IBL HDRI image flicker

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Old 10 October 2012   #1
IBL HDRI image flicker

Hi

I have a problem surface. Approx 45 degrees to camera and quite high spec values.

Why would some HDRI maps render ok and some will flicker on the same object?

The probes are the 'Usual Suspects' from http://www.pauldebevec.com/Probes/

Do I need to clamp some values or blur the image?

Here are the test videos

broken Grace probe
https://vimeo.com/51081733

broken kitchen probe
https://vimeo.com/51081201

working Uffizi probe. Yay!
https://vimeo.com/51081200

thanks in advance
 
Old 10 October 2012   #2
All 3 seem to have the same flicker. Not sure if it's low fg rays or simply it's the IBL being reflected and what we're seeing is correct. It's possible because the vehicle is full of flat surfaces. Maybe try lowering the reflection glossiness? What is it set to now?
 
Old 10 October 2012   #3
One way I use to troubleshoot scene flicker is to beak up the shot into the basic aov passes and then render over a 10 or 20 frame portion where you have known flicker. This way you can at least identify the area that needs further investigation, ie is it indirect, reflection or a specular issue?
 
Old 10 October 2012   #4
@ pap87

reflectivity = 0.35
glossiness = 0.5

metal highlight = on

IOR = 50

I'll try lowering these values

@ JasonA

good idea. I'll get rendering those passes

cheers
 
Old 10 October 2012   #5
What version of Maya?

Are you just doing turntables?

A lot of the probes on Debevec's site (especially the Grace Cathedral) have a lot of contrast and hot spots.

Couple things to remember about lighting with FG only:

1. FG doesn't really generate shadows. Just a soft occlusion-like effect. You will lose some details with this method
2. High value and contrast HDRs introduce more variance/flicker

If you are using mental ray 3.7 and up I would suggest using the Native (builtin) IBL for mental ray. This isn't the same as the Maya IBL emit light options, these are string options.

It's an importance sampled dome light that bakes whatever is plugged into the camera environment, including a procedural or texture in the Maya IBL slot. Then use FG with much lower values for accuracy to fill in secondary lighting effects.

You will find renders with the Native IBL (or user_ibl in Maya 2013) are much more detailed and stable. Typically omitting a need for ambient occlusion passes as well. It may take longer for a frame at first glance but you'll notice the quality is much much higher than what you could achieve through older methods in the same time per frame (so basically: it's faster). It's also fairly automatic so you can hit 'render' without much tuning at all.
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Old 10 October 2012   #6
@bitter

I'm using Maya 2013 and just rendering simple turntables at the moment

I found your tutorial http://elementalray.wordpress.com/2...bl-shaders-pt1/ and although I haven't tried the usr_ibl node yet I did try a few of the free HDRI from http://www.hdrlabs.com/sibl/archive.html

Again some of them worked and some of them didn't!

so next thing to try is the usr_ibl

@papa87 I turn off spec/reflectivity and I still got the flashing artefacts
 
Old 10 October 2012   #7
Originally Posted by uncle_frankie: @papa87 I turn off spec/reflectivity and I still got the flashing artefacts


Then it's definitely FG accuracy. Try using user_ibl like Bitter suggested, that'll nuke the problem away.
 
Old 10 October 2012   #8
Originally Posted by Bitter: Couple things to remember about lighting with FG only:

1. FG doesn't really generate shadows. Just a soft occlusion-like effect. You will lose some details with this method
2. High value and contrast HDRs introduce more variance/flicker


^^^ this ^^^

the classic trick is to create a low resolution blurred HDR file from the one your using via HDRshop or something simple. So if your hdr is 1600x1200 pixels downsize it to 400x300 pixels and apply blur (all this is do-able in hdrshop quickly) then save the file. That tends to create a softer FG result (ie much less flicker).

also the point interpolation option in Final Gather render options really helps things out too for smoothing some of the flickering.


have a look at this thread below.. low res HDR/FG result but high res HDR/FG reflections:
http://forums.cgsociety.org/showthread.php?t=957013

overview is there are 2 methods... use a nurbsSphere to create reflections and plug the low res hdr into the usual ibl sphere... or (2) use the mib_rayswitch (post #4 in the thread linky)


finally i would say as bitter says you dont get the solid shadows so it is always worth doing a shadow pass setup as necessary. I also like to do an ambient occlusion pass but tbh i stick all that in an openEXR file now really and fire it out in one go then comp it later.
 
Old 10 October 2012   #9
ok - two images for you

the first is with the standard IBL - approx 2.5 minute render

the second using the new usr_ibl with high glossy sample to reduce graininess - approx 10 minute render! and the shadows look terrible!

So next questions

can I reduce the render time for the usr_ibl and improve the shadows?

ps I owe you all a pint





 
Old 10 October 2012   #10
I would have to see the HDR, are you blurring it?

user_ibl does no baking of the HDR or acceleration cache. It directly samples the texture. (This is why I tend to prefer the Native IBL. It's a string option, not the IBL with "emit light")

I would also be sure to use Unified Sampling. You will get the best details this way.
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Old 10 October 2012   #11
@ anthonymcgrath

thanks for long post. I followed your instructions and the help from the web links on the forum posts but still no joy.

I'm going wrong somewhere - just have to find it!

@ bitter yes I'm now blurring the HDR

I'll tried again with the unified sampling options (and other string options) and blured hdri but my model still flashes crappy artefacts

I think I should maybe just use the hdri that works, render in passes and tweak in Nuke
 
Old 10 October 2012   #12


That image is user_ibl. I could render it with passes and Unified sampling at about 22-25 minutes a frame. The HDR is unblurred.
__________________
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Old 10 October 2012   #13
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