I've been trying to use the +UA (output Alpha-channel data) option to
create sprites with transparency information.  Normally, it works like a
charm.

But if I want to render, say, a glass sphere with something opaque
inside it, the opaque bit still turns out transparent.  It looks as
though POV-Ray just takes the transparency data of the first surface it
strikes, and though it continues to calculate the RGB values by
splitting the rays, etc., it doesn't seem to update the transparency
component any further.

Is there a way around this?


-- 
Jeff Lee         shi### [at] gatenet        http://www.gate.net/~shipbrk/

  "The only thing that helps me maintain my slender grip on reality
  is the friendship I share with my collection of singing potatoes."

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Jeff Lee wrote :
> 
> I've been trying to use the +UA (output Alpha-channel data) option to
> create sprites with transparency information.  Normally, it works like a
> charm.
> 
> But if I want to render, say, a glass sphere with something opaque
> inside it, the opaque bit still turns out transparent.  It looks as
> though POV-Ray just takes the transparency data of the first surface it
> strikes, and though it continues to calculate the RGB values by
> splitting the rays, etc., it doesn't seem to update the transparency
> component any further.
> 
> Is there a way around this?
> 

You're right: I have the same problem in a scene with a planet (sphere)
surrounded by a semi-transparent ring.

Here's my solution: I create another scene with the same camera and objects. You
remove the light_source.
And: the background is black, the opaque objects are white, and transparent
objects vary from rgbt<1,1,1,0> (transmit=0 in the original scene) to
rgbt<1,1,1,1> (transmit=1 in the original scene).
Of course, don't forget to set finish { ambient 1 diffuse 0 }.

POV-Ray will directly render a 256-gray picture: the alpha channel you need.

It works, but it needs more work than automatic alpha-channel rendering.

___________________________________________________



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On Sat, 24 Apr 1999 03:35:10 GMT, shi### [at] gatenet (Jeff Lee) wrote:

>I've been trying to use the +UA (output Alpha-channel data) option to
>create sprites with transparency information.  Normally, it works like a
>charm.
>
>But if I want to render, say, a glass sphere with something opaque
>inside it, the opaque bit still turns out transparent.  It looks as
>though POV-Ray just takes the transparency data of the first surface it
>strikes, and though it continues to calculate the RGB values by
>splitting the rays, etc., it doesn't seem to update the transparency
>component any further.
>
>Is there a way around this?

That's the behaviour that lead me to believe that more powerful alpha
channel control will be of use. Ever tried the infamous
chrome-sphere-on-a-checkered-floor scene with +ua? If you only need
the sphere, you'll need to make another scene with it only. See the
other reply for a solution.

---------
Peter Popov
ICQ: 15002700

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