Huge bug - double rendertimes with black backdrop

[Pavlov]

Hi,

in Corona we miss a very important feature, which is Unpremultiplied Alpha.
Without it, any compositing operation will have issues.

So we have to use an "old" workaround, which is rendering with black visible backdrop. Anyway, as soon as you try to do it, you will get double rendertimes.
So we have a missing feature and a bug in the unique workaround possible.

Hopefully this will get a fix, it's like that since Corona 8 or so.

Best
Paolo

[Aram Avetisyan]

Hi,

Aren't you able to to the premultiplication of the Alpha in the compositing software?
Corona provide the alpha channel data, given you save in the corresponding format, which you can use anyway you want, if the software lets it.

How exactly are you using the black backdrop?
There is no way it should double the render times. You can simply use it as environment overrides, e.g. for direct visibility (background) in the Render Settings > Scene tab.

[dj_buckley]

Hi,

Aren't you able to to the premultiplication of the Alpha in the compositing software?
Corona provide the alpha channel data, given you save in the corresponding format, which you can use anyway you want, if the software lets it.

How exactly are you using the black backdrop?
There is no way it should double the render times. You can simply use it as environment overrides, e.g. for direct visibility (background) in the Render Settings > Scene tab.

It absolutely does result in a huge render time increase.  I posted about it back in 2021 https://forum.corona-renderer.com/index.php?topic=34688.msg191184#msg191184

[maru]

"Unpremultiplied Alpha" - That's not a Corona/3ds Max feature, but the 2D editor's, right? You can render an image with an HDRI background, or anything else, without using the black background solution, load it in a compositing software, and specify premultiplied/unpremultiplied alpha there. There were many forum threads in the past where we shared how exactly the same data loaded into Photoshop looks "wrong" compared to Fusion or Nuke.

"Scene rendering longer on a black background" - The link points to a thread where the issue was related to using the noise limit. Can you confirm that everything is working as expected with pass or time limit? That is, the render time is higher when using noise limit only, and when using pass limit or time limit, there is no performance drop when using a black background.

[dj_buckley]

"Scene rendering longer on a black background" - The link points to a thread where the issue was related to using the noise limit. Can you confirm that everything is working as expected with pass or time limit? That is, the render time is higher when using noise limit only, and when using pass limit or time limit, there is no performance drop when using a black background.

Is there any way this behaviour can be improved?  I'd be interested to see, maybe run a poll, on how many users user Noise Limit over the other options.  If I had to guess I'd say noise limit comes out on top by some distance.

[maru]

We will be happy to help with both issues. Do you have some simple scenes which you could share?
Regarding the black background slowdown - I was able to find a report for Cinema 4D and it's scheduled to be investigated (fixing it based on a C4D report would also fix it for 3ds Max). Of course we can log additional reports with additional files, so if you have an example or simple reproduction steps, please do share them.
Thanks!

[dj_buckley]

Regarding the black background slowdown - I was able to find a report for Cinema 4D and it's scheduled to be investigated (fixing it based on a C4D report would also fix it for 3ds Max). Of course we can log additional reports with additional files, so if you have an example or simple reproduction steps, please do share them.

By asking us to submit scenes, are you suggesting that you're unable to replicate the slowdown at your end?  Surely you have a tonne of scenes by now, submitted by users that you can test on?  Simply open any scene, render, then render again with environment override directly visible set to black and compare the render times?  The heavier the scene the better.

[maru]

User scenes demonstrating specific issues are always valuable for us. We will also try reproducing this based on the steps you mentioned, thanks.

[romullus]

The heavier the scene the better.

Isn't this the other way round with this specific issue? I think complex scenes where background takes relatively small portion in terms of calculation time, are not affected as much as lightweight scenes, particularly single object renderings against monotone background.

[dj_buckley]

The heavier the scene the better.

Isn't this the other way round with this specific issue? I think complex scenes where background takes relatively small portion in terms of calculation time, are not affected as much as lightweight scenes, particularly single object renderings against monotone background.

I was thinking along the lines of this, a heavy scene, the black background takes up roughly half the image, a scene that already takes a while to render so any shifts in rendertime will be noticeable rather than negligble.

[maru]

Here is one test, Corona 11 HF1. The scene looks fairly simple, but it has lots of objects and takes over 20 GB while rendering.
The one with black direct override actually rendered a bit faster. Other than that, the stats are almost the same (rays/s).

[dj_buckley]

Here is one test, Corona 11 HF1. The scene looks fairly simple, but it has lots of objects and takes over 20 GB while rendering.
The one with black direct override actually rendered a bit faster. Other than that, the stats are almost the same (rays/s).

What's it like using noise threshold instead?  Also 20GB is small :)  The example I posted used over 120GB

Also could it be the massively overexposed sky causing the other one to render slower?

[maru]

I could reproduce the issue when using the noise limit. It was discussed here and Romullus correctly explained what is going on: https://forum.corona-renderer.com/index.php?topic=34688.0
With a black background, there is more contrast in the scene, so even if you render the scene to the same quality, the reported noise level will be different.
I agree that this is problematic since the same noise limit value should guarantee more or less the same quality. Using a black background override is a common workflow, for example when replacing background in post.

As a workaround, I would suggest using the noise limit with an additional pass and/or time limit. For example, you can set your noise limit to 2%, but you may not want to render your frame for more than 1 hour or 200 passes.

We do have this reported internally.
(Internal ID=1310588860)