HDRi exposure problem

[Terri Brown]

Hi everyone,

 

I tried to search the forums for an answer to my problem - either i'm a bad searcher or the answer isn't out there . I'm pretty new to Vray (been on MR for years), but as a standard setup I use HDRi lighting with a Vray Dome light, LWF workflow (after 2 years still trying to wrap my head around that one) and Reinhard color mapping. (I used Peter's tutorial as a general guideline-unfortunately his site is down at mo so can't insert the link).

 

Here's the problem:

I am experiencing very blown out backgrounds, and no amount of tweaking to my HDRi map or Dome light makes the slightest difference. (as you'll see my settings are much lower than would be expected). The only thing that makes any difference is my 'Burn control' amount under Reinhard settings...however I never seem to get a realistic result...even with minimal Burn control settings.

 

I will attach two test renders - one with burn at 0.55 and the other 0.05. Still way too overexposed - My sky is a dusk blue - very little light! I want to get a nice contrast between the blue dusk sky and the warm interior lighting. The thing is that i want to be able to tweak my HDRi separately without using a global setting such as Burn that will affect all my lights.

 

Does it make sense?

 

PS pls excuse the bad renders - still in test phase!

 

Have attached ALL renders & settings.

 

Please help!

 

thanks soooooo much

T

[cg_Butler]

It seems very odd to me that your getting such burn out with such low multipliers for your HDRI. Are you using a Vray camera?

[Terri Brown]

yip. if you look at the attachments my vray physical camera settings are there...do you think my shutter speed settings are too low?

[cg_Butler]

Oh yes they are. Sorry I missed that.

 

They aren't that anything odd to be honest. What I do find odd is all of your multiplier settings being so low, yet everything is so bright....... Have you got anything else that might be effecting the environment? Any environment overrides? Background ambient light or vray sun?

[Chris MacDonald]

You're using a render engine that is based on real world physics.

 

This is exactly what happens when you are in a dark room, with a bright light source coming from outside (i.e. the sun & sky), it's just that your eyes adjust to it far better than a camera ever will. The fact is you will get blowout when you expose an interior correctly, below are two shots I've taken of our office - One has the exterior exposed correctly, the other has the interior exposed correctly:

 

 

You'll see how extreme the difference between the two is, and this is with a room with enormous windows. If you have smaller windows, the more extreme the difference will be because less light from outside is able to get in.

 

There are therefore two or three solutions to your predicament:

 

1) Fudge it. Use unrealistic lighting values to achieve what you want to achieve.

 

2) Render two separate exposures, OR save as a 32 bit exr/hdr and adjust the exposure on the windows separately in photoshop.

 

3) My personal favourite (because it's real!) - embrace the blowout! Add a slight glow/glare to it using the VRay Lens Effects, it looks ace.

[cg_Butler]

@Macker, would you not agree it's odd she's having this issue with such low multipliers of her hdri and dome lights?

 

I think, whilst your post is totally accurate, I have never had such low multipliers, yet such blowout.........there must be something else in the scene in my opinion.......

[Terri Brown]

oooohhhhh my... guys, i feel like a bit of an idiot. I totally forgot that there was a vray sun in my scene - hence the lack of change when adjusting my hdri! i admit it...i'm tired!

 

So sorry to waste everyone's time...but thank you andy for spotting the issue in the first place! And Chris, i agree - I'm all for blowout...i just want to be able to control if i need to

 

will post a re-render inabit to show the effect

 

thank you as always guys!

[Chris MacDonald]

I do think the combination of both the HDRI AND light multipliers being so low, yet blowing out is surprising, but given that the exposure is 1/20th of a second at f5, that will brighten the image a little bit. Doesn't explain these results though.

 

As it stands the room is massively underexposed and that should be the priority. Firstly I'd get some rough colours on the walls/items, because everything being white/grey will bounce a lot of light around and will throw off all your settings when it comes to adding materials later on down the line. Once this is done I'd set the light/HDRI multipliers back to 1 (at least we're then at a point we can all relate to and troubleshoot from) and render off a correctly exposed interior shot, regardless of windows blowing out.

 

Once you've got the interior exposed correctly you can then tackle the problem of the blowout - which as I said in the previous post can be done in a number of ways.

 

[edit] I see you solved the problem whilst I was typing this! haha

[cg_Butler]

I totally forgot that there was a vray sun in my scene

 

thank you as always guys!

 

I knew it!

 

Now put all your multipliers back to 1.0 and then either adjust only the Vray dome multiplier or the HDRI multiplier not both and only after you are certain on your camera settings are suitable for this interior scene.

 

I don't think we get enough credit for what we have to know about so many things as artists! Photography, architecture, lighting, science, scale, textures, the list goes on!

 

Glad we could help though. I always get a sense of gratification when I am useful, I'm sure Chris is the same.

[Chris MacDonald]

I don't think we get enough credit for what we have to know about so many things as artists! Photography, architecture, lighting, science, scale, textures, the list goes on!

 

Glad we could help though. I always get a sense of gratification when I am useful, I'm sure Chris is the same.

 

Hit the nail on the head!

[Terri Brown]

sheesh i tell you! when i started 3D i specialised in modelling and post...being an Arch Viz whiz on your own is insane. That's why this community is so often my lifeline.

 

All my settings are back to normal - doing some test renders as we speak. What would u guys put as realistic camera settings? F-stop 5? Shutter speed approx 20? Or is it totally scene-dependant?

[Chris MacDonald]

Defnitely scene dependant, but there's nothing unusual about the settings that you've got on your camera.

 

I tend to stick to "standard" f stops though, so f 1.4, 1.8, 2, 2.8, 3.5, 4, 5.6, etc - but that is purely because I'm used to using a dSLR camera, there is no reason you can't just use whatever number you want.

[cg_Butler]

Totally agree. If it's realism you want, and vray is based on a physically correct, real world system, then using camera settings that can be replicated on a real camera makes sense!

 

What some people don't realise is that different combinations of f stop and shutter speed give you the exact same exposure in an image. The noticable differences would be in there are moving objects. These would become more or less blurred depending on the settings.

 

This image is a useful chart to show you the respective f stops and their shutters speeds to gain the exact same exposure.

 

[Terri Brown]

great. thanks for the info. Will keep that chart handy!

[Tommy L]

If you want to use the same image for GI and background then you can use the environment override slots in the Vray render panel. Or you can have 2 Vray domelights, one visible and one not, tweak to taste...

[Terri Brown]

Thanks Tom, but what is the difference between using one domelight with an HDRI map as a texture to what you suggest in the environment slots? are both methods not doing the same thing (using the same image for GI and background)?

[Tommy L]

Thanks Tom, but what is the difference between using one domelight with an HDRI map as a texture to what you suggest in the environment slots? are both methods not doing the same thing (using the same image for GI and background)?

 

It enable you to have separate control over all aspects of each (GI and reflections). Whether you use the overrides or the double dome method depends on which set of numbers you have a better handle on. I like using two dome-lights because you can use the built in visibility and function controls/multipliers in the dome-light object.

If you are not doing it already, you should be using a smaller, blurred version of your HDRI for environmental lighting. Seems arbitrary, but its really quite important as it integers samples over fewer values for a smoother finish.

Your BG should only be your HDRI dome in a pinch, they are not high enough res for stills and for animation they have no parallax. Skies for stills are easy in post, fully CGI skies for animation are another subject though...

[Terri Brown]

thanks for the tips

 

just one thing: not sure what you mean by "in a pinch"? lost in translation here...from american to english ;)

[Terri Brown]

as promised, here's another draft render...am not happy with image in general, but at least the exposure problem is much better now.

[Tommy L]

'in a pinch' means when you're in a rush and dont really have any good options.