View Full Version : please help me out checking for a bug

07-23-2003, 09:57 AM
I have an object that is 100% transparent.
it renders completely transparent normally, but when I turn on Raytrace transparency, It renders completely black and opaque. the ray cast render takes 2 minutes, the raytrace takes 3 seconds.

the only change i made to the default surface is transparency, and to set the object as double sided.

can anyone duplicate this?

07-23-2003, 11:36 AM
tried this:
added a sphere to empty scene file.default dir light/ambient/default ray recur/4threads
setsurf to slight spec/fulltrans/double sided.
F9....black sphere(reflecting whats around it...black background color )
back to render options, check raytrace trans
get the same as before?no diff.
both take same time(eyeblink)on dual 1000/7.5/x.2.6
hmmm? would try your scene file here if you wish.
gave it a try anyway...
did i miss something?

07-23-2003, 12:19 PM
dont think you missed anything.
i did a test just after posting this.

3 cubes with 75 % transparency ray cast and raytraced as expected.
I then gave the original Object without the scene file to my co-worker on A PC.
His results were worse than mine, half of the polygons were raytracing transparency incorrectly. Its definetly something to do with the object.

thanx anyway.

further research shows that, as of right now Raytraced transparency and refraction is Gone. it does not work (period).
the cube tests i did, have a completely opaque alpha channel, which means that they are not in fact transparent, and the original model actually renders the backdrop color in its place, through objects that should be visible.

this is consistent between restarts, and re-launches of the app. i am now going to remove my pref files, if that doesnt work, then I am forwarding this [email protected] to the bug team.

Julian Johnson
07-23-2003, 12:31 PM
I don't know if it's relevant, but when you turn on Ray Trace Transparency the Environment options for refraction in the surface editor come into play even if you have no value set for refraction and refraction turned off. For example, if you have a black backdrop colour, a white targa as your background image, and your refraction environment set to backdrop only and render a 100% transparent ball with Raytrace Transparency off you'll just see white. If you turn on Raytrace Transparency the ball is completely black (because Raytrace Transparency forces the transparency calculation to use the backdrop colour by default).

Is it possible your weird object has some stray settings for the refraction environment turned on that are getting activated when you turn on Raytrace Transparency? A spherical refraction map or something?


07-23-2003, 12:56 PM
you tell me...

07-23-2003, 01:01 PM
i should give you guys a visual.

this is the cube test. it corresponds with the above Surface panel series

on the left it appears to be 3 cubes inside each other with some slight transparency.
on the right is its Alpha channel.

btw: the alpha of a ray cast Transparency render is "correct".

julian, i noticed this problem because the transparent object completely obliterated Multiple objects behind it.

Julian Johnson
07-23-2003, 01:54 PM
Hi eblu - I'm not sure how much this will help but anyways. The default behaviour that Lightwave has had with refraction since 6.x is to make the alpha opaque. So, as soon as you turn on Raytrace Transparency you're effectively telling Lightwave to treat the surface as a refractive surface irrespective of whether it has a refractive index (or whether refraction is on). Because of this, you get opaque alphas with raytrace transparency on. It's a limitation of the current render engine and architecture. With your surface settings, the alpha is being made opaque and the black from the background is being used as the background to the transparency.

As soon as you turn off raytrace transparency, the refraction 'mode' is turned off and the surfaces become genuinely transparent.

The classic example is a volumetric behind a transparent object. With RTT off, you can't see the volumetric but you do get a properly evaluated alpha channel. With RTT on, you can now see the volumetric but the alpha is opaque. Compositing is the only way.

It's certainly not what you would intuitively expect, but it is the way the current renderer works. Whether it's a bug or a generic issue with all raytrace render engines is probably open to debate...


07-23-2003, 02:12 PM
thanks for the sanity check. My guess is that its a bug in the logic of the renderer, that is being marketed as a feature.

having put that to bed, lets take a look at the original object:

the image is of 2 renders with raytraced transparency on in one and off in the other. The alarming thing is that the Object With transparency completely obliterates everything behind it, and uses the backdrop as a 2d fill color. This cannot be the appropriate behavior of the rendering engine. It basically makes ray tracing Useless.

Julian Johnson
07-23-2003, 02:40 PM
Hi eblu - is the grid structure behind the transparent objects actual geometry or is it a backdrop image?


07-23-2003, 02:47 PM
no thats all geometry.
the backdrop is black.
there is no background image.

Julian Johnson
07-23-2003, 02:58 PM
OK. If the box objects in front of the grid are transparent, have their refraction environment set to raytracing and backdrop, and RTT is turned on you should be able to see the geometry behind them. RTT should only give that result if your refraction environment options for your transparent surfaces are set to backdrop only....


07-23-2003, 03:13 PM
so, i guess its time for me to file a bug report.
thanks julian,