Splotchy HDRI

[ngrava]

Hi folks,

We are working on a commercial that incorporates CG characters into live action. I shot a bunch of light probe images and made HDRI's out of them. I've found a bunch of tutorials on the web about using HDRI with Vray. So far all I've managed to get is really splotchy results. Does anyone know what causes this? It looks like an issue with inaccurate sampling but setting the Irradience map to higher levels actually has a more pronounced effect.

Here's an example:



Thanks for any help you can give me,

-=GB=-

[jujubee]

Are you using a HDR dome light? IR/QMC?

If not and your settings have been turned way up, you could always throw in a small fill light to try to offset some of these splotches. Also, check to see if your HDRs are set to correct/matched size dimensions (max custom units) in respect to the size of the scene. The other thing that could have happened is that you didn't obtain a high-enough resolution for your HDRs (which happened with my first photo-based HDRs.) You may want to also try to blur the maps if this is the case.

[flipside]

Can you show all your GI settings? I guess what you refer to as higher levels just isn't high enough.

Also the lighting in that room is so different in various places, that I think it will be very difficult to use hdri in this case. Especially with small artificial lightsources (like a lightbulb) hdri isn't easy and can be avoided by simply using a vray light instead. That will be way faster and splotch free. Remember a hdri map is the lighting info from one point in space only. So if the animal will need to go from one end of the bath to the other, you cannot light it with the same hdri map.

[tom schuelke]

Hi ngrava,

You might ask Vlado for a vray 1.418 build. this includes the vray skydome which gives you the opportunity to use hdri as a map.

vray uses now priority sampling which makes with domllight much more accurate results compared to hdri as an environement as i think.

also, you can get a very nice llight without using gi.

for animations this is a very valuable adittion.

so ask him maybe. for a newer build.


Tom

[Morbid Angel]

I have to agree with the others, using hdri as gi emittion is not a good idea. So what I would suggest is to use the plate as a reflective environment, I would build a reflector cube with the plate mapped on each, best if you have different angles. Then just light it using area lights or standart lights.
Or, if you want to use hdri, well then settings must be very high and good luck in render time.

[panthon]

Hi!

I´ve used the hdr as a lightsource without much problem. The only thing u need to do is creating two versions of the hdr. The full res one is for reflections only, and the one u wanna use for lighting must be blurred AND resized to a minimum. However, u should be able to recover the light sources from ur blurred version, so dont overblur it too much! You don´t need a high resolution HDR to light a scene!
The idea of using vraylights to mimic the lights on your real studio is a good idea too, but in addition to the hdr. U will have to grade it in post anyway, so working in layers (reflection, refraction, gi, etc) would be a clever thought!.

I´d love to do commercials with vray too, but, unfortunately, i´m only into archpreviz at the moment . Good luck!

[ngrava]

Thanks for all the responses guys!

I've tried a variety GI settings using Light cache + Irradience map and QMC + Irradience map. It's funny because when you turn up the hemisphere subdivisions it only makes the noise more pronounced. The feeling I get is that with an environment that's made up of geometry, the sampler can locate areas of the scene that are going to appropriately add to the illumination of the scene and concentrate more rays there. That's what I believe importance sampling is all about. But, with an image, it has a harder time because all it has to go on is the contrast of the pixels and the illuminance contained within each one. So, when the sampler sends out rays to gather illumination from the environment, you can get wildly different values but no way to know which ones are the important ones.

I was hoping someone would say, "hey you need to set this parameter to such and such to make it work." but that doesn't seem to be the case.

Flipside: One thing that's troubling me is that I've tried your tutorials and was never able to get the results that he shows in the images on your site. They always came out looking blotchy with whatever setting I used. Have you tried your own tutorials lately with the newest versions? The reason I ask is that I wonder if there isn't some kind of bug or some other big change to the engine recently.

I'll try and post an image of the settings when I get a chance (I'm not at work right now).

Panthon: That's a good idea and I'll give it a try. About how much smaller should the GI environment image be? What you're saying kind of goes with what Flipside said about the image having to much variance in it. Making it smaller might cut down on that. By the way, the HDRI images are of a mirror ball on the set, pointing toward the lights but there is still quite a bit of the set in them as well.

[flipside]

I really think you're better off using 3d lights instead of hdri

What tutorials do you mean? The hdri one? I made them with 1.47.03 and I'm still on that version. But for any tutorial, make sure you use exactly my settings. I get a lot of emails with questions, and 90% of the time it's because people discard some settings I make because they think it's not important.

Resizing a hdri is actually very similar to blurring. The less sudden variance between two neighbouring pixels, the less trouble vray has with the map. But note that by blurring/resizing you will loose shadow sharpness (but you need very high settings for sharp shadows anyway).

What you say about more noise with higher hsph, this is normal too. If you use really too low settings, the whole GI solution is blurred a lot, so it looks smooth but it is nowhere near the real lighting. If you use higher settings, more shadow detail will come out, but you will notice noise a bit more. So at that point, you need even higher settings to get a blotch free AND accurate shadows rendering.

Take a look at this one:
http://www.aversis.be/extra_hdri/flashlight.htm

Here I used a blurry small version of the map for skylight. Since it is such a bright and small light in the hdri, you need very high settings to clear out the blotches. This render uses 2 to 3 times higher hsph values than all of my other hdri's. Without blurring this map, you can't get a clean render.

[panthon]

Ngrava, i think that hdr as a lightsource must contain the set too, not only the lights. If u only need the lights, use vraylights, but the extra benefit of using a hdr with the set contained in it, is that u will have sort of color correction on your 3d, making it look more integrated in the real scene, before u start compositing it. It can speed up the composition a lot later on. You could always use both methods, vraylights+hdr lighting, lowering them conveniently. This way u´ll have the subtle color variations of the hdr plus the strong and sharp shadows of the vraylights. All this will change with the arrival of the HDR MAPPED DOME LIGHTS announced to be part of the newer versions of VRAY, though. I hope so, anyway!



Hey flipside, i´ve taken a look at your website and it´s really awesome. Your tutorials are quite in detph and well explained. I see u master the creation of hdrs and i´m curious about the process u follow. I have my chrome ball and my cam but can´t get proper results creating the hdr. Have u considered the possibility of writing a tutorial detailing the process of taking the photographs (changing f-stops, or shutter or whatever) and then using any app for that (hdrshop, photoshop cs2, photomatix?). I´ve found hdrshop particularly difficult to use, and photshop cs 2 extremely easy, but the results were weird.

Maybe ngrava could benefit from it too, because i´ve seen that most of the problems when using your own hdr images are related to bad adquisition of the pictures or bad assembling-tone mapping of the resulting hdr...

Thanks!

[flipside]

Well, I use only hdr shop, but tried CS demo once and results were very similar. Make sure you shoot your images without moving (I use timer function so I don't need to touch the camera's button). I they are not lined up, you need to do that manually in PS and export to multiple files again.

Shoot with desired interval (1 stop, 1/2 stop, 1/3 stop depending on your camera). I usually shoot 1/3 stops. This results in more images but I got better results that way.

Then use the camera curve calibration function in hdr shop. This should only be done once for your camera. There is a tutorial on the hdr shop site for it. Just press the 1/3 stop button and al values will be correct. Cut of the extreme values and let it run, adjust the slider a bit until you have a nice curve as in the tutorial. Save it.

Then use the assemble function. Load your camera curve. Load images, pres 1/3 stop button and assemble it. Then crop the image so it fits perfectly. Now use +/- to go to a 'medium' exposure and go to 'pixels/scale to current exposure'. This will be the default exposure now (in vray this corresponds to a value of 1.0).

Note that the display gamma in hdr shop is set to 2.2 by default. In max this is 1.0. So your hdr will look much darker in max than in hdr shop. This is because the hdr itself is a linear image, not a gamma 2.2 like most of your textures and original pics. That is why in max your renders with this hdri will be very saturated, blown out lighting all over the place. You need to use the 'linear space workflow' to get best results with your hdr map, and leave the gamma for this map set to 1.0 (in the linear workflow you would normally adjust your normal maps to 2.2). See this thread for a comparison:
http://www.chaosgroup.com/forum/phpB...r=asc&start=25

Look for the teapot images.

This is really crucial, I only found out about it a few days ago. Most people use hdr's wrong resulting in very high contrast lighting, and you need to adjust the saturation of the hdri etc...

[ngrava]

Thanks guys!!

This is invaluable information! I didn't do any of this camera calibration at all. I just assembled them in HDRIShop and cropped them there as well. This probably has a lot to do with how my images are turning out. Thanks again guys, I'll give it a try.

[flipside]

I'm not sure how important this camera calibration is... Never tried it without!

[panthon]

Thanks FLipside for your help!
I´m shooting a short duration film and it will be very useful information, because i need to take lots of hdr´s from real locations

Thanks!