Help bouncing photons (and other stuff)

[jsunandmax]

Hello, Corona newb here so apologies in advance if this has been answered elsewhere but i couldn't find it.

1)  How do i get more "bounce light" into my scene?  I have an exterior scene with a building w/ windows i want to see inside (see image).   I need more light to 'bounce' inside.  I can't fix w/ exposure because that would blow out the exterior...

2)  I"m finding white materials get blown out w/ Corona sun intensity set to 1.0.  Is this normal?  Is it better to try and correct this w/ extreme exposure settings or just lower sun intensity (i've gotten down to .2 or .3).  Or does it matter?

3)  Is there any way to 'force' an old-school reflection map?  I would like specific materials to reflect an image (as opposed to the environment, or some HDR sky model used for lighting).

As always, thanks in advance!  Loving the renderer so far and looking forward to gaining some competence.

-jason

[Brock_Lafond]

Have you placed any light portals in the scene?

If you want the interior to be brighter, but maintain the exposure for the exterior, you wil have to fake it. I would do this kind of thing in post. Render to a 16-bit or 32-bit, including a masking layer and adjust the various exposures in a compositing app. Having this kind of exposure will make the image feel unnatural. Look at some reference photographs and you should see what I mean.

I would definitely avoid playing with the Sun intensity. Keep it at 1 and adjust exposure as necessary. Keep it physically real and physically based.

[jsunandmax]

Thanks Brock,

Seems like there SHOULD be a way to do this without post/composite but i defer to then knowledge on this forum.

Maybe I should ask this differently;  is there an 'ambient light' value anywhere?

I suspected as much re: Sun intensity....i'll try to leave it.

Thanks again, any future opinions welcome also!

-j

[romullus]

First of all your scene is severely underexposed. By a 2-3 EV at least. Should you expose properly, you'd see that interior would be much brighter than it is now. If you still need more light there, then best option would be to save to 32bit float format and use tonemapping in external application.

Also check albedo pass, i see quite strong colour bleeding, probably your materials are set up incorrectly. Take a closer look at those two chrome torches - i suspect them to cause those nasty fireflies in your image.

[jsunandmax]

Romullus, thanks for the reply.

As mentioned, simply fiddeling with the exposure is not the solution.  To debug this, i deleted the sun and made a new one default values.  Then i created a new CoronaMtl and left it default, assigned to every object in scene.  Finally I hit 'reset settings' in the Render Setup -> Scene tab.   This is my results:

Any help appreciated.

-j


ps. including my first render, an interior where things 'just worked' without much fiddeling

[Juraj]

You can use a negative exposure as well (it's exposure bias parameter, it can be 0, -2, +2,etc.. ) and you should use tonemapping (highlight compression >1).

There is basic lighting tutorial on Corona's Youtube channel.

[maru]

I just want to fix some things from this topic:

-I think (not sure, though) that you have wrong gamma settings. Please ALWAYS use these settings when using Corona:
https://coronarenderer.freshdesk.com/support/solutions/articles/5000515647

-light portals will not affect the scene's appearance in any way, they will only make rendering faster in certain conditions. Portals WILL NOT make anything brighter or darker.

-in OP's pictures the SKY is extremely dark. So do not change sun color, change sky! If you want to keep dark sky but have bright light illuminate your scene anyway, you should use bright color in 3ds max env and effects (8 key), and the dark bitmap in Corona's "direct visibility override" in render setup > scene


-working in Corona usually requires a bit different thinking than working with other renderers - it is best to keep everything as physically correct as possible, so for example in OP's pictures the sky is very dark, and sun is strong - this doesn't often happen in real life, right? But you can for example decrease sun's intensity and increase its size (this is not entirely physically incorrect as sun in real life could be behind clouds, then it is weaker and more blurred). Think of rendering with Corona more as photography - with any camera you would probably get either very dark interior and fine exterior, or fine interior and overexposed exterior.

[jsunandmax]

I'm still confused.  If i have reset everything to 'default', and i get that image called "bright" up above, whats wrong?
I didnt START adjusting sun/exposure etc.  thats where I wound up because defaults ("real" physical settings), give me blown out white image.

suggestions welcome,
j0

[-Ben-Battler-]

Yes, and now increase Highlight Compression to something higher than 1.0, just like Juraj wrote. The biggest difference in HC happens between 1.0 and 3.0, so you should start with value 3.0, then you should be able to see something. Further you can decrease Exposure if the scene is still too bright.

I think your problem could easily be solved by adding a CoronaSky to the Environment Slot and setting a reasonable Highlight Compression. Maybe you would have to decrease the reflectance of the windows a bit. As maru said, your sky is very dark. Right now you are trying to illuminate a whole room only with the direct light from the sun. I think it would help if some skylight from a CoronaSky reached in there too.

[romullus]

I'm still confused.  If i have reset everything to 'default', and i get that image called "bright" up above, whats wrong?
I didnt START adjusting sun/exposure etc.  thats where I wound up because defaults ("real" physical settings), give me blown out white image.

suggestions welcome,
j0

In order to successfully render with any physical based renderer, you have to have at least very basic understanding what exposure is. There is no realistic or unrealistic exposure. Your render may be exposed correctly or underexposed / overexposed. In Corona you can adjust exposure at any time with arbitrary EV number or with photographic exposure controls. Once correct exposure is achieved, you can further tweak it with tonemapping controls, which are quite limited in Corona, but can do pretty decent job under certain circumstances. Those controls are Highlight compression and Contrast. I'd suggest to watch this introductionary tutorial:

[jsunandmax]

yes, i've done all the basic tutorials.  I am a photographer and understand "exposure".  I included the interior image to show i am capable of using software.
Guess i'm not going to get an answer...thanks anyway.

[Juraj]

But people gave you fantastic, in-depth (Romullus) answers.

The default "exposure" in framebuffer is 'arbitrary' (ok it's not I know,..but it's not "sunny f/8", or something common,etc..), so if you place your physical Sun (AND sky in environment, it's one button click within Sun now),
it will be over-bright because:

1) It's fully linear, ie: Highlight compression=1.  Solution: Set this to higher number, for example 8.


2)       A) Exposure parameter in Framebuffer is set at 0. Set it -2 for example.
     OR B) Use photographic exposure (Shutter/Aperture/Luminance) and it will match to your real-life expectations.