View Full Version : Compositing Issue with Backdrop Radiosity

Chris S. (Fez)
04-05-2005, 10:29 AM
If you render with Backdrop Radiosity and Textured Environment how do you get around the compositing issues of using a non-white or non-black background? In Max there a way to get Radiosity to detect the textured Backdrop while still rendering to a black background. Can we do this in Lightwave?

04-05-2005, 10:34 AM
render your image out as a 32-bit targa. that will include the alpha channel, so you can then composite to your heart's delight :D .

Chris S. (Fez)
04-05-2005, 10:54 AM
Thanks for the response but I am asking how to get around the premultiplication/alpha issues that arise from rendering a Lightwave scene against a non-white or non-black background and then compositing.

The best would be for Lightwave backdrop radiosity to detect the textured backdrop, but still force a black background. This is what Max does to get around the issues.

04-05-2005, 11:27 AM
Hmm I can't say that i fully undestand what you want so can you show me screen grabs of project you are working on ?

But i do have few questions :)

If you are using BkgRadiosity with some gradient (non black/white) and you don't want that to be showed on render/alpha tell me why you want different background color and then you are want it excluded on render/alpha :)?

You can setup ball object and texture it with gradient like on backgorund texture so you can exclude it from camera and alpha so you will get same effect as on BckRadioisty and exclude color from alpha channel. There is numerous ways of excluding but i'd like to see what you are trying so i can give you example :)


04-05-2005, 11:51 AM
I am doing this right now. I have rendered AVI no compression and it includes an alpha channel, that if I don't use premultiplication, but straight alpha in After Effects, it does a good job of taking out the background. Also, I have rendered alpha images which I can use in After Effects to take out the background (this is a bit better).

Does this answer your question?

Chris S. (Fez)
04-05-2005, 12:03 PM
Hey Lewis. Thanks for the response. Sorry, I cannot post what I am working on, but to answer your questions:

I need to use backdrop radiosity because it is much faster than using the ball object method.

I am compositing an object against a background plate. For increased control when compositing, it is necessary to keep the object separate from the background plate. I am using a gradient in Textured Environment combined with Backdrop Radiosity to mimic the colors and lighting in the background plate.

Working with non-black premultiplied renderings results in subtle highlights and rims along antialiased edges when compositing. This is a universal issue with compositing in 3D. Therefore, I need a way for Lightwave to detect the textured backdrop but still render against a black background. Of course, any other workarounds would be welcome :).

Chris S. (Fez)
04-05-2005, 12:04 PM
Let me try that gordon and get back to you...

Chris S. (Fez)
04-05-2005, 12:35 PM
Nope. There is a rim whether the image is set to multiplied or not in the compositor. I think this is exactly why there is the BlackBackdrop option in Max.

Try this if you want to see what I am talking about. Render an object against a black background (native settings). Save the image as BlackBackdrop (32 bit tga. or anything else with an alpha). Now add a Gradient in Textured Environment with the Gradient set to Pitch. Give the gradient a wild variety of colors. Render and save as TexturedBackdrop.

Open up both the BlackBackdrop and TexturedBackdrop images in your compositing package. Zoom in and compare the edges of the object in both images. With or without premulitplication there is a rim around the object in TexturedBackdrop. Right? It is subtle but it is there and poses significant problems when compositing and color correcting. There has got to be a way around this in Lightwave.

04-05-2005, 12:43 PM
You could use a single backdrop color, or imageworld for the image you wanna use for lighting the scene. Then use a blackor white backdrop image in the composting tab. This way you background color will be black or white, easy for premultiplying, and you're using another image for radiosity.

Chris S. (Fez)
04-05-2005, 12:55 PM
It worked! Thanks a million, Ronald.

04-05-2005, 04:35 PM
Good to see it's solved :)

You just needed to say "glow" or "ring" around object and i would understand what you mean :). Now i know exactly what issue you had and toonafish already told you solution - power of LW community ;).


Chris S. (Fez)
04-05-2005, 05:05 PM
Yep. Doesn't get much cooler than the Lightwave community :). Thanks again for your efforts, Lewis and Gordon. Much appreciated.

04-05-2005, 05:50 PM
You could use a single backdrop color, or imageworld for the image you wanna use for lighting the scene. Then use a blackor white backdrop image in the composting tab. This way you background color will be black or white, easy for premultiplying, and you're using another image for radiosity.

Very cool tip.

Save this one for 1001 Tips Vol 2! :)


04-06-2005, 08:55 AM

I am interested in this, and am planning to do more of it see http://vbulletin.newtek.com/showthread.php?t=35437 for my first attempt at compositing.

What I want to know is do you not get the same problem but with black or white pixels round the object. I have not tried it so I will get on it tonight. Also, what is best black of white?

Chris S. (Fez)
04-06-2005, 03:20 PM
Hi Gordon. Cool composite! Always, always, always render against black whenever possible. Some compositing packages like Combustion offer special settings just in case you render against white, but others don't, so render against black just to be safe.

Assume you render your plane against a black background to a 32bit tga. Premultiplication basically means the transparent parts of the plane have black in the exact same areas as the Alpha. Zoom in and you will see the antialiased edges of the plane are transparent. In both the RGB and Alpha, where does the plane end and the background begin? Lucky for us we can rely on the compositing package to answer that for us.

I am not a professional compositor but I have worked with professional compositors and been asked to composite in a crunch. When i was working in Shake the two golden rules for compositing were to color correct unpremultiplied images and to transform and filter premultiplied images. Shake was great because it gave you absolute control over premultiplication. Other packages, like Combustion, basically offer a magic premultiplication button, as well as color correction, filter and transform tools that take premulitplication account "under the hood." Which is probably adequate for 9 out of 10 compositing cases.

So, long story short, in Lightwave, be sure to render against black and turn on premultiplication for the footage in your compositing package.