View Full Version : Alpha Channel-Unaffected by Object-What's that mean?

02-18-2006, 12:12 AM

What is the Alpha Channel setting "Unaffected by Object" in the object properties panel for?

I was revisiting some scenes I created last summer in LW 7.5 which have a group of oilfield mixing and pumping trucks surrounding an oil well. I opened one of the scenes in LW 8.3 and did an F9 render of one of the frames. It took 5 minutes 34 seconds to render the 864x486 frame, with the antialiasing set to Enhanced Low. This was a lot longer than I remembered, so I opened that exact scene in LW 7.5 and rendered the same frame and it only took 43 seconds, which is closer to what I remembered.

After some troubleshooting, I isolated the problem to the Alpha Channel setting I'm using on the truck models. For post production purposes I'd checked "Unseen by Alpha Channel" in the object properties in LW 7.5. When that scene opened in 8.3, the Alpha was set to "Unaffected by Object", and this accounted for the very long renders. When I set the alpha to "Constant black" the scene rendered quickly, as it should.

The 8.0 manual shows the correct Object Properties panel but only mentions the obsolete "Unseen by Alpha Channel" checkbox. I can't find a description of "Unaffected by Object" in either the 8.2 or 8.3 addendum.

Just curious.

Shawn Marshall
Marshall Arts Motion Graphics

02-18-2006, 01:21 AM
Far as I know it means the same thing. Interesting find though, I'm going to check that out - the alpha shouldn't have much of any effect on the render time. Are there any special effects in the scene, like refraction or volumetrics?

02-18-2006, 09:41 PM
Thanks for the reply, Toby.

There's no refraction or volumetrics or any other special effects in the scene. I'm using ray traced shadows and a soft filter; that's pretty much it.

The slowdown in the rendering time correlates to the number of polygons in the object; the more polygons with an alpha set to "Unaffected by Object", the slower the render. The truck models have around 290,000 total polygons; the scene has around 500,000 polys. If I switch the alpha of the trucks to "Constant Black" the render speeds up significantly, even if a few of the lower-poly objects are still set to "Unaffected by Object".

If you try to duplicate this issue I'd suggest using a fairly high poly object.

Shawn Marshall
Marshall Arts Motion Graphics

02-18-2006, 11:40 PM
I see why it takes longer, it's actually rendering the furthest polygons first, the opposite of a normal render. That means that it's rendering a lot more polygons and all their shadows, then they just get covered up because they're in the back!

I don't know, maybe it's the easiest way for the render engine to keep the object out of the alpha?

If you really need to use this feature, try putting a 100% transparent polygon in front of the camera and turning on raytrace transparency. This forces it to render only what is seen by the camera, and might speed it back up again - but it does depend on the scene, so don't hold me to it!

02-19-2006, 02:27 AM
Well, since I really still don't know how "Unaffected by Object" is different from "Constant Black", I'll just use "Constant Black" from now on if I want certain objects to render without alphas.

The old "Unseen by Alpha Channel" checkbox seems a lot more straightforward.


Shawn Marshall
Marshall Arts Motion Graphics

02-19-2006, 06:50 AM
Unaffected by Surface is exactly that, the surface won't affect the alpha channel, constant value allows you specify an alpha channel value (Black 0 - White 255) Surface opacity uses the opacity of a surface for the alpha channel values, and lastly shadow density will use shadows on that surface as the alpha channel values.

But you're right, constant value of 0 yields the same results. I'm sure there might be a situation where the relationship of the objects in a scene might need Unaffected by Surface because Constant Value would wipe out any other settings on other surfaces. Or something like that! I just can't think of one right now!

02-19-2006, 12:06 PM
Actually he's talking about the alpha settings in object properties -

I'm sure that 'unaffected by object' means that the alpha will look normal for any other objects in the scene, whereas 'constant black' will black out other objects' alphas if it's in front of them. I don't think 'unseen by alpha' gave you both options.

btw that raytracing trick I mentioned doesn't work, it makes the alpha visible again :P