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View Full Version : Constant Alpha Value does not seem to work in LW8?!



Deep_Thought
09-18-2005, 06:35 AM
Hi folks! I am just playing around with Dave Jerrards explosion tutorials. Take a look first:

http://user.cs.tu-berlin.de/~andrasz/data/Explosion_0043.jpg

Obviously the shock ring obscures the glowing sparks behind it. To avoid just that the alpha value of the surface has to be set to constant 0 via Surface-> Advanced -> Alpha Channel -> Constant Value = 0.
I did that to the effect of absolutely nothing. The ring still is opaque.
Quite the same goes for the explosion sphere in the center. I have applied an Edge Transparency Shader, so it looks a bit better than the ring. But it still suffers the same behavior, i.e. obscuring the sparks behind it. You can see that the center of the explosion is somewhat blackened and less particles are visible.
Does anyone know how to resolve this? What am I missing? It does not sound too complicated to me to simply set up the constant alpha value, but obviously I did something wrong.
Any help is highly appreciated.

toby
09-18-2005, 04:49 PM
Changing the alpha of an object will not make it transparent in the render. Only in the compositing program or Photoshop. Apply the ring texture to the transparency channel, you may need to invert it.

You may still have problems though, because both the particles and the glow are post-processes - rendered after the 3d objects.

Deep_Thought
09-21-2005, 06:24 AM
Alas, it seems that you are right. Well, I paste two pictures from the tutorial and the text of Dave:

>>>
15a: http://user.cs.tu-berlin.de/~andrasz/data/15a.jpg 15b: http://user.cs.tu-berlin.de/~andrasz/data/15b.jpg

Figure 15a. The shock ring combined with the particle explosion. Note how the ring obscures the glow effect on the particles behind it.

Figure 15b. By setting the ring's Alpha Channel to a value of 0, the glow effect shines through unobscurred. <<<

That's exactly my problem and when I used this tutorial first, it worked out just fine. That had to be with LW6 IIRC. I mean, how did Dave generate those pictures if not like he said he did?
It seems that LW8 handles the Alpha Channel different than the older version.
All my experiments with transparent textures and so on yielded the same result: Object eats glow effect of everything behind it, no matter if the object is transparent or not.
I think what I am really looking for is not transparency but rather some kind of dissolve which can be applied by a texture.
The color textures of the ring applied to its transparency channel made the black part of the ring indeed transparent, but it still obscured the glow effect _and_ altered the colors of the objects behind it. Even so at 100% transparency.
So I am looking for a technique which allows me to generate pictures like the 15b of Dave, i.e. allow a glow effect to shine through transparent objects.

MonroePoteet
09-21-2005, 03:06 PM
From my experiments, this looks like more of a bug in ray-traced transparency than alpha channel processing. I haven't done the tutorial you're talking about, but made up my own little scene and goofed with it. Ray-traced transparency allows the HV's to show up, but trashes the glow (it also trashes Partigons altogether!).

In a similar scene (big bang and shock ring), I ended up using a rough-edged clip map on the ring object just inside where the transparency went to 100%. That fixed the issue for me.

mTp

Dave Jerrard
09-21-2005, 03:48 PM
****. Looks like this got broken over the past few years. The article was done with LW 5.x, so I have a feeling that after LW6 came out, the alpha trick got busted. Hopefully this can be fixed before 8.5 comes out. I'd like to be able to use this trick again myself.


He Who Hates To See His Old Tricks Get Broken Like That.

Deep_Thought
09-21-2005, 03:56 PM
Oh thanks man, you're assuring me I am not mad. :-)
I _knew_ it worked a few years ago!
It looks like the whole transparency thing is screwed up since it eats up the glow effect every time. Maybe that went overboard together with your alpha trick.
Anyway, your tutorial is awesome and gave me lots of hours of fun. I am still playing around with it and try to blow up something nicely.

>>> We all like to blow things up and the bigger the boom, the better! <<<

Oh how right you are! :-) Keep up the good work!

toby
09-21-2005, 09:59 PM
Figure 15a. The shock ring combined with the particle explosion. Note how the ring obscures the glow effect on the particles behind it.

Figure 15b. By setting the ring's Alpha Channel to a value of 0, the glow effect shines through unobscurred.

You sure he wasn't talking about combining the shock ring with the particles in Photoshop or AE?

* Oops - didn't read carefully enough - sorry :D

There were some changes to transparency / alpha stuff in 8, raytracing refraction or transparency used to make the alpha opaque. They fixed that (finally :)), I guess this is the trade-off