MR/Maya : Still getting blotching with Very high FG settings ... any thoughts?

[Chog Burley]

hi all,

 

have just done a rough set up for some interior views in Maya2010 rendering with mental ray - physical sky / sun with one portal light.

 

was getting quite alot of FG blotching in my tests, but usually upping the settings takes care of this. in this case however, despite v high settings (for me at least) blotching is still apparent.

 

FG Accuracy - 500

Point Density - 2

Interpolation - 50

 

typically i could render with 100, 1, 30 and be ok.

 

any thoughts on what i could try?

 

- upping the interpolation smooths it out but starts to lose details in the corners a bit.

- could try the min/max raduis but this usually causes me more problems than it solves!

 

attached are two views (one with adjustments to bring out blotches) and the FG settings.

 

cheers,

 

Cb.

[Dragonearl]

Hi CbCb,

 

To start with i would recommend that you use at least 2 portal lights for the scene 1 on each window.

 

The main problem with this scene is that there is no direct light into the room as the sun direction is from the front illuminating the room at the rear. If you can i would change the lighting direction.

 

If you can not the best way you can solve this is use an area light on the ceiling.

 

thnx

[Chog Burley]

thanks for your reply Dragonearl,

 

unfortunately there's not alot of scope for changing the light direction as it's a real site and the rooms at the rear don't receive direct sun light at any time of day.

 

also, the view in the front room (with direct sunlight) still has blotchy areas.

 

i'm really hoping there is something i'm missing here. surely just because an area is lit with diffuse light doesn't mean it has to be blotchy?

 

good point with the portal - i'll try adding some more and see if that helps.

 

thanks,

 

CB

[Dragonearl]

OK so if the light has to be a fixed position at the front there are a few things that you can do.

 

The main problem is that the blotches are caused from the fact that mr can not bounce the light around enough. there are a few tricks you can try.

 

1. add a plane outside from the walls all the way until you fill up the view, then make sure its primary visibility is turned off in the render stats (so that it does not show at render time) this will enable the fg rays to bounce off this and in through the windows giving you more rays.

 

2. Your glass may be blocking rays as well if you are using mia glass. I little workaround for this is, make a fg map that has the glass hidden and then use that calculated map with the glass turned on in the final render. This will get more light in and allow the fg to pass in the windows freely.

 

3. If you still have a problem add in area lights to the ceiling and enable the light shape option as rectangle and balance the light with the scene (you will need large values in your scene with light shape i.e 50,000 +)

 

hope this helps

[Chog Burley]

cheers mate, good tips.

 

yep, mia_glass on the windows. i was thinking of turning of 'cast fg' for the glass objects which should acheive a similar result?

 

the other tips are useful too so will give them a shot.

 

i always kind of want to get things working 'purely' though if you know what i mean. no fill lights, no bounce cards etc. never seems to work though ...!

[Dragonearl]

np fella i wouldn't turn of fg casting before trying a map as it may cause more problems that it fixes.

 

The quest for perfection is a wonderful premise but untill you have real world values with physically accurate rendering and materials that use corect light density's with correct reflection etc values its a pipe dream.

 

But pure is ts fake with a kick up the backside and telling everybody you did it without cheating i.e LIES lol

[Chog Burley]

portals seem to be doing the trick, so thanks for that. had used them a lot in the past, but then on one particular project they seemed to make little differecne to the quality. that space had rooflights though so perhaps that allowed enough rays in. each project is different ...

 

always tricky getting them right, how they sit with the window frame and the shadows, noise. etc. adjusting the shadow ray extension helps but still not ideal.

 

will post some follow up images later.

 

cheers,

 

CB.