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Rendering transparent objects with Alpha

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Greg Platt
Rendering transparent objects with Alpha
on Apr 24, 2011 at 4:11:19 pm

This seems like it should be an easy thing but we'll see. I'm trying to render a transparent object to composite into another scene and I just can't seem to get the alpha channel to behave like I expect. (maybe my expectations are off?)

In the image below for example, I've got a simple torus with a blinn shader on it at about 50% transparency. In the render view it shows up with nice specular highlights. However the alpha channel is simply a straight 50% grey, completely disregarding all the lovely highlights. Taking this into AE or photoshop of course gives me a very dull image at 50% opacity. How can I get the transparency to render appropriately?

Am I over simplifying this? Is it not possible to render a transparent object without using render layers and getting complex?



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Tibor Bernscherer
Re: Rendering transparent objects with Alpha
on Apr 24, 2011 at 5:06:53 pm

Hi,
The alpha channel contains information about light, so this picture is actually perfect, as you can see wherever it has any kind of light, it is "colored". I'm not exactly sure what you want to achieve but alpha channel isn't the appropriate to show transparency, it is just not designed for that.

Hope it helps


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Greg Platt
Re: Rendering transparent objects with Alpha
on Apr 24, 2011 at 6:11:26 pm

The alpha channel appears to be looking at the transparency value of the material and is applying that value across the board. However in actuality, some parts of the image should be more transparent than others based on where the light is hitting it. ie. In the specular highlight, I shouldn't be able to see through the object just because the material itself is transparent.

If I take this render and apply this alpha channel to it, the entire image is going to be 50% transparent, which isn't how light would really react. Does this make more sense?


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Tibor Bernscherer
Re: Rendering transparent objects with Alpha
on Apr 24, 2011 at 9:46:33 pm

Hi,
Not really, but mit might be only me. I only use alpha channel for compositing in photoshop or after effects if I need to, then I render a colour and and alpha channel and mount the two layers together using overlay or multiply, digital tutors had made up a really good tutorial covering this topic (lighting). As far as I know alpha channel does not contain information about transparency, when I render an alpha channel, it's usually snow white, but I might be wrong, sorry.


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Greg Platt
Re: Rendering transparent objects with Alpha
on May 1, 2011 at 9:30:02 pm

Thanks for the replies Tibor- still trying to figure this out. Here's another image to demonstrate what I'm seeing. This sphere has a blinn texture with 50% transparency. It looks fine in the render view but taking it in to any other program (Photoshop/After Effects) basically just gives me the RGB information at 50% opacity. Clearly not what I need.

How would I render an object with transparency to be composited with a different program?


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Steve Sayer
Re: Rendering transparent objects with Alpha
on May 2, 2011 at 8:02:04 pm

Greg, everything you've said certainly makes sense.

Unfortunately I think you're going to have to use render layers. If you're doing something as complex as compositing transparent objects together (and that IS a complex task, for exactly the reasons you're having difficulty with this--transparency is not a simple phenomenon), you will be glad of the extra control anyway. Split your render up into a diffuse and specular pass, and screen the specular results on top of the composite--that's the way to get accurate and controllable behaviour.

(That being said, there MAY be a way to change how Maya calculates that alpha channel, but I'm not sure off the top of my head what it would be.)

-Steve


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Greg Platt
Re: Rendering transparent objects with Alpha
on May 2, 2011 at 8:44:31 pm

Thanks for the reply, Steve!

This definitely makes sense if my final product will be a full screen composited frame. Here's the tricky bit though. Suppose I need to hand off a single composited asset for use in other mediums such as a video game. I need to be able to comp these two render passes together and hand off a single file. Even if I screen the specular pass on top, the background will still be 100% black.

This definitely seems to be one of those "trickier than it seems it should be" type of tasks...


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Tibor Bernscherer
Re: Rendering transparent objects with Alpha
on May 2, 2011 at 11:53:06 pm

So is that an animation you have to hand in? If it's a single picture, all you have to do is just bring the layers into ps and composite them using overlay composition and save it as jpg or any other format. If it's an animation, you'll have to batch render each and every layer in Maya and then bring it into ps and batch export them. Unfortunately in this case you won't be able to use After Effects or video editing software as they don't like alpha channels (as far as I know) but I might be wrong. I tried it and didn't work for me.

Good luck


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