Alpha Contribution ignored Affect Channels on Refractive Material

[beat_reichenbach]

My file has a VRayEnvrionmentFog, a box with a default material, and a window with a glass material.
The glass material uses the Affect Channels: Color+Alpha.
This behavior works well when rendering the scene normally, the box's alpha is white, and visible through the glass, and the glass is basically see through on the alpha channel. (See image top half)When I set the alpha contribution to -1, the alpha is now black, instead of see through. (See image bottom half)
The documentation says: "whereas if the object is fully transparent, it will not influence the alpha at all." But in my example the object is transparent, but it does influence the alpha.

This would solve the alpha issue in this case, however for the rgb channels, I wish there was also a "see through option" if the color is see through, but that is a different topic that might not make a whole lot of sense out of context.

[vladimir_krastev]

Hi this effect is related to the environment fog. Currently this is the way it affects the scene's alpha. Here is an old thread with a similar setup:

https://forums.chaos.com/forum/v-ray...-matte-objects

The fog affects all objects alpha channel and when you set the box's alpha to -1 it still is affected by the fog's alpha.
Try to switch off the environment fog and everything starts to behave as in the docs.

[beat_reichenbach]

Consider a scenario where you want to render the fog as a separate pass to composite on top of other renders. This is a very classic setup for a fog pass where you set everything to matte objects so that you can merge this render over another render.
Now consider the box being a normal object and the glass being a car window, I would like to see the fog through the car window. With the current settings, it just turns opaque though and we can't see the fog through the window. One option would be to hide the car window but that is not optimal in case where you have a texture on the glass that controls the alpha (for example a sticker).

So telling me to remove the fog, doesn't help this problem at all. The fog is a crucial part of the problem.

[vladimir_krastev]

Yes what I meant was the opaque alpha that we end up with is not due to materials refraction channels.
There is a feature request for the environment fog's alpha contribution in our system under VMAX-9793.
I will add your thread to it and bump the priority.
As a work around you could render twice - with fog on/off. Then use the VRayAtmosphere render element to compose the final image.

[beat_reichenbach]

Ah thanks for the clarification!