Retroreflective paint (lightwave or maxwell)?

SebastianP

Here's a tricky question for the Lightwave experts out there... How do you create retroreflective paint, the kind used for road stripes and signs, license plates and emergency vehicles (in Europe at least)?

The paint in question reflects light unevenly, but in an opposite way from a shiny surface - it reflects light mostly towards the light source, so that the closer the observer is to the light source, the brighter the paint gets. Very frustrating for news photographers wanting to take night pictures of police cars and ambulances, cause they'll appear nearly inverted (the regular white paint turns up dark, while the rest lights up like it was on fire...),
For extra points, since I imagine the end result will involve gradients etc, how would I go about applying it to part of a texture (say a hull number)...

Anyone care to throw some hints my way?

Cheers,

SP

IRML

you can make a universal light incidence node by plugging a lambert shader into the input of a gradient, then using the gradient to control the brightness and tightness of the light, then you can add that to your reflection shading with a mixer node

SebastianP

OK, I've tried that, and I get results like what I'm looking for but only when the light and camera angles are close...

I think the way it's supposed to work purely mathematically something like this: abs (cameraIncidence - lightIncidence), piped into an inversion gradient (low value in, high value out), piped to the diffuse channel. Except stock LW doesn't have a camera incidence node... I've seen one somewhere, but I can't remember off hand where that was. I'll have to go dig for it....

In the mean time, I've at least determined that using straight diffuse values above 100% is a quick way of doing dayglo painted surfaces. They won't glow on their own, but they'll take what light there is and make really good use of it... :-)

Cheers, and thanks for the help.

SP

L2K

maybe, instead of a black to whit gradient, you could use a black-white-balck-white-black-white-black gradient.
so you'll have the same results as the begining, only with more camera angles.

IRML

SebastianP wrote: »

OK, I've tried that, and I get results like what I'm looking for but only when the light and camera angles are close...

I think the way it's supposed to work purely mathematically something like this: abs (cameraIncidence - lightIncidence), piped into an inversion gradient (low value in, high value out), piped to the diffuse channel. Except stock LW doesn't have a camera incidence node... I've seen one somewhere, but I can't remember off hand where that was. I'll have to go dig for it....

In the mean time, I've at least determined that using straight diffuse values above 100% is a quick way of doing dayglo painted surfaces. They won't glow on their own, but they'll take what light there is and make really good use of it... :-)

Cheers, and thanks for the help.

SP

ok I didn't put much thought into it I'll admit, but you can combine a regular incidence angle (which is basically a camera incidence) with light incidence like I said, there'll be some sort of maths involved which I don't really know right now, but you get the idea

SebastianP

I finall figured out the way, way easy method of creating the effect in distance shots: set the diffuse value way high (several hundred percent), then set the specular value way low (negative several hundred percent), so that the total of the two values end up in the 100-odd percent range. That gives a big brightness bump when the light and camera angles are close, while cutting the brightness as these angles separate... the only minor problem is that a perpendicular surface will reflect less light than a glancing angle, but as long as the total is still over 100% that won't show. I'll need to figure out what kind of adjustments to use for the fresnel shader, as well, in order to widen/constrain the reflectivity angle, but that should be a cakewalk now. ;-)

Cheers

SP