Alpha VRayMtl Blend problem with domelight

[snivlem]

Sorry, maybe I forgot after the xmas break. But how do I stop the dome light from appearing in the alpha channel?

alpha.zip
I have attached an example for convenience.

Domelight is checked to be invisible.

Maya 2014x64
[2014/Jan/15|10:21:52] V-Ray: V-Ray for Maya version 2.45.01, revision 23970 from Dec 17 2013, 06:21:13
[2014/Jan/15|10:21:52] V-Ray: V-Ray core version is 2.00.02

[snivlem]

Just to clarify, this is happening when looking through a glass material. So I assume this is a bug.

Even though I have invisible ticked, the domelight is showing up through the alpha and beauty channel. I didn't expect it to do this.

[tashko.zashev]

Hi,

You see the Dome light reflections over the objects. If you uncheck "Affect reflections" of the Dome light parameters you can avoid it.

[snivlem]

Thank you Tashko,

I would not of thought "Affect reflections" is linked to alpha visibility.

[tashko.zashev]

Thank you Tashko,

I would not of thought "Affect reflections" is linked to alpha visibility.

Hi,

Its because the reflection takes away the refraction strength. In other words lowering the reflection strength of your material will give you more refraction.

[snivlem]

I understand, but it does not make sense to me that the alpha should be 100% opaque if a dome light happens to be directly behind the glass.

It seems I do not have the same issue if I delete the dome light and with the exact same material settings use a V-Ray sky instead (which shows expected alpha channel results).

I'm really confused.

[vlado]

The alpha seems to come from the reflections, specifically the "reflect on back side" option. The invisibility of the light only affects the light when it is directly visible, or when it is visible through purely refractive bounces. As soon as it goes through a reflection bounce, it becomes "visible". Not sure what would be the "correct" behaviour here.

Best regards,
Vlado

[snivlem]

Hello, something just doesn't seem to "add up" for me.

For instance, this is a current very early WIP scene (with rough draft settings so hence the noise).

My windows are highlighted in the below picture. Since they are not full 100% transparent they should be taking away from the pure white I have set as the background. (The domelight is hidden)



In the beauty pass, this is confirmed as measured in PS, the windows are 77% white in RGB. Everything is working as per expected so far.

However when I get to the alpha channel, surely I'd expect to see the same result? ie roughly 33% black or 77% white?



Instead it is 100% white (100% solid quite obviously) which doesn't correspond to the beauty channel. This is just confusing for me.


However, I figured out since starting thread that I can set the offending materials reflection's "Affect Channels" to color only. This makes the alpha channel work as expected for me. I get an alpha channel that is not pure 100% white but arount 5% white.

[svetlozar.draganov]

It seams that this is a bug with a DomeLight/Meshlight.
I've done a few tests with different type of lights and this issue only persists when Reflect on Backside is turned on and Dome/Mesh-light are used.
It doesn't persist with Rectangular/Sphere lights or with Environment Override textures when Reflection Backside is ON.

I will forward this case to our developers for further investigation.
Meanwhile you could avoid it by turning off Reflect on Backside option in Reflection parameters of the glass material or using Environment Override option instead of DomeLight.

[snivlem]

Thank you Svetlozar. I thought I was going crazy.

[snivlem]

Hi I found a better way, just plug the glass shader into the wrapper material. There is no alpha that is linked to opacity but you can customise it and the best solution.

turning off "backside option" does not always work.

[snivlem]

Hello, I was just wondering if this bug been fixed?

I'm still only on 2.4 - but I'm having a nightmare rendering an interior at the moment on a project.
The "reflect' backside option doesn't fix it in this case. Nor does setting the window with .001 diffuse. V-Ray just renders glass with a solid alpha channel.

My suggestion above unfortunately removes the glass entirely I discovered so we lose that nice reflection from glass.

[tashko.zashev]

Hi,

We have added "Affect Alpha" option for the V-Ray Dome light, which will be very useful for your case. Unfortunately this option is only available in version 3.0
Would it be possible to send me the scene file just to check your setup and try to find a workaround for you?

[snivlem]

Hi, thanks for the reply. Sorry but it took a long time but I figured out this is a bigger bug than just the domelight.

I switched the project over to vray sun + sky but it turns out that it still happens without a dome light present, so I gave up and just removed windows from my project entirely.

[2014/Nov/19|09:18:11] V-Ray: V-Ray for Maya version 2.45.01, revision 25052 from Oct 10 2014, 08:18:04
[2014/Nov/19|09:18:11] V-Ray: V-Ray core version is 2.55.01
2015 W7 Ext1 SP5

See attached for sample scene for a reproducible error - the windows should render as a solid alpha

alphaFail.ma.zip

[snivlem]

If you delete the pCube1 primitive, the alpha renders correctly which makes no sense as the glass is clearly not opaque as seen in the beauty render, it seems the reflection is way too strong and physical camera is not taken into account.