I am interested in the property of reflective/refractive surfaces generating GI: Two in particular glass and water. I’ve noticed that if you turn on refraction and reflection on a material you don’t get that hot spot of redirected light on any nearby surfaces. As I understand it caustics describe this effect in mathematics terms as such: In differential geometry a caustic is the envelope of rays either reflected or refracted by a manifold. So in my case I’m turning on caustics on a water material I created and getting all of those interesting light patterns. Great but no gradient of bounced lighting decaying at inverse squared back up my bridge piers. My work around has been to create a vray plane light the size of a bridge, orient it facing the sky and cause it to act as a skylight excluding the water and all geometry other than the bridge. Without the skylight the underside of the bridge is really dark compared to my source pics, in fact to compensate for the lack of gi I had to increase the GI output of the water to 20 in a VrayMtlWrapper. However, this caused my water to change from a murky New Jersey ocean water to a Caribean like water so for now the skylight is a solution, but ideally I want a material solution. I’m sure you guys have walked by a reflective glass wall and been blinded by the highlight, even felt the heat from the redirected rays. Anybody’s whose been on a boat knows that you can get a sunburn quicker on the water than on the land from all of the reflected light, and it’s not those caustic like patterns so much as it is a more uniform distribution of reflected energy. Can anybody help me solve this?
A sample scene would be helpful, really.
The concept is clear, however there may be a good number of things gone astray, or it may be a renderer limit, but it’s impossible to tell just by the description of it.
Lele
File
Sorry it took so long, I had strip this thing down so I could host it on the net. One thing, under light cache I forgot to change it from file to single frame. I also did away with the ground texture and just made it gray since the bitmaps were too large. Shouldn’t effect the reflective gi conversation though
Reflective_Gi
Here are some pics I took of the phenomenon:
http://au.pg.photos.yahoo.com/ph/unrealmirage42/album?.dir=2d0fre2&.src=ph&store=&prodid=&.done=http%3A//au.pg.photos.yahoo.com/ph/unrealmirage42/my\_photos
fixed a few settings that were odd to me, turned caustics on, used vray sun and sky, a physical camera, and a quick hack to mimic my .255 method for outdoors (blend with the bridge multi-sub).
Stopped the rendering to post, as i have to go out now.
here’s the max file:
http://www.scriptspot.com/Lele/Vray\_forum01.max
Lele
Sky
I really don’t want to use vray physical sky, I’d rather use a bitmap with clouds in the background. Will that effect the scene?
I don’t get it, can’t you just turn on reflective GI caustics in your GI settings (instead of using photon mapped caustics)?
Will try your file also
It would be hard to get these caustics as GI caustics; the chances that a random GI ray will bounce from the bottom of the bridge, reflect off the water and hit the sun to produce the caustics is quite small. It can be done, of course, but the render times might be quite high (e.g. a few hours with the “universal settings”).
Best regards,
Vlado
as Vlado said.
I used photon mapped ones because they are precise and quick, and ARE reflective caustics.
PPT is another method to get them as direct without a huge speed impact, but of course the method’s intrinsically slow and not optimised specifically for caustics.
Lele
Yeah indeed, it takes forever 
With large light sources it works better though. But if you use a direct light as a sun, you have no other choice than to use photon mapped caustics anyway.
Ahh caustics, one of my favourite things
But yeah as the others said.. refractive GI, just not that usefull when you need things done quick and accurate.
In a shot like this it may be easier to fake it with a direct light and a projection map.
Or even with a single direct light for casting caustics so its more controllable and you dont waste subdivs where you dont need them.
With a bit of noise/displacement in that water you will get some nice caustics happening.
Second that! While doing it the “real” way is fun, faking it is much more effective and controllable.
What I would do is, render one PPT render for a reference and then render a sequence of images with Caustics Generator (found here http://www.lysator.liu.se/~kand/caustics/). Then just use projector light to project the rendered image seq to the bottom of the bridge.
Mare
I guess I would have to fake it, because if I still want my refraction and fog color to work the caustics seem to disappear. If the water is only reflective the caustics seem to work pretty well, but I still want to see the refraction around the piers.
You might just need to increase the caustics multiplyer.
As soon as you turn on refraction you loose alot of the caustics reflection power, as most of it will just be used for refraction. So up the MP to see how it goes.
You could always pre-calc the caustics using a pure reflection water then use that saved map and your prefered water.
would reflective caustics reach the bridge that high up. i would think they would diffuse to a blur
They do, DaElf.
I had to reduce the number of photons to interpolate a lot not to have them look like a blur
Lele
physical sky clouds
StuidoDIM, nice sky in the original file, however I am forced to use a bitmap envrionment because I want clouds. I am using DreamScape to get the sky tint across the image background from the Dreamscape sun, looks ok but really the gradiation doesn’t look as good as your vray physical sky. I found a site that talks about how this is overcome for mental ray, is there someway this can be utilised in vray? zap's mental ray tips: Clouds with mental ray sky Maybe this topic has been hit on allready somewhere else.
Increasing the caustic multiplier adversely affects the water, really blows out the water color. Instead of feeling like the scene is in New Jersey it looks like the bridge is on some Tahitian reef. I’ve tried setting the water to not receive caustics but create them and playing with the object properties for receiving gi, etc. Caustics, reflective gi, and refractive gi all seem to over brighten the water. That’s why I was thinking about faking it. I can get the light to bounce up and hit the bridge but it messes up my water, if it wasn’t for the refraction I’d be set.
You want to set the ground beneath the water to not recieve caustics, not the water itself.