Problem with alpha channel in front of transparent plane - Polyloop

Tassilo · 8th February 2007, 11:18

In a scene I'm working on there's a girl sitting before a transparent and reflective glass plane. The girl is of course not really a girl, but a .pz3 with an .hr2 on it's head. But that does not matter, important is just, that the hair is made up of alpha mapped strands.
Behind the glass plane you can see the HDRI map.
When I render the scene, I get misrendered alpha channels in the hair, as you can see in the crop. When I hide the glass plane (or when I make it just reflective, not transparent), the rendering goes fine.
By setting it to just 70% transparent and nothing else I finally found out, that it's the transparency that's messing the image up.

Anybody got a clue what this is and how I could work around it?

I'm working on a G5 Mac (3.5Gigs) with C5.1 Pro.

Thanks a lot in advance!

bwtr · 8th February 2007, 11:35

Why not make life easy and just have the frame for an imaginary glass plane?

LoneGunman · 8th February 2007, 11:50

If you are using GI, turning indirect lightning ON should solve your problem, I think

Tassilo · 8th February 2007, 12:34

Quote:

Originally Posted by LoneGunman

If you are using GI, turning indirect lightning ON should solve your problem, I think

No, no GI here. I will try to render it with Indirect lightning anyway, thanks.

rickei · 8th February 2007, 17:18

not to ask a stupid question, but you do have light through trans. checked, in your render options right??

Tassilo · 8th February 2007, 17:51

Quote:

Originally Posted by rickei

not to ask a stupid question, but you do have light through trans. checked, in your render options right??

Yes, I do ;)

rickei · 8th February 2007, 18:33

You are using HDRI without GI?
That will only provide you with a backdrop. you have to at least have Sky Light checked for thr HDRI to illuminate.
So you must have other lights in the scene. right?
I would try to hide my lights and then render with Sky Light or Indirect light, and let the HDRI do the lighting for you. it could help???

Other than that all i can see from your renders, is that maybe the hair color, does not extend past the alpha chanel.

another stupid question. Your alpha map is in the Alpa chanel of the shader? Sometimes people stick it in the tranparancey chanel by mistake?

Tassilo · 8th February 2007, 19:49

Quote:

Originally Posted by rickei

You are using HDRI without GI?

Yes, I'm only using it for more realistic reflections. Lightning is done by two directs and two bulbs plus a little bit ambient.
I tried the scene without HDRI background, same problem.

Quote:

Originally Posted by rickei

another stupid question. Your alpha map is in the Alpa chanel of the shader? Sometimes people stick it in the tranparancey chanel by mistake?

It is in the alpha channel, yes.

For now I helped myself this way: I removed the transparency out of the glass plane and gave it an alpha value. That way, the hair rendered correct.
So the problem seems to be alpha in front of transparent.
I'm going to do some test renders in a blank screen later to find out more about the problem.

Thanks a lot so far for your assistance!

jones2000u · 15th February 2007, 17:12

I might have had a similar problem that was solved by Mr Mark Bremmer.

I was wrapping a transparent alpha around a bottle and got artefacts and a haze in the bottle at render stage, even though it looked completely transparent in preview. The solution was to increase raytrace depth in the transparency and related shader values to around 16. Then my alpha became completely transparent in the render.

Tassilo · 22nd February 2007, 12:00

Quote:

Originally Posted by jones2000u

I was wrapping a transparent alpha around a bottle and got artefacts and a haze in the bottle at render stage, even though it looked completely transparent in preview. The solution was to increase raytrace depth in the transparency and related shader values to around 16. Then my alpha became completely transparent in the render.

Thanks a lot! That fixed it. Seem to have been too many transparencies above each other.
You made my day!

jones2000u · 22nd February 2007, 12:06

Happy to pass on the knowledge!

8th February 2007, 11:18	#1 (permalink)
Tassilo Illustrator Join Date: Mar 2006 Location: Cologne, Germany Posts: 62	Problem with alpha channel in front of transparent plane In a scene I'm working on there's a girl sitting before a transparent and reflective glass plane. The girl is of course not really a girl, but a .pz3 with an .hr2 on it's head. But that does not matter, important is just, that the hair is made up of alpha mapped strands. Behind the glass plane you can see the HDRI map. When I render the scene, I get misrendered alpha channels in the hair, as you can see in the crop. When I hide the glass plane (or when I make it just reflective, not transparent), the rendering goes fine. By setting it to just 70% transparent and nothing else I finally found out, that it's the transparency that's messing the image up. Anybody got a clue what this is and how I could work around it? I'm working on a G5 Mac (3.5Gigs) with C5.1 Pro. Thanks a lot in advance! Attached Thumbnails

8th February 2007, 11:50	#3 (permalink)
LoneGunman SILO abuser Join Date: Jan 2006 Location: Louisville, KY USA Posts: 556	If you are using GI, turning indirect lightning ON should solve your problem, I think __________________ My missions are not impossible, I just make them look that way

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8th February 2007, 11:35	#2 (permalink)
bwtr Brian Join Date: Jul 2005 Location: South Australia Posts: 2,094	Why not make life easy and just have the frame for an imaginary glass plane?

8th February 2007, 17:18	#5 (permalink)
rickei Respect the Dawg! Join Date: Dec 2005 Location: USA - Conway, South Carolina Posts: 791	not to ask a stupid question, but you do have light through trans. checked, in your render options right??

8th February 2007, 18:33	#7 (permalink)
rickei Respect the Dawg! Join Date: Dec 2005 Location: USA - Conway, South Carolina Posts: 791	You are using HDRI without GI? That will only provide you with a backdrop. you have to at least have Sky Light checked for thr HDRI to illuminate. So you must have other lights in the scene. right? I would try to hide my lights and then render with Sky Light or Indirect light, and let the HDRI do the lighting for you. it could help??? Other than that all i can see from your renders, is that maybe the hair color, does not extend past the alpha chanel. another stupid question. Your alpha map is in the Alpa chanel of the shader? Sometimes people stick it in the tranparancey chanel by mistake?

15th February 2007, 17:12	#9 (permalink)
jones2000u NURBS Booleans are your friend Join Date: Jun 2006 Posts: 134	I might have had a similar problem that was solved by Mr Mark Bremmer. I was wrapping a transparent alpha around a bottle and got artefacts and a haze in the bottle at render stage, even though it looked completely transparent in preview. The solution was to increase raytrace depth in the transparency and related shader values to around 16. Then my alpha became completely transparent in the render.

22nd February 2007, 12:06	#11 (permalink)
jones2000u NURBS Booleans are your friend Join Date: Jun 2006 Posts: 134	Happy to pass on the knowledge!