How do I put a backplate that isn't affected by color mapping & color corrections?

[Alex_M]

I'm trying to integrate some stuff into a photo. How do I put a backplate that isn't affected by color corrections and color mapping? This is really easy with Corona, you just use the CoronaOutput map - https://coronarenderer.freshdesk.com...color-mapping- Sorry for the long link, I'm on a proxy because I can't access the forums and the formatting bar at the top is gone.

How do I achieve this with Vray? I don't want to use the "Background Image" functionality in VFB because it's causing nasty artifacts around the edges of objects and it also doesn't work with my workflow - I'm not using the sRGB button and the background image becomes dark if I disable the sRGB button.

The only workaround I found so far is to use a bitmap with screen mapping in the Environment slot and then I spend time adjusting it in an Output map with IPR until I sort of match it to the original photo which I have open on my 2nd monitor.

Yea, it's a pain and I've got the feeling that I'm missing something very simple. I'll admit it, sometimes I am stupid enough to miss something that's been in front of my eyes all along. Any ideas on the "button" or the "map" that I'm missing? Thanks in advance!

[Alex_M]

Sorry, I forgot to add some images but the proxy won't allow me to edit my posts so I'm forced to add new replies. I should be twice as careful with my posts from now on...

Using the "Background Image" function:



Original photo:



How it looks when I don't use "Background Image" but the Environment slot instead. It sort of works but it's not ideal and takes more time to get right than I would like:

[thanulee]

If i understood correctly, maybe affect background off in color mapping is what u want? cheers

[Alex_M]

If i understood correctly, maybe affect background off in color mapping is what u want? cheers

Thanks for the suggestion, it does sound like the logical solution but the fact is it still doesn't work. Even if I have it turned on, all color corrections in VFB (exposure, curves, etc) affect the background. In Corona with the CoronaOutput map even if you make adjustments to exposure, curves, color balance etc. the background image doesn't change one bit.

[thanulee]

Well it does work, i mean color mapping doesnt affect ur bg. But yea, vfb is logical to affect ur bg image since its like doing it in post. I d suggest to dont use vfb corrections and do those stuff in post. Finer control anyways, and if u just use those controls wont take u more than few minutes to set em up in post. cheers

[Alex_M]

That's certainly a solution but not really for me because I want to do changes to the scene interactively while rendering with IPR and I need to know how the objects look in the context of the backplate while setting up the scene. It would really break my flow to jump from one environment to another constantly. I hate having to switch to other software for simple projects like this when it should be easy enough to do right in Max/Vray. I just need the backplate to not get affected, that's all. There must be a way. I checked all maps in the help pages and it seems that none works like corona's CoronaOutput map. I also checked for any sneaky button I might have missed in VFB or in Render Setup but I didn't find one either. Don't tell me I should switch to corona for these types of jobs.

I feel like the "Background Image" option in VFB is the thing I'm looking for but if this is really the analog of CoronaOutput map in Vray then I think it needs some looking at because it doesn't really work for me. 1st, when I turn sRGB off, the background image is affected too and I don't want it to since I don't use sRGB corrections in VFB, I simulate it with the curves and exposure. 2nd, for some reason using curves cause white halos around the objects. And last but not least, the background image is pixelated which is undesirable. Attached is a screenshots showing the halo and pixelation problems:

[LudvikKoutny]

There's currently no good way to do that. Closest is the "affect background" checkbox, but its use is very limited, and it does not work with progressive sampler, only bucket one. It also does not counter VFB adjustment, only those Reinhard adjustments in render settings.

[thanulee]

Yes Alex i get what u mean.
In situations like these personally, i change my lights in order to blend things better. And that is the logical way to do this if u wanna composite objects and not try to match em mostly in post. Post should always be complementary. If we talking a sky here, or a window opening it can completely done in post/vfb, but if we talking about proper compositing on a backplate from a shooting, u shouldnt do this through vfb adjustments.
Maybe ask the support for a better implementation of the background image in vfb if that fits ur specific needs better. have a nice day!

[^Lele^]

Alpha Channel?

[Alex_M]

Alpha Channel?

I'm not following you, can you elaborate?

[^Lele^]

sure, here.

Notice how exposure affects the render, not the BG.

[LudvikKoutny]

This almost looks like ultimate solution. It does exactly what it should when it comes to not being affected by color mapping, but it has two dealbreaker problems:

A, It does not filter image in any way. If you put there image of double resolution, you will get terrible aliasing.

B, It's just preview, it does not save with output when you save the file.

And before anyone says that, no... doing it in post is not a solution. The entire point here is to avoid postprocessing (going to different software to do this particular task).

[^Lele^]

oh, you want an inverse CM map, i see.
Nevermind my previous replies, i read further down the thread when the objective was a bit more muddled, EDIT: and i got your question wrong. Also, i was writing this while you replied.
it shouldn't be terribly hard to implement in principle, but i'm not the one to know, yet it still wouldn't save you from any VFB corrections you'd apply after the render, hence me saying save your alpha channel, and do a proper composite, if it's a backplate you need.
If you have edges you're likely doing something wrong on the comping side, or whacking both RGB and alpha so out of the original range that edges will come up and will have to be dealt with. Proper (un)pre-multiplication and separate channel work has been proven to work for many decades now.
If you end up with fringes, then you're likely asking too much of the rendered data (ie. too much brightening or darkening), but that only becomes apparent after a couple of stops either way of the rendered result.

See the attached Contactsheet (download is likely needed.).



EDIT: While inversing the CM for a given bitmap is surely doable (that's in essence what a vrayLUT or vrayICC do, given a forward descriptor), as i said you are then bound to whatever you will apply to the VFB.
Any "automated" implementation of the composite work i showed above inside the VFB (ie. separate BG controls, saving it with the render, and so on) would *not* sidestep the issue with the contrast gradient on semi-transparent edges being steepened by different CM applied to foreground and BG.
That is why one uses a post app: to have the tools to afford latitudes which require more severe alteration of the input data (ie, alpha work, edge recovery, and so on and so forth), where the tech at the avail simply can't achieve a satisfying (albeit correct, mathematically!) result for the specific needs.

[LudvikKoutny]

No, no, actually, when it comes to not being affected by color mapping, it works correctly, that's not what I meant.

What I meant is following:
Problem A:
If you put background on in VFB, VrayVFB will not properly scale the background using some filter. It will just pick nearest pixel to the grid, and use that. So if your BG doesn't match rendered resolution 1:1, you get ugly aliasing:


Where as if I use the very same map in Environment slot, even with bitmap filtering completely turned off, it still gets correctly scaled and antialiased when rendering:


The second problem is that if you want to utilize VFB as your final image delivery tool, you can not, because even if you display BG in VFB:

If you click save, it will saw raw image, without background:


Since Alex_M is asking about how to render unaffected background right inside of V-Ray, then obviously he is not interested to hear about how much better and flexible it is in post. If he was rendering it separately with alpha, and comping in post, then he would not need to worry about figuring out how to have BG unaffected by color mapping in the first place, would he?

[LudvikKoutny]

To kind of package my thoughts together into something comprehensive. Vlado stated on several occasions that he's not interested in improving affect background feature. But I had no idea that background feature of VrayVFB handles this workflow as well as it does. It correctly works even with shadow catcher and colored shadows.

So, if the problem with background image scaling causing aliasing was resolved and we'd got a checkbox that would allow us to save background within image file, the "affect background" checkbox in render settings would not be necessary and requested anymore.