HDRI question: why isn't it giving enough illumination

Jonstark

Doing some experiments using HDRI as a background coupled with skylight and global illumination, and I'm curious, because I've noticed that solely using the hdri to light the scene, with GI at 100% and skylight at 100%, I'm still getting much less illumination for the objects in the scene than the background suggests I should. I have to turn the skylight up to something close to 250% to get the strength of light that seems about right. Any particular reason why this should be?

I was guessing that for outdoor HDRI's the reason that it could be a mostly clear blue sky with maybe a few little clouds in the afternoon, and still look like my objects in the scene are covered by their own dark cloud (kind of a murky light) was that even an hdri cannot accurately mimic the effect of sunlight, in other words the hdri has an image of the sun, but it can't possibly be bright enough to actually equate to what the real sunlight strength would be.

But if that's true of outdoor type hdri, then I can't figure out why indoor hdri shots have the same problem. No sunlight involved, just regular indoor lights, I would think that a single hdri should be enough to accurately light the scene with realistic light levels, but yet I still have to crank the skylight intensity way up to get something approximating what looks like the right illumination strength. Anyone have any idea why?

JimmyC_2009

I don't know the answer to your question, as I am a new Carrara user, but I can recommend this:
http://www.daz3d.com/carrara-realism-rendering-training-video

Excellent set of new videos by PhilW and 30% off at the moment.

DustRider

Hey Jon,

I usually have to increase the intensity of the HRDI a fair amount (under Backgound in assembly room). For last months challenge render as I recall I had it set to 250%. For the second attached image, which is tthe one that I posted as a part of the quick tip for the September Challenge, I set the intensity to 500%. For the first image below I used a different approach, the HDRI intensity was set to 100%, and the Gamma (in the render room) was set to 2.2.

With the first image I think the contrast of the HDRI is to intense, whith the second it's a bit washed out. You could use a combination of the two to get the perfect balance with a bit of trial and error. Oh, sky light is not enabled in either image, only indirect light, and it is at the default values.

Hope this helps!!

Edit to fix the image references

evilproducer

Jonstark said:

Doing some experiments using HDRI as a background coupled with skylight and global illumination, and I'm curious, because I've noticed that solely using the hdri to light the scene, with GI at 100% and skylight at 100%, I'm still getting much less illumination for the objects in the scene than the background suggests I should. I have to turn the skylight up to something close to 250% to get the strength of light that seems about right. Any particular reason why this should be?

I was guessing that for outdoor HDRI's the reason that it could be a mostly clear blue sky with maybe a few little clouds in the afternoon, and still look like my objects in the scene are covered by their own dark cloud (kind of a murky light) was that even an hdri cannot accurately mimic the effect of sunlight, in other words the hdri has an image of the sun, but it can't possibly be bright enough to actually equate to what the real sunlight strength would be.

But if that's true of outdoor type hdri, then I can't figure out why indoor hdri shots have the same problem. No sunlight involved, just regular indoor lights, I would think that a single hdri should be enough to accurately light the scene with realistic light levels, but yet I still have to crank the skylight intensity way up to get something approximating what looks like the right illumination strength. Anyone have any idea why?

Part of what you say is true, about the sunlight in an HDRI. Most things I have read about using HDRIs is that you'll want to use a directional light with them, as they (HDRIs) tend to produce a softer light, which is great for fill, but not to make the scene elements pop. You won't want a 100% bright light as it could wash out the highlighted areas. I've had good luck starting at around 50% for my scene light.

Jonstark

JimmyC_2009 said:

I don't know the answer to your question, as I am a new Carrara user, but I can recommend this:
http://www.daz3d.com/carrara-realism-rendering-training-video

Excellent set of new videos by PhilW and 30% off at the moment.

Thanks Jimmy, I actually already managed to purchase Philw's new training a week ago over at infinite skills, watched it straight through immediately and been re-watching and referencing it ever since. It is superb :) He does mention the fact that it's necessary to de-gamma HDRI files who have been gamma corrected as a default, to work with the linear workflow, but I'm referring to regular HDRI files, even after being corrected. I think it's just that '100%' in the render room for skylight intensity isn't based on any sort of real-world math, and doesn't reflect the correct amount of 'oomph' that the hdri needs to actually drive realistic illumination within the scene.

dustrider said:

Hey Jon,

I usually have to increase the intensity of the HRDI a fair amount (under Backgound in assembly room). For last months challenge render as I recall I had it set to 250%. For the second attached image, which is tthe one that I posted as a part of the quick tip for the September Challenge, I set the intensity to 500%. For the first image below I used a different approach, the HDRI intensity was set to 100%, and the Gamma (in the render room) was set to 2.2.

With the first image I think the contrast of the HDRI is to intense, whith the second it's a bit washed out. You could use a combination of the two to get the perfect balance with a bit of trial and error. Oh, sky light is not enabled in either image, only indirect light, and it is at the default values.

Hope this helps!!

Edit to fix the image references

Interesting that you came to similar numbers as me, makes me think maybe my eyes are guessing about right :) I went through a ton of different hdri's and came up with (if I have both indirect light and skylight intensity to 100%, and gamma to 2.2) that right around 250% intensity seems pretty close to how much light the background should be giving, as opposed to how much it was giving at only 100%. Also these are settings applied in the render room, I know we can also increase the intensity in the 'scene' tab but this will also brighten the background image of the hdri itself, whereas increasing it in the render room leaves the actual background image at it's normal level and only increases the effect/intensity of the light it puts off.

Part of what you say is true, about the sunlight in an HDRI. Most things I have read about using HDRIs is that you'll want to use a directional light with them, as they (HDRIs) tend to produce a softer light, which is great for fill, but not to make the scene elements pop. You won't want a 100% bright light as it could wash out the highlighted areas. I've had good luck starting at around 50% for my scene light.

Thanks Evil, that's close to how I've always done it in the past too, using an extra light at between 40% - 60% to drive specularity and harder shadows. It also really makes sense to use a directional light for outdoor scenes and match it up directionally to the 'sun' image on the hdri, because I think it only makes logical sense that the hdri brightness, even at it's brightest point where the sun is, isn't going to be more than a white color, which won't have much greater intensity than the rest, and real sunlight is... well... really bright, brighter than a mere white color on an image can convey :)

DustRider

evilproducer said:

Jonstark said:

Doing some experiments using HDRI as a background coupled with skylight and global illumination, and I'm curious, because I've noticed that solely using the hdri to light the scene, with GI at 100% and skylight at 100%, I'm still getting much less illumination for the objects in the scene than the background suggests I should. I have to turn the skylight up to something close to 250% to get the strength of light that seems about right. Any particular reason why this should be?

I was guessing that for outdoor HDRI's the reason that it could be a mostly clear blue sky with maybe a few little clouds in the afternoon, and still look like my objects in the scene are covered by their own dark cloud (kind of a murky light) was that even an hdri cannot accurately mimic the effect of sunlight, in other words the hdri has an image of the sun, but it can't possibly be bright enough to actually equate to what the real sunlight strength would be.

But if that's true of outdoor type hdri, then I can't figure out why indoor hdri shots have the same problem. No sunlight involved, just regular indoor lights, I would think that a single hdri should be enough to accurately light the scene with realistic light levels, but yet I still have to crank the skylight intensity way up to get something approximating what looks like the right illumination strength. Anyone have any idea why?

Part of what you say is true, about the sunlight in an HDRI. Most things I have read about using HDRIs is that you'll want to use a directional light with them, as they (HDRIs) tend to produce a softer light, which is great for fill, but not to make the scene elements pop. You won't want a 100% bright light as it could wash out the highlighted areas. I've had good luck starting at around 50% for my scene light.

Don't want to contradict you, but ...... don't believe everything you read :cheese:. A properly made high quality HDI can reproduce the light that was in the "scene" so to speak very very well, and extremely accurately, If your HDRI scene is of a bright sunny day, you will get the exact same shadow effects you see on a bright sunny day. Where the washed out non-directional lighting comes into play is when an image that was not captured as a true full range HDRI, is called and HDRI (there are a lot of them out there). This is true for improper settings when using HDRI images as well (even when it comes to the high quality hdri's, they are not all created equal and need to be "adjusted").

Now, I'm not saying that at times it's not a good idea to use additional lights with and HDRI, because they often help to perfect the lighting. It's just like you would do if taking a photo or filming in that same "real" environment, You might use reflectors, low intensity spots, and any number of other lighting tricks to perfect the natural lighting. These same principles apply when using full range HDR images.

The first attached image is a perfect example. I added one very low intensity spot light to help the translucent "glow" of the dragon's wings, but that glow was already there. I also added a very low intensity spot to help brighten the girl's face just a bit. As I recall these were both set to 5% brightness (or less). I often use a low intensity spot light to bring out the face/eyes, just like a photographer would use a reflector for the same purpose.

The second image (done in Octane- this was actually done for a similar discussion in another forum) is lit with only an hdri (note the position of the sun in the reflection on the ball). The shadows aren't razor sharp, but note how they start out next to the figure and sphere quite sharp, and the blur increases as you get further from the subject - just like they do in real life.

So can you light a scene with an HDR image and accurately reproduce the lighting conditions it was captured under without additional lights? With a high quality HDR - most definitely. Will you want to do this with every scene? Just like a professional photographer, probably not, because the natural light often doesn't work perfectly for every situation/photograph.

ringo monfort

I need to test a few things with HDRI, Indirect Lighting and Global Illumination maybe I found a settings bug but I am not sure.

evilproducer

Great examples Dustrider. Do you get similar results to the second image using Carrara's renderer, or is Octane better at IBL?

evilproducer

I just thought of the perfect place for this discussion, and I'm sure Dustrider would agree. It may even provide some useful WIPs and info. for those of us that participate.

http://www.daz3d.com/forums/discussion/45561/

;-)

ringo monfort

RingoMonfort said:

I need to test a few things with HDRI, Indirect Lighting and Global Illumination maybe I found a settings bug but I am not sure.

Okay, False alarm. I found no bug.

Regards

Szark

it s all about the quality of the HDRI being used, Many freebies don't have enough light information in them to light a scene as you would think. I found DT's Yosemite HDRIs to be excellent http://www.daz3d.com/hdr-prosets-yosemite-pack-two and http://www.daz3d.com/hdr-prosets-yosemite-pack-one but even those needs a Sun to give good directional shadows as pointed out already. The one I used for this image http://www.daz3d.com/gallery/#images/36420/ was 111 MB many on the net are only 10 - 15 MB which should tell you something. It is all about how many light step that are included in the HDRI...I am not an expert in these thing though. DT has explained this in one of his product threads which I can't find at the moment.

Szark

found it http://www.daz3d.com/forums/discussion/37851/P45/#560846

When you download an HDRI from the internet it will most likely be made from 3 exposures because of the consensus that this is the minimum (which is even more likely if it's high resolution), it's less likely that the HDRI will be done with 5 images and less with 7 images etc. Even those 7 images probably wont reflect the possible tonal depth of an HDR, or a 32-bit Floating Point TIFF (7 exposures are what I use for the HDRs in these sets). From what I've come to understand 15 exposures is the upper threshold of bracketing and I don't believe the HDR format would actually benefit from more than that, but I could be wrong and the number/quality difference obviously depends on bit depth of the original images (14-bit RAW etc). I know that there are cases where way more bracketed shots are put together like astral photography of nebula etc but I believe that is a different format and process

DustRider

evilproducer said:

Great examples Dustrider. Do you get similar results to the second image using Carrara's renderer, or is Octane better at IBL?

Sorry for the slow reply, I had to blast off to class.

This isn't using the same HDRI as the Octane image above, so the lighting is different. It's the same as the chrome ball above with a plane primitive added to catch the shadow (gamma set to 2.2, intensity 100%). So yes, you can get similar results in Carrara. Working with HDRI is easier in Octane, mainly because you have control over the exact rotation, the gamma (of just the HDRI) and the intensity (power) of the HDRI.

This image was done with one of DT's Yosemite HDRI's that Szark mentioned above (really great sets). Good HDRI's in Carrara can generate excellent results.

DustRider

evilproducer said:

I just thought of the perfect place for this discussion, and I'm sure Dustrider would agree. It may even provide some useful WIPs and info. for those of us that participate.

http://www.daz3d.com/forums/discussion/45561/

;-)

Great idea EP!! I added a link to this thread in the Quick Tip for using HDRI 's in the challenge WIP thread.