@Troyan (Troyan) posted a new topic, subject "Transparency in a volumetric light". It read:

   Ok, here's another one. I've made an object that has a gradient in the Alpha channel to make the bottom of it fade out. Looks fine in a normal render. Problem is if I put the object in a volumetric light, you can faintly see the transparent parts of the object. I've attached a simple object rendering to show what I mean. The bottom of the sphere should be completely transparent. I've tried a compositing tag but nothing seems to help. Any suggestions?
    
    Thanks!

To which @JoelDubin (JoelDubin) replied

    Do you have dust turned up on your volume light?

To which @Troyan (Troyan) replied

    No pwest@example.com. Not even any noise, that's just volumetric light. try it yourself, shouldn't take more than 15 seconds to set up. Happens even without using a gradient, just put 100% black in your alpha channel.

To which @JoelDubin (JoelDubin) replied

    top image is dust set to 100  
    bottom–dust set to 0
    
    maybe thats the culprit?  
    You should theoretically be fine using an alpha and volume lights.

@JoelDubin (JoelDubin) replied

    hmmm–just saw your post after posting the images.
    
    Well–it should work (works here)  
    Im on 11.0 however.

After which, @JoelDubin (JoelDubin) added

    You're right–solid alpha causing a slight ghost of the object. Don't have the answer at hand unfortunately.
    
    I think this has come up a few times over the years—if you havent done a search maybe give it a shot.
    
    EDIT!
    
    I take that back. I still had dust turned on in my light when I did the last test. Removing dust makes my sphere with a solid black alpha render totally invisible.

So @JoelDubin (JoelDubin) replied

    heres an example illustrating my edited comment above.  
    top image dust =100  
    bottom dust = 0
    
    You mentioned that dust is turned off in your light, so not sure what else it can be.

@Troyan (Troyan) replied

    Sorry it took so long to reply, we've had a lot of company.
    
    Send me your simple setup. I tell you, we can't get 100% transparency in volumetrics here, and there is no dust. Take a look at the attached images, including the settings. The outline is not as clear as yours when the dust is cranked up. It's faint, but it's there. It's really noticeable in the bone scene we have, which is one of the attached images. All I'm doing is dropping a light in and turning on volumetric visible light. That's it.
    
    Don't turn on any noise, just straight volumetric.
    
    EDIT: Make sure you use a spotlight and not an omni. I think it intensifies it.
    
    EDIT EDIT: Under your visibility tab, uncheck “Use Falloff”. You should be able to see it then. With falloff, the closer the light is to the object, but more intense the transparent areas become.

@JoelDubin (JoelDubin) replied

    Heres mine. Looks fine to me with a simple sphere and light, but It might help to see your scene. Maybe theres something up in the materials. Not sure whats happenning.

@JoelDubin (JoelDubin) replied

    heres the same scene with use falloff unchecked.
    
    I tried this in my 11.5 demo, and getting the same results.
    
    Im on a mac. Maybe if you post your simple file others can confirm as well.

@Troyan (Troyan) replied

    JoelD:
    
    > Heres mine. Looks fine to me with a simple sphere and light, but It might help to see your scene. Maybe theres something up in the materials. Not sure whats happenning.
    
    Yep, looks fine here too. Now, I've tweaked the angle of your light and the wideness of the cone. Render the attached scene and tell me you don't see the bottom of the sphere, very faintly.
    
    EDIT: Each one of your images, the light is behind the object. Light it from more of a frontal angle.

So @Troyan (Troyan) replied

    Ok, I've dropped another shape into your scene that I tweaked and filled the alpha channel with pure black. Can you tell from the attached image what shape is in the light?

So @JoelDubin (JoelDubin) replied

    Try reducing your sample distance from default (25) to something like 5 or 6.

To which @Troyan (Troyan) replied

    JoelD:
    
    > Try reducing your sample distance from default (25) to something like 5 or 6.
    
    Yep, that did it. The original bone scene is a little more intense and I had to crank it all the way down to 1 to get them to disappear. I had cranked it down to 5 earlier today but was still seeing a slight outline, so I dismissed that as not it. Thanks for the help, Joel :). Something like this can be maddening to find the answer.

@JoelDubin (JoelDubin) replied

    Maybe check your scene scale (you could just scale everything up). Odds are your bones might be very tiny in relation to the light scale. When I work with imported objects I try to scale them up so they are at least as big or larger than 200m (when possible), as you will begin to have shadow map issues.

So @CGTalk_Moderation (CGTalk_Moderation) replied

    This thread has been automatically closed as it remained inactive for 12 months. If you wish to continue the discussion, please create a new thread in the appropriate forum.


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