View Full Version : hdri for FG in Mental Ray

01 January 2011, 08:58 AM
I'd like to create my own hdri to use with final gathering.

I'm wondering from the maya angle what kind of image/s do I need. Do I need like 16 raw images shot from all angles at 3 different exposures up down front back left right and stitched together in photoshop and saved as hdr and plugged into IBL in Mental ray?

I did a test with a cube and mapping on a jpg for each side, upping the ambient value and turing on FG and it was pretty nice already. Is this a pretty good method? I know it's all relative but if someone could tell me how they use FG and IBL for renders I'd much appreciate hearing from someone with experience.

is there a place to get free hdri files?

Thanks a lot!

01 January 2011, 03:28 PM
in Max you could use the Skylight with the HDRI and it works OK with FG.

In Maya.... you have to really blur out the HDRI in HDRI Shop (e.g 50 pixels) and save as a small image size (e.g 128x128) and use that image for the IBL to create the FG but then you have the problem that your materials no longer have sharp HDRI reflections. If you put the environment reflections back to the no blurred HDRI image (via the materials's Advanced Reflection> End Color) then the orientation of the HDRI maps no longer lines up and i can't see any way of working out how to fix this.

Best of luck, if you are working then its probably best to just chuck some fake lights into your scene.

01 January 2011, 03:55 PM
smartIBL is here for you: :)

It does what twazzer described (small blurred texture for lighting, sharp high rez texture for reflections).

01 January 2011, 07:09 PM
Great. Thanks a lot.

This is what I was hoping to get. Some experienced users with tips around Maya.

01 January 2011, 10:46 PM
wow, that link was amazing!!! unlike my last post (sorry i've got the flu), the work around for Maya is-
Create a poly sphere, make it bigger than your scene, apply a new lambert and add your small/blurred environment hdri image as incandescence (turn diffuse off, also turn off visible as- reflection,refraction, primary, double sided but not opposite from inside the Render Stats tab of the sphere). Now the FG will come from this sphere but the reflections will come from your IBL texture. Then just rotate the sphere to match the IBL sphere

Job done. Or just use the sIBL script from which is obviously much better than my way.

01 January 2011, 11:15 PM
yeah hdrlabs is a real find!

thanks twazzer for the breakdown. Makes me appreciate hdrlab even more, haha.

I understand the idea of hdr images, that the have depth in a sense. I'm not sure why hdr creates a better light on my object then a jpg though?

I guess I'll have to do some test to see the difference.

01 January 2011, 04:20 AM
I understand the idea of hdr images, that the have depth in a sense. I'm not sure why hdr creates a better light on my object then a jpg though?

@gauranga108: It's because a higher-range file such as an .hdr or .exr or even .bmp or .tiff has more light-contrast information per-pixel. A .jpg can only have 8 bits of brightness spread over three channels. A nice 32-bit .exr can have millions of levels of brightness between the darkest pixels and the brightest - so when Maya's looking up the brightness, there's a much greater contrast and therefore more accurate and developed lighting solution. Especially using an IBL with Photon Emission.

01 January 2011, 04:17 PM
walkthrough of hdri preparation for mental ray IBL (

02 February 2011, 06:21 PM
I was wondering about IBL with SSS without a light.

Has anyone used IBl with SSS without a light?


02 February 2011, 06:55 AM
We've used SSS with just an HDR pretty constantly. It works fine but be sure the lightmap is set to catch indirect light since you do not have a direct source.

02 February 2011, 06:41 PM
This is like the coolest thing ever, except for space.
Going to try it out asap.

I've tried using images for reflections by plugging it into the shading group instead of using an HDR-file for both reflections and light, but it has always looked really strange. So what this one does is making a dome for both reflections and light, so when you do FG, it only takes the light information from the low res image, but reflections from the huge image(?)

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02 February 2011, 06:41 PM
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