Environment and Exposure settings changed automatically between renders

[RebelSaffa]

I have this really annoying problem with my environment and lighting settings.
Ill set up the scenes lighting, do a test render and its all perfect. I then go carry on modelling and setting up other objects.
When i go back to render from my cameras perspective the settings are exactly the same as the first render but my image is either overexposed or under exposed. Or a mixture of both but not the same settings as the first render.
The settings are also exactly the same so i cant work out where these settings are being changed or why.
I've disabled Auto Exposure from the environment tab thinking that maybe when i exit the camera and use the render in viewport mode that it may somehow affect my camera settings. but again it changes the lighting.
I'm using the HDRI OSL with a background image attached.
Any help with this would be appreciated. I have absolutely no idea how to fix this and its counter productive setting the lighting just right only to come back and it be off again.

[romullus]

I'm not sure if that's your issue, but you should not use any exposure control in enviroment and effects window when rendering with Corona. The renderer does not use those settings and it's better to turn them off.

Render settings should not change on their own between renders and exposure should remain exactly the same, unless you change geometry to an extent that it affects scene's lighting significantly.

[TomG]

Some example screen grabs to show what you mean would be useful. It would be useful to know what you mean by "setting up lighting" - does this involve adjusting the tone mapping? (so perhaps there is some override somewhere that undoes this later, such as the camera overriding the VFB). Do you use LightMix? (are you checking the LightMix view rather than Beauty later?) Is there a tone mapping override someplace (where effects only show after a render is restarted, which could mean restarting the render later gives a different result).

[maru]

Maybe the issue lies somewhere in the OSL map used for the lighting? What exactly are you using there?

Another thing that comes to my mind is how the Max Sample Intensity affects the rendered image. It can result in the perceived exposure differences, like this: https://coronarenderer.freshdesk.com/support/solutions/articles/5000517899

[RebelSaffa]

He Guys thanks for the response.
In terms of what i mean by setting up the lighting its an OSL HDRI with background attached, corona sun and the tweaks i did inside the corona tone mapping and environment tab.
In really simple terms. Something is changing either the Environment lighting or the exposure.
I cant be sure which one as none of my settings look different, everything has stayed exactly as i left it. Its either super bright or super dark when i go back to render.
So something is cause these settings to change somehow between rendering.

Ive attatched my first render that was how i wanted the lighting and the second two are screen grabs of what happens without me touching anything between renders.

[TomG]

Can you share a grab of the OSL? Why use it btw rather than just a straight HDRI in a bitmap?

[RebelSaffa]

In all honesty its inexperience with Corona and 3ds Max. I switched from blender and its a completely different way of doing things.
I started the lighting using the OSL and as i could just attach a background image to it i managed to get the lighting the way i liked it.
So i wouldn't know how to replicate this exact look using a different approach.

[TomG]

I'd try without that OSL, since it has things like exposure etc. in it. See https://coronarenderer.freshdesk.com/support/solutions/articles/5000501978-how-to-set-up-hdri-environment-

(basically just drag and drop the bitmap into the Max Env slot or the Corona Env slot)

[RebelSaffa]

If i drop the HDRI in the Env slot how do i go about setting my background up?
When i place the background in the over ride slot it is super dark.

[TomG]

That page has the info on how to add a background override (e.g. for placing a backplate in the scene, rather than use the HDRI as the background image). Are you sure you need to override the background though, the HDRI doesn't seem to have much detail in it? As a note the HDRI will be both the background image AND the lighting by default, which may be all you need.

Generally, overriding the background that is seen is done using a backplate, a regular image, rather than an HDRI.

Anyway if you choose to use a backplate with an override you can adjust it using the Corona ColorCorrection control (or directly in the Output section of the Bitmap parameters) and then avoid it being affected by tonemapping changes by using the Corona Tonemap control, allowing you to avoid having the backplate affected by changes in exposure etc. in the VFB. See https://coronarenderer.freshdesk.com/support/solutions/articles/5000522247-corona-tonemap-control-map

(so backplate into bitmap optionally into ColorCorrection if you want to use that to adjust its setting, then ColorCorrection into the Tonemap, and then the Tonemap into Direct Visibility override).

Mostly here though the first question is just do the random changes in exposure stop happening once you are not using the OSL shader (even if the results aren't what you want yet - let's eliminate the possible culprit, and if that is the cause, then you can explore how to set it up to your liking without it :) )

[RebelSaffa]

@Tom G,
Sorry for the late reply we've been put in another lockdown in the UK and haven't had a chance to reply to this till now.
Thanks so much for that information, ill give that a go and see if it stops the Exposure problems I'm having.

Again thanks for the detailed explanation. Appreciate the help.

[RebelSaffa]

I made the switch and it works really well.
But one thing i found that I'm not quite sure how to change is my background image comes out super overexposed when i place it in the direct visibility over ride.
Is there a way to keep the image identical to the one I've got underneath the preview window?

[TomG]

Great! Must be something odd in that OSL that was causing it.

Assuming there is no adjustment to the JPG in the bitmap, that's where the tonemap control comes in. Uncheck the aspects of the VFB that you don't want to affect the image (e.g. Exposure, but perhaps also tone mapping and LUT). One thing to remember is that even with the tonemap, if you change the exposure in the VFB (e.g. during IR), you will need to restart the render for the background image to go back to being unaffected by that exposure.

[RebelSaffa]

Sorry that was my mistake, i forgot to uncheck them.
Works so much better then the OSL. Its pretty strange that the OSL can just change exposure values on its own.
But yeah this method is solid.
I have one last question regarding the Background over ride.
Is there a preferred way of adding control to the images brightness?
Would i plug the Tone Map into a Corona Colour Correction node or something similar?

[TomG]

There is the Output section of the Bitmap itself that can be used, or you can use a Color Correct if you prefer (but BEFORE the tonemap :) ). Glad all seems to be working as intended now!