Corona Renderer Forum

General Category => I need help => Topic started by: zules on 2017-11-08, 18:59:52

Title: Real Cool Alpha
Post by: zules on 2017-11-08, 18:59:52
Hi,

i'm getting a bit crazy here as I cant never have real alpha cutout on my renders.
Whatever I try I always get an edge all around, even when using Cache>Remove Stroke (that is for me a non-solution : they shouldnt be here).

Am I the only one here getting tropuble with alpha cutout ?

Thank you !
Title: Re: Real Cool Alpha
Post by: cecofuli on 2017-11-08, 19:04:07
You have to render with black background.
Title: Re: Real Cool Alpha
Post by: zules on 2017-11-08, 19:04:54
You have to render with black background.

Really ?
Title: Re: Real Cool Alpha
Post by: maru on 2017-11-08, 20:06:02
Yes, really:
https://corona-renderer.com/forum/index.php?topic=17065.msg107040#msg107040
Title: Re: Real Cool Alpha
Post by: Jadefox on 2017-11-08, 20:28:36
i've noticed with volumetrics on the black overide is not completely black, obviously it has a haze to it
Would this impact the above ?
Title: Re: Real Cool Alpha
Post by: zules on 2017-11-08, 20:33:18
Yes, really:
https://corona-renderer.com/forum/index.php?topic=17065.msg107040#msg107040

Thanks to both of you, i'm trying right now (on a final render :/ )
Title: Re: Real Cool Alpha
Post by: Njen on 2017-11-09, 02:23:04
And remember if you need to grade or colour correct an image with alpha, first you must unpremultiply the image, then grade, the premultiply the image, otherwise you will be colour correcting the partially transparent pixels incorrectly.
Title: Re: Real Cool Alpha
Post by: zules on 2017-11-09, 11:44:41
And remember if you need to grade or colour correct an image with alpha, first you must unpremultiply the image, then grade, the premultiply the image, otherwise you will be colour correcting the partially transparent pixels incorrectly.

Mmm not sure to understand...

But, actually rendering on a black BG worked well, thank you !
It's not very convenient (imo) because :
- it adds a setting to get what the alpha is made usually for
- when using a black BG, it's visually disturbing while rendering to have a black sky for example (and much more to use Lightmix as a correct exposure of the sky is imo important for an overall correct exposure)
- if a client want finally to change your sky but you rendered without a black BG, you're trapped.
Title: Re: Real Cool Alpha
Post by: Juraj Talcik on 2017-11-09, 12:27:26
Imho this was always a very poor limitation that could possibly be bypassed by introducing a pass that will act as if black override exists in the background but is at same time replaced by the actual Sky.
This would also alleviate the issue with Bloom&Glare which doesn't exist with black override. The current work-around forces you to choose and brings in ton of drawbacks. It's poor.

The second issue than Njen writes can be bypassed if you use mask as opposed to multiplication. (So for example in Photoshop, load you image as "with alpha" option and then use that alpha as mask, not opacity).

Title: Re: Real Cool Alpha
Post by: zules on 2017-11-09, 16:24:14
Imho this was always a very poor limitation that could possibly be bypassed by introducing a pass that will act as if black override exists in the background but is at same time replaced by the actual Sky.
This would also alleviate the issue with Bloom&Glare which doesn't exist with black override. The current work-around forces you to choose and brings in ton of drawbacks. It's poor.
>>> this is typically what Alpha channel is made for no ?

The second issue than Njen writes can be bypassed if you use mask as opposed to multiplication. (So for example in Photoshop, load you image as "with alpha" option and then use that alpha as mask, not opacity).
>>> I always work with alpha as a mask, I don't even know how to use it as opacity...