SU,
ask ko lng po nabanggit po kc dito na pde mgamit ung rec light sa pin lights, is it necessarily round un pinlight, if ever possible, panu po mga sir?

yung receptacle ba ng pinlight ang tanung mo. medyo malabo.

opo sir nomer hehehe mejo malabo po ba hehe salamat po sa answer sir... im trying to use omni light instead... di ko pa kc gamay paggamit ng omni naguguluhan ako dun sa bilog na lumalabas eh, nahihirapan ako mgscale... enx po and more power ^_^

scale mo bro using su tool. tapos i pasko mo sa loob ng light receptacle.

galeng tlga ni sir nomer. awww!!!