hdri for reflections only

[Dave Buckley]

this may be a really simple answer and might just be me being stupid, but how would one use a hdri for reflections alone??? and is this good practise?? i should i use the hdr to contribute to the lighting aswel?

[shaneis]

It's not a stupid question, kudos to you for trying to extend your knowledge.

 

By using a HDRi map for reflection only, you're essentially making it a spherical LDRi texture. You can add it to your scene the same way as you'd add an HDRi and turn off all the connections to the Final Gather solve (how you do this depends on the app you're using). Similarly, you could open the HDRi in Photoshop, bring it down to 8-bit and map it to a sphere (remembering to reverse the normals on the sphere). This is probably the preferred method as it will allow you to adjust the 8-bit version so as to control the reflection to your liking. Some people use a 16-bit TIF format instead which allows you to push the value into the super-whites which will avoid clamped hotspots in your reflections. I've been playing with this approach over the last few days (16-bit) and it seems to work quite well, so long as you're rendering to 16 or 32 bit.

 

It is good practice if you already have a good light rig and simply want a spherical reflection map. You can also mix two spherical maps, using the LDRi for reflections and an HDRi for your Final Gather - even more control.

 

Have fun,

 

S.

Edited July 15, 2008 by shaneis

[Dave Buckley]

ok thanks shane, (always trying to increase my knowledge and something tells me i shall be doing that forever

 

ok so i have my light rig (using max design 2009 by the way) and i have a .hdr file.

 

i know about loading it into the environment map but now you have just thrown me with 8bit, 16 bit, clamps, hotspots, super-whites and rendering to 16bit

 

all i used to do was load .hdr as a bitmap in environment slot, alter whitepoint values until the pink over exposed areas had gone in the setup screen that appears. then copy this to material editor, change to spherical environment and then increase the number of RGB output to match that of the whitepoint value i got from the previous setup screen.

 

i'm just about to start lookin at the hdri handbook. and also looking into creating hdri's from my existing 3ds max scenes, interiors and exteriors, by rendering out images with different exposure settings along with the wraparound lens shader. not sure if its going to be possible yet tho or if its necessary but hey

[shaneis]

To save me from writing an essay, here's a couple of links to get you started,

 

http://en.wikipedia.org/wiki/Reflection_mapping

http://en.wikipedia.org/wiki/Color_depth

 

If you have the HDRI handbook, that will answer all your questions. As for Max-specific techniques, I can't help you there - I'm a Mayan. Sorry. Maybe a Max pundit can help you with the details.

 

All the best,

 

S.

[johnvid]

Hi Dave,

 

Not sure why Shane is talking about Ldri, am i missing something, if you where producing HDRI images as final output why would you want your objects to have a Low dynamic range reflections?

 

Surely when you start messing with the image in post, it wouldn't look very good if the panoramic BD or Reflections didn't alter at the same rate/range.

 

Not sure if Mental Ray(long time since using it) works in the same way as VRAY with a dedicated Reflection Environment overide slot, and the standard max environment slot is for the background when used with vray (maybe reflection also?)

 

but if the HDRI Reflection image was on an invisible sphere, but visible to reflections and maybe self illumination needs to be on...does this offer any more control over the way mental ray works... Does Mental Ray have its own Environment Slot?

 

It should be be able to use the HDRI without clamping it.

 

You should use it also for the lighting, but you can use a lower rez blurred version, but once again not sure how you seperate in MR, but if you can use the full rez for the lighting & reflections, without any memory/speed problems, then just use the full version.

 

CHeers John

[shaneis]

Hi Dave,

 

Not sure why Shane is talking about Ldri, am i missing something, if you where producing HDRI images as final output why would you want your objects to have a Low dynamic range reflections?

 

Surely when you start messing with the image in post, it wouldn't look very good if the panoramic BD or Reflections didn't alter at the same rate/range...

 

Shane is talking about the option to use LDRI as a reflector because if your LDIR doesnt have pixels at 0,0,0 or 255,255,255 then once rendered to a HDR format, it will make no difference. You would only need to include a float/ half-float reflector if your values exceeded 1.0.

 

I mentioned these options as it can help with file size and if you don't have access to image manipulation software that can work in 16 or 32-bit colour space, then it's handy to know that you can get by using 8-bit for your colour maps. Remembering also to gamma-correct the files if you're using LWF.

[johnvid]

Hi Shane, but the question was HDRI for reflections...at the moment that is the problem, surely your method of using LDR images for reflections would be better when rendering to 8bit.

[Xoote]

Thanks for the info

[shaneis]

Hi Shane, but the question was HDRI for reflections...at the moment that is the problem, surely your method of using LDR images for reflections would be better when rendering to 8bit.

 

Ahh, so it was. Sorry Dave, sorry John. Hope I haven't confused the issue. I'll have to ramp up my coffee intake again, methinks.