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View Full Version : Weird render artifacts - bright spots on corners of small objects



dsol
04-05-2014, 12:44 PM
Hi chaps,

I'm tearing my hair out trying to work out what's causing this issue in my latest LW project. It's an array of mobile phone screens with an image overlaid (done using instancing of course). It is all working really well, apart from one massively irritating and distracting issue, which is that as the camera pulls back and the individual phones get smaller, their corners suddenly become intensely bright white spots.

Here's a render of one frame:
121195

Note the bright spots on the corners. I wondered if it was a render artifact, so put on limited region and rendered it at 400%. And then it got REALY weird. Here's what I saw:
121196

So you can see the spots are super-bright. I've checked all the surface settings. The surface that the spots seem to be centred on is a very basic one - it's just using basic settings (no nodes or materials), with a specular setting of 9%. Has anyone got any idea what might be causing this? It's really distracting on the finished thing and the client has noticed it! Anyone who can solve it gets bought a LOT OF BEER :)

spherical
04-05-2014, 03:14 PM
Turn on Limit Dynamic Range.

What Color Space are you in? If you haven't chosen sRGB, then the lighting will be way too strong.

On a different topic, it may not be what I think it is but what sticks out at me is that, if the screens are showing reflection of the environment, those reflections should all be orientated the same, not follow the rotation of the instance.

XswampyX
04-05-2014, 04:09 PM
It looks like a 2 point poly problem to me.

http://i465.photobucket.com/albums/rr16/xXswampyXx/Phone_2pPolys_zps26132767.jpg (http://s465.photobucket.com/user/xXswampyXx/media/Phone_2pPolys_zps26132767.jpg.html)

Open the model in modeler and press 'w', do you have any polys with 2 vertices? If so click on the little '+' next to them and delete them.

Try your render again.

Maybe. :D

dsol
04-05-2014, 04:33 PM
Thanks fellas. I tried all those things - in fact, I'd already turned on "Limit HDR" anyway as I am rendering to PNG (I'm rendering the final thing on an external renderfarm, so downloading loads of EXR files would take too long for my deadline!).

In the end I think I worked it out - it seemed to be caused by the background environment map having some super-hot spots. I fed it into after effects and dulled down some of the worst offenders and it seems OK now. There's definitely something quirky about how the LW renderer handles small areas of detail though - is this a micro poly thing perhaps?

spherical
04-05-2014, 10:51 PM
Unsure why the Limit Clamp wouldn't have taken care of this.

What Reconstruction Filter are you using?

dsol
04-06-2014, 03:41 AM
I tried a couple of different reconstruction filters (gaussian, classic, lanczos). All had the same issue. I would've tried rendering in EXR format, then clamping in post, but (I thought) it would have taken too long to download the finals. Though the issue has resurfaced in the final render (from the farm), so now I wish I had!

spherical
04-06-2014, 03:09 PM
So you're using a cloud farm? That's why the reference to EXR/PNG, above. Well then, more info is needed.

dsol
04-06-2014, 05:05 PM
Naw, these bright spots appear on my own renders. They also appear on the farm's renders. It's an HDR image probe thing, I'm pretty certain. If I rotate the imageworld backdrop around 180 degrees it goes away. But it is annoying that this happens in LW. I suspect it's something to do with too much energy being concentrated in too small a spot

- - - Updated - - -

The promo this is for is about to go out anyway, so as soon as it does, I'll happily share the project files if anyone's curious to pull it apart and find out what caused it ;)

dsol
04-06-2014, 05:06 PM
I also did a test rendering in EXR format (for one still), with HDR limiting removed, then tried clamping it in After Effects. That didn't work either - which makes me think LW has generated an image with spookily high concentrated levels of HDR super-whites in those corner spots.

Thanks for all the suggestions though guys - I love this community! :)

spherical
04-06-2014, 09:07 PM
The promo this is for is about to go out anyway, so as soon as it does, I'll happily share the project files if anyone's curious to pull it apart and find out what caused it ;)

Looking forward to it, as now I'm really curious.

Andy Webb
04-08-2014, 04:09 AM
Saw your promo last night on Discovery, very nice. :thumbsup:

You certainly seem to have solved the artefact issues.

:D