Coloured Glass Issue With Alpha Channel

[rsuleyman]

Hello,

I'm having an issue with coloured glass. When I save the image with the alpha channel enabled, the glass has no colour. When I save it with alpha chanel disabled, the colour is there. The problem is that I don't want the HDRI included as I want to place a background image separately in Photoshop.

I've added some crops to explain what I mean. As you can see, the image with alpha channel diabled shows the blue glass as it should be but the one with alpha channel enabled shows it without any colour.

I'm using absorbtion medium to get the colour but I've tried it with transmission and it has the same result. I'm using the Pathtracing kernel and I have fake shadows enabled. Switching off fake shadows has no effect. I've also tried a separate HDRI for the background set to Visibility Environment but with no success.

Does anybody have any suggestions?

Thanks,

Rico

Alpha Disabled

Alpha Enabled

Composited Image

Alpha Enabled With Affect Alpha Disabled

[bepeg4d]

Hi rsuleyman,
enable the Affect alpha option in the Common tab of your specular material:

ciao beppe

[rsuleyman]

enable the Affect alpha option in the Common tab of your specular material

Thanks Beppe,

But I should have mentioned that the first three images in my initial post were with Affect Alpha enabled. The problem is that the colour of the glass isn't there when the alpha channel is applied.

Rico

[bepeg4d]

Hi Rico,
sorry, my mistake, I have not read carefully your post.
I have tried to play a bit with render passes and visible environment but I wasn't able to have back the absorption in the alpha part.
Standalone behaves in the same way, so I have sent a simple scene to the devs, let's see if they have the answer.
I'll keep you informed.
ciao beppe

[mojave]

We are aware of this and it is in our todo list, at the moment this is a limitation, so the background should be rendered as an environment node.

[abstrax]

We are aware of this and it is in our todo list, at the moment this is a limitation, so the background should be rendered as an environment node.

To be more specific: It is not possible to compose colour filters using normal alpha compositing. You would need an alpha channel for each colour channel (i.e. 3 alpha channels) and if the result is saved with pre-multiplied alpha, it's possible to achieve what you want to do. As far as I know, none of the compositing tools allow something like that. Please correct me if I'm wrong here.

[bepeg4d]

Hi Rico,
I have made some experiment about this issue and I think to have found a different solution that doesn't involve compositing.
The idea is to use the Visible environment for placing any kind of image as backgrond and use it aslo for refraction reflection.
Here is an example scene:

AffectAlphaRL.zip

(2.45 MiB) Downloaded 456 times

The Primary environment is the Physical sun (but can be also an HDRI):

the Visible environment is a Texture environment with a generic 8bit image file:

The projection is changed from Spherical to Prospective with negative Y axis:

All the Visible environment option are active so the background is also visible in refraction and reflection.
What do you think about this solution?
ciao beppe

[rsuleyman]

Thanks for your help Beppe.

I had tried that, and it worked. The problem is that I need to be able to adjust the background separately in Photoshop. I don't want to have to re-render the image if the client wants the background brighter or moved a bit.

I managed to add the colour manually in photoshop anyway and it has worked for now. I just wondered if there was something I was missing. I was sure it was possible in previous version of Octane but I could be mistaken. Perhaps I'm thinking of VRay.

Anyway, thanks for all your help.

Rico

[GIOLETS]

I have tried to download the scene with 3.06.2 but the results are not correct, I have about half image showed

Could you specify how to load correctly a background image that fit exactly the scene resolution? I've tried also to link the camera into perspective projection but with no success.
I ask that because every day we have to place a background image in a scene that render correctly using "keep background" feature.

Thanks a lot for any help.

Luca

[GIOLETS]

The solution proposed By Bepeg4d should be perfect, in that case we can have a correct background through glasses, and also see post effects ...
The only ptoblem is that I can't re-create the solution since if I try to load my image don't fit the live viewer (or picture viewer) window...