VFB: Background image feature causes sharp edges on Glare and Bloom

[kosso_olli]

Hey,
I have an issue here, where enabling the background image inside the VFB causes hard edges in places where Glare or Bloom are visible. Take a look at the screenshots for a better explanation.
It looks like the Lens Effects are not correctly blended over the background. Is this a known issue?

Max2017, V-Ray 3.60.03



Regards

Oliver

[Miro]

Thanks for the pointer.
We have the issue logged in our bug tracker and will post in this thread when we have an update.

[kosso_olli]

Can this be fixed at all? The issue is still there, even with the new lens effects system. I already talked to Asen about this. I hope a solution can be found.
Attached you can find three images with the new lens effects. The shot on black looks correct, but falls apart when the background feature in the VFB is enabled.

[^Lele^]

I am not sure a solution would be easy to do in the (current?) VFB.

The lensFX RE has no alpha as such (it's meant to be plussed), while the image has the one alpha which pertains to the rendered content.
From a logic standpoint, the lensFX would have to generate its own alpha to be blended with the image's alpha each time, making that alpha useless for further compositing work if it was saved that way, or the order of operations would need to change (add BG, then calculate lensFX), which would still result into a broken alpha as far as the lensFX go (ie. the alpha wouldn't change from the original, and the image would look just as it does now when loaded anywhere else.).
The third option would be to make the Bg image overwrite the alpha (to white), which again negates further compositing options.
As for the "composite" looking one way in the VFB, and save as another kind on disk (ie. fiddling with the alphas in the VFB only, to save them as they were originally), i'd call it wrong from a data handling perspective.

This is exceedingly trivial to do in compositing, but that is because the user knows what the result is going to be (for both the image and the alphas).
I'm really unsure on how this should be tackled to not be an awfully confusing, and potentially dangerous, piece of technology.
The new VFB may have the options to handle this stuff, but it wouldn't be a checkbox, or a default behaviour, requiring just as much work as doing it in post (ie. set layers, fiddle alphas, and so on).

How would you expect it to behave, having read the above?

[kosso_olli]

How would you expect it to behave, having read the above?

Well, since you mentioned the lens effects should be plussed: Why not do it that way? The lens effects have the ability to be generated as a seperate layer. This could be added on top of the rendered image and background internally, no? This would avoid any masking of the lens effects, or am I totally wrong on that one?
I didn't check how this is handled in other renderers, yet.

[^Lele^]

Well, since you mentioned the lens effects should be plussed: Why not do it that way? The lens effects have the ability to be generated as a seperate layer. This could be added on top of the rendered image and background internally, no? This would avoid any masking of the lens effects, or am I totally wrong on that one?
I didn't check how this is handled in other renderers, yet.

You are right, in that respect: i was mentioning this when stating that the order of operations should change (ie. bg merged with Render, plus lensFX. Alpha wouldn't change.).
What is complicated is what should happen on file save.

*) Say we did this, and we saved the image with the lensFX, and the BG, but the original Alpha channel.
It would mean having the same issues as now in the VFB if you tried and do any compositing on it (say, change BG again, or some selective CC. And because some have post departments, it's not straightforward to reload the "comp" in the VFB and make changes there, provided they could be done at all.).

*) Say we created a lensFX alpha channel through the image luma, and merged it with the original alpha in the VFB, you'd then be unable to remove the lensFX alpha from the "geometric" one after saving the file.
The alpha is intended to be in 0-1 range. we could make it go above that, and then one could remove the lensFX alpha. but it wouldn't be canon, and i am not sure it'd be well accepted, as other things would break.

I have no idea of how others deal with this specific aspect yet, but it's a bit of a catch 22 regardless.

[kosso_olli]

Well, when saving a single RGB image, I would save what is displayed in the VFB (background added with rendered elements, plus lens effects added afterwards, so we get no crazy fringing)

When saving all channels as seperate files, the VFB should always output the lens effects as a seperate render element, which then gets added in post in whatever the application is with the same math as in the frame buffer. I would not fiddle around with any alpha for the lens effects, that makes things complicated.
Maybe it is because I am missing knowledge, but to me this should all be very simple

[^Lele^]

It's two different behaviours within the one save action, i think it as inherently wrong from a data handling perspective, on top of being confusing.
You have this issue right now, but think of all those which wouldn't have it, and face the double behaviour still. (The save single image also saves an alpha.)

[kosso_olli]

Well, see it this way.
With the following render elements available: Background, RGB, Glare and Bloom I can easily get the correct result in any application (for us it's Photoshop).
Why the VFB is unable to display this correctly is beyond me. It has everything it needs, but it can not use it properly.

[^Lele^]

I explained to you why it isn't as trivial as you make it to be, and why it's very doable with any other post application.
The current VFB isn't a compositing application, it was never marketed as such, and it's not going to turn into one: you'll have to wait for the new VFB to be ready.
Then, you'll have to make just as much work as you would in any other compositing app, but you'll be able to do it from inside the VFB.
For now, i'd say PS (or equivalent) is your best bet.

p.s.: Corona doesn't modify the picture alpha either. And it doesn't have a background option available.
We should gray the BG option out when lensFX are used, i would say, for the time being.

[kosso_olli]

We should gray the BG option out when lensFX are used, i would say, for the time being.

No, don't do that! For quick preview purposes I would rather live with the error then not having a background in the image.

[kosso_olli]

Hi! Are there any improvements on that matter?

[yavor.rubenov]

You can now put the Background layer in the Composite:



In this way it should work with the Lens Effects correctly.

[kosso_olli]

Thx, did not know this. Will try it!