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Mental ray interior question
show user profile  CAD Monkey
Hi guys,

I hope someone can help:

I'm working on an interior for a friend and it's of quite a gloomy extension for a house. There isn't a huge amount of natural light in reality, but she wants to show farrow and ball cauliflower white paint on two of the walls (it's a very pale teal green) The thing is that whatever I seem to do the walls either come out looking grey and drab or bleached completely :S I'm using the MR daylight system, with photographic exposure control and mainly lit by FG.

Is there anything I can do within object properties that will just allow the true colour of the bitmap swatch to show through? I'm going to have a try with render passes(not really sure what I'm doing) but on my underpowered PC this could take a week!

Any ideas gratefully received!

Cad monkey

Today is a gift, and tomorrow is not a promise, so love life!

read 753 times
4/28/2014 12:01:46 PM (last edit: 4/28/2014 12:01:46 PM)
show user profile  mike_renouf
Some tips.

Throw some photon mapping in there and use sky portals too. That should get more light into your scene without cranking FG up to ridiculous levels.

Post a screen grab of what you're getting now, and maybe I can suggest some options?

read 746 times
4/28/2014 1:01:55 PM (last edit: 4/28/2014 1:01:55 PM)
show user profile  CAD Monkey
Cheers Mike,

I'm using sky portals, but I've had to take the multiplier up to 10 for each whereas usually 1-1.2 is fine :S

I've also turned GI off so that I can just light the scene with FG (GI killed the render times by hours!)

Here is where I am so far:

The 2 walls you can see should be this colour:

Here are my renders settings:

Here is the other view of the room:

The left hand wall and ceiling are meant to be white, with the other 2 walls cabbage white / cauliflower white.

I hope you can help mate as I'm struggling like buggery with this and I want to just get it finished!

Many thanks in advance!!


CAD Monkey

Today is a gift, and tomorrow is not a promise, so love life!

read 716 times
4/28/2014 7:58:06 PM (last edit: 4/28/2014 8:02:43 PM)
show user profile  mike_renouf
Well there are a bunch of things that could be improved with your render - none of which are really going to help directly, but I'll list them anyway a bit lower down.

To get the walls the correct colour your quickest and easiest option will be to photoshop your scene. There's usually going to be some post production required even in a really great render if you want to match real life finishes to a good level of accuracy.

The main issue you have with your scene is that the indirect illumination (final gather and/or photon map) is very low resolution, blurry and speckled. The other factor is that the indirect illumination is doing exactly what it should do based on real physics - its showing colour bleeding. Your floor is yellowy brown, hence the white surfaces show a yellowy brown tinge from the bounced light coming off the glossy floor.

1) a quick fix would be to calculate the final gather map while you have a plain white override material applied to the scene.

2) The other is to use a ray-type-switcher material on the floor, so that it doesn't cause bounced light to use its colour information when calculating indirect illumination:,topicNumber=d30e410439

3) The thing that really stands out though is the "initial FG point density" of 0.1 is way too low for a good render. At that low level essentially you're bouncing a few samples of light into the room and then blurring them around so much that everything comes out with the same level of illumination. Its just one big smudge of light. You can see the problems easily when you look at the corners of the rooms - they're not shaded correctly and there are no defined shadows from the windows. Also there are no contact shadows at the base of the cylindrical stools.

4) Use ONLY the arch&design materials with mentalray.
5) Turn on ambient occlusion within the a&d materials to get you some cheap clean shadows at the corners of the rooms
6) for the walls you can get away with "use highlight and FG for glossiness" or whatever its called.

These are just some general tips.

This is what you REALLY need to study:


Here's an old scene of mine. I've shown the Indirect illumination settings next to it, but couldn't face letting it render for a whole hour. Note the clean shadows around the edges of the walls and the ceiling.
FG settings photo FGsettings.jpg

read 701 times
4/28/2014 9:54:31 PM (last edit: 4/28/2014 10:00:16 PM)
show user profile  CAD Monkey
Hi Mike,

Thanks for taking the time to detail all of that for me - you're a star!

I will have a go at changing the FG settings. I seem to have gone from one extreme to the other, but I found this tutorial on Youtube giving some instructions:

This seemed to help but I think I'm still on low settings :(


CAD Monkey

Today is a gift, and tomorrow is not a promise, so love life!

read 677 times
4/29/2014 12:44:53 PM (last edit: 4/29/2014 12:44:53 PM)
show user profile  CAD Monkey
Hi Mike,

Sorry to be a pain, but when I have run a final gather map should it look perfectly smooth (as you'd want the final render)? I ran one at 800x600 with your FG settings (0.8,100,100,1 & 4 bounces) and my finished render (of the FG map) looked awful - like made of lego. Is this normal?

Sorry for all the questions, and thanks again for the help!!

Cad monkey

Today is a gift, and tomorrow is not a promise, so love life!

read 659 times
4/29/2014 8:54:34 PM (last edit: 4/29/2014 8:54:34 PM)
show user profile  mike_renouf
It should be pretty smooth at those settings. I expect that the interpolation of the FG points is not working correctly. Difficult to say what the cause is.

One thing to bear in mind is that if you save an FG map you can try reusing it with different interpolation settings without recalculating it repeatedly.
Try keeping the FG Map and then just progressively increase "interpolate across no Points". Essentially the Lego blocks you described are the actual light samples FG has recorded. The interpolation is the smudging of these blocks to create a useful illumination solution.

read 657 times
4/29/2014 9:11:38 PM (last edit: 4/29/2014 9:18:07 PM)
show user profile  CAD Monkey
Cheers Mike,

I'll take a look. I'm generating a FG map at 680x460 with the AA turned down a bit. I'll run both maps and give the renders a go. I've also checked that AO was selected with all the mtls. I know it was on for the rug, but as you say - if the settings weren't high enough it won't show up.

I've got the Initial point density up to 2.0 now so hope that should be ok.

Thanks again.

CAD Monkey

Today is a gift, and tomorrow is not a promise, so love life!

read 646 times
4/29/2014 9:41:50 PM (last edit: 4/29/2014 9:41:50 PM)
show user profile  CAD Monkey
Here's my FG map :(


Today is a gift, and tomorrow is not a promise, so love life!

read 639 times
4/29/2014 10:06:28 PM (last edit: 4/29/2014 10:06:28 PM)
show user profile  mike_renouf
Think I might have confused you - sorry. That actually doesn't look too bad for the FG map - if its just the FG map. The FG map itself won't be smooth - the final render will be thought. Don't forget - the final render process will blur the FG map depending on the interpolation settings.

If thats your final render, then there's a problem with the anti-aliasing, not the FG.

Decent shadows will now depend on your sky portal samples, and you can "cheat" by using the AO settings on the materials, to bring out detail and contact shadows.

read 627 times
4/29/2014 10:47:25 PM (last edit: 4/29/2014 10:51:42 PM)
show user profile  mike_renouf
There is one other issue too which I missed before. At join between the floor and the window there is a light leak. This could be fixed with a false bit of geometry behind the bottom part of the window to block the sunlight.

read 623 times
4/29/2014 10:56:58 PM (last edit: 4/29/2014 10:56:58 PM)
show user profile  CAD Monkey
Thanks again Mike!

I noticed the light bleed, but will have to PS it to get it done - should have modelled a block or threshold on the outside of the door :(

I'm running the renders at the moment with the FG map and will post up the results and also the photoshopped images - let's hope I can get this finished :)


CAD Monkey

Today is a gift, and tomorrow is not a promise, so love life!

read 601 times
4/30/2014 11:20:52 AM (last edit: 4/30/2014 11:20:52 AM)
show user profile  CAD Monkey
Hi Mike,

Thanks for all the help. Here's what I ended up with after a bit of PS. They're a long way off where they need to be, but were passable for what was required:

Thanks again!!

CAD Monkey

Today is a gift, and tomorrow is not a promise, so love life!

read 568 times
5/2/2014 4:59:20 PM (last edit: 5/2/2014 4:59:20 PM)
show user profile  mike_renouf
Nice result! Glad you got the renders to where they needed to be.

read 545 times
5/2/2014 8:21:45 PM (last edit: 5/2/2014 8:21:45 PM)
show user profile  Nanne
What was the final render times for these images? :)

Kristoffer Helander
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read 541 times
5/2/2014 9:41:16 PM (last edit: 5/2/2014 9:41:16 PM)
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