reflectivness based on value of objects?

[andrewjohn81]

I'm wondering if there is a way to do that? Of course we don't all use real world lights and we often use hdr or even ldr images in our environments.

I wonder if I could tell an object that it is reflective, but it would Not reflect dark images. This is not physically correct of course, but unless you have accurate lighting and reflection maps then you might want to have a little help getting what you want, right?

So what I'm hoping to be able to do is use some sort of threshold, maybe using a falloff map or gradient somehow, to control the value that is reflected. Dark reflections make the object darker, but in real life things aren't perceived like that. When you look at a white reflective object you see the bright white highlights and some of the really dark things and you pretty much miss out everything between. Although, sometimes you may not even want to see the dark reflections.

Is there a way to control what values are reflected and not just objects? What if I wanted to say blue tints reflect less than red ones? I might want only midtones reflected to reduce contrast, or increase contrast by saying only bright or dark areas are going to reflect at a high value.

I'm just wondering if someone might know a way of doing this.
I think this is especially nice for refractions. That way I could control the refraced object without having to use a different map for reflection than refraction, something that I don't even know how to do right now. Is that even possible in one pass?

[joconnell]

Well, what normally happens is two things - one is that the object isn't as reflective as you'd think. Second of all the bright bits getting reflected in your objects are much brighter than you think so it lets you get a really wide range of colours in your image - For example vray lights are a nice way to go. They use an inverse square falloff which pretty much means that the light gets weaker the further away from the bulb it travels. Personally I used to have problems getting that range in my images where I'd have really bright crisp highlights but not burn the image out to all hell and the main thing was lighting position in this case. If you have a light with a high multiplier close to an object, you get a nice reflection from it but it'll burn out the diffuse since it's being hit with too much light. As a result you turn down the light so it doesn't burn the diffuse but you may have lost some of your reflection. This time you turn the reflectivity up but now your object is reflecting everything around it which you don't necessarily want.

One way to balance this is to keep your reflection values low, make your light very, very bright but move them further away from your objects - the highlights will still be nice and bright since reflections don't die off over distance (they just get smaller) but your diffuse will go back to something a bit more normal - you'll get a better, more photographic exposure.

What you're talking about with contrast and so on i'm not quite sure about - normally contrast is easier to add in a grade or post process - Can you give us an example or a picture showing what you want to achieve?

[andrewjohn81]

Very nice reply, btw, but didn't quite get the heart of my problem which I probably didn't explain well.

I've been setting up scenes quickly with only GI, and maybe a couple lights for fake specular that contribute no light. That way I don't have to worry as much about how the lights are affecting each part from the different objects. The shapes are all different, but the designer wants each animation to be lit exactly the same. Also, just the data (has inside parts as well for a washing machine) reaches 1.5gigs of memory and my farm only has 2gigs right now. We're talking about upgrading now. I don't want to add more lights to the situation. I had to lower light cache just to get it to render, but have a little noise now.

So I'm using an HDR, but those don't really have falloff since it's just a map. Everything on the map is equally distant from every object. I'd like to make it so that only very bright objects from that are visible to the reflection, or really dark objects. But I don't want to change the reflective value of the object because it looks correct when reflecting other objects, just not the maps.
I could make a different HDR for each material I need to be different, but then I run out of memory. It seems that it loads the map more than once if it isn't instanced. Maybe not, but when I doubled up on the same map I crashed. Once I used an instanced map it was fine.

Maybe on the next project I will have to use vray lights and just try and get it to work. I know that many objects need to be barely reflective though. Although, what if you need a value between something like 9 and 10? 255 values just doesn't seem to cut it for that. Can I use vray color for that and use a decimal value?

Thanks for the help