View Full Version : HDRI - dark areas and shadows

04-25-2005, 10:46 AM
First the example image rendered:
~8 minutes with Fprime. No lights, just HDRI image MC 8 bounces.

And the questions:
1. The greatest problem with using HDRI and in more general Radiosity is that I don't have control over the intensity of the rays bounced on each surface. In the example image the balcony area is too dark and stays dark even if you boost the radiosity intensity (i thing 200% in this example). Most times I am forced to add a distant light looking up to brighten the areas. Is there any other technique I am not aware of?

2. How can you have HDRI and hard shadows? HDRI + light source boost the contrast of the image undesirably...


04-25-2005, 11:59 AM
In image world, you can adjust the heading of the image you used for the backdrop. Try changing it.

You can supplement with lights to brighten the areas you need to make brighter.

You also have image controls in the image editor to adjust the image you are using for the backdrop as well. You might have to make those adjustments in the HDR EXPOSURE image processing filter, but I've never used it before. I hope this helps!

- Ben

04-25-2005, 01:41 PM
Well first of all the reason, I'm guessing, you have dark areas on your balconies is because that area is supposed to be shadowed according to Lightwave's radiosity solution.

I would suggest you actually light your scene first. Don't even touch radiosity or HDR or anything until AFTER you light your scene the way you want it.

Then use your HDR as a compliment to your lighting rig. You are never going to have more control over your lighting when using only radiosity. Yes you can rotate the position of your sphere in ImageWorld, but your results will never be as good as someone who understands how to light a scene properly and THEN use the HDR image as a compliment to that lighting.

HDR lighting is great, but IMHO should never be used as a replacement for your lighting.

04-25-2005, 07:43 PM
I personally think that the way it turned out is pretty accurate. It's an outdoor shot, anything indoors will tend to be dark or the exterior will be too bright, depending on the camera setting, or the human eye for that matter.

I realize that this is undesirable for arch-viz, clients want the inside and outside to be equally visible, but that's unrealistic and HDR won't give you that on it's own.

If you want hard shadows without adding lights, you'll have to have an hdr with an extremely bright sun that covers a very small part of the sky. The HDR would have to be created that way from the start.

04-26-2005, 01:39 AM
Thanks for the replies.

By watching the new Maxwell Renderer threads I can see that the dark areas of rendered scenes are brighter than in LW/Fp. IMO this is the main reason people think that it is more realistic. If I have to add lights to build the bounce effect it is not that easy to keep real. I think a bounced rays intensity control would be essential!!!! A slider or something in the radiosity panel... Would that be possible? Hope someone from Newtek is watching.


04-26-2005, 09:29 AM
I agree with Toby but there is one cheap fake to do in lw/fprime and that is to raise ambient light. Ambient light will raise secondary bounces and if your using 8 bounces (why? ) you have to raise it to ridiculous hights to see the effect. My suggestion is to use 1-3 bounces and some ambient light 25-400 depending on bounces.

hehe i forgot to ask... Is it a client project or is it possible to borrow that house for some Kray renders? :-)

04-26-2005, 11:50 AM
That's a good point, I remember hearing on this forum from one of the programmers ( Arnie Cachlin?) that ambient light works really well to substitute multiple bounces, he said 40%. I never liked the look, too little contrast, but that may be what you're after.

I realize that you don't always want absolute realism ( I we did, there'd be no need for 3D! ) , but I still think your render looks 'accurate', and you will always-always have to tweak to get the look you want. There are a fair amount of arch-viz guys on this forum that wish LW had better radiosity too, they can probably steer you better than we can - they hang out in the Architecture and Visualization forum -

04-26-2005, 02:51 PM
ok toby... is that a render? Cannot really tell. D*** I wish I could do this in the time I have. The ambient trick kinda works, and might do the job. Its just that most of the times I need to brighten the dark areas and at the same time have control over the contrast.

silverlw: send me your e-mail please, I'll see what I can do (it is a client project)


04-26-2005, 03:51 PM
no! no! I'm sorry, it's not a render, I should've been clearer, it's a photo I found on the web, just to show how dark the recessed areas are, just like your render.

Speaking of which, that's a problem I noticed while working for an arch-viz guy, there's not a single client that will allow you to use the techniques to make a photo-real image. Dirt? Absolutely not! Irregularities in the materials? Hell no! Old cars? Are you nuts? Cracked sidewalks? What is this, gang territory? ... so why can't you make it look more real?

BTW, have you heard about the motion blur trick for super-fast radiosity? It works really well for stills.

04-27-2005, 03:21 AM
BTW, have you heard about the motion blur trick for super-fast radiosity? It works really well for stills.
Do Tell... :)

04-27-2005, 03:39 PM
I will try to find the link again.

It involves using Interpolated with high tolerance settings which gives you blotchy results very quickly, and adding motion blur with lots of passes. Using motion blur makes LW recalculate the radiosity at each pass, then averages the blotches out.

What I remember is :
Start with tolerance at about .232 ( .1 seems to always take too long and not look much better, sometimes longer than monte carlo )

Rays can be set high, they don't affect render times very badly

Evaluation Spacing depends on your scene, but start high, like 50cm.

AA on enhanced (optional) high, adaptive sampling off.

Shading noise reduction, optional. I don't like it, it's either muddy looking at low radiosity settings or too perfectly smooth at high settings, and takes time.

04-28-2005, 02:11 AM
Thanks, I'll give that a whirl :)

Captain Obvious
04-28-2005, 03:53 AM
In my testing, I could turn up the bounces in Interpolated to about 3-4 without a big render hit, but after that it got a lot worse quickly. Five bounces took maybe 50% longer than four, and six bounces twice as long as five. Seven bounces and I cancelled after a long while... never bothered to try eight.

The number of rays per evaluation shouldn't be set TOO high. It will increase the render time. A high value will not produce more accurate radiosity, only more "full." With few rays, you'll see lots of small separate patches of light. With many rays, the small separate patches of light will be of a greater number, but the size will be the same. Interpolated radiosity can sometimes make a mess of detail (shadows in particular), and turning up the RPE doesn't really help much, in my experience. It just means you need fewer AA passes to get an acceptable radiosity.