View Full Version : Reading Alpha channels in Lightwave

06-21-2009, 03:10 PM
I rotoed a person in shake from some archived footage and exported with the alpha channel to lightwave. I had created a plane that fit the aspect ratio and size of my original footage and applied my video with the alpha as the texture.

I enabled the alpha in the image editor and tried to render, but what I got was the person over a gray plane. My desired effect is to have the person masked out in lightwave so I can add the cg elements as I see fit. The closest example I can think of is the video for the Black Eyed Peas "Hey Mamma."

The last time I tried this, I had to create the video plane and then render it as a separate element and roto it in shake.

I'm looking to do this effect in an upcoming music video. I plan to shoot it on a key wall and save the rotoscoping effort, but I'll still be in the same boat.

Any help would be greatly appreciated.


06-21-2009, 03:50 PM
banged my head against this a few times...
lightwave wants an alpha channel applied to the transparency channel too of your surface on the plane- and i mean an alpha channel that is purely black and white image what is white will show up, what is balck will be treated as transparent and use auto sizing to match both images to the plane, and each other- like a cookie cutter, as the guide book says- so you may have to generate this also as a different file- like i say, pure black and white...i do this for static images in photoshop, and just take the original file and play with 'brightness and contrast' until the original image has become purely black n white....you may have to either do the same in after effects, or shake, or generate an image sequence and do it in photoshop...but thats your problem- lightwave doesnt like images without a SEPERATE alpha file, to control the alpha from within the transparency channel of surface editor- this does give you a lot more control and options later, but the process is slightly longer

good luck with your shoot

06-21-2009, 09:34 PM
In the color channel you can only control the color of the surface. if you think about it, it makes sense. Besides that gives you the option of overlaying different textures on top of each other as long as they have alpha.

You can clone the image in the image editor and force the clone to be "alpha only" and map that clone to the transparency channel. (it will come in inverted so you need to check the invert box)

06-22-2009, 07:30 AM
Danner and Skate23,

Thanks guys for your advice. Looking at it now, placing the alpha in the transparency channel makes perfect sense. I was hung up on trying to accomplish the effect through the image editor. I had actually rendered the alpha channel as a quicktime movie, but found myself staring blankly at my monitor trying to figure out where to apply it.

It does make sense about the color channel. Basically all I had done was applied a "racing stripe to a car model" so to speak. The background was cut out, so the color of the plane was still visible behind the rotoed person.

Thanks again for all of your help,


06-22-2009, 04:38 PM
mighty welcome my friend, makes a change for me to be able to offer any wisdom on this forum, rather than pleading pathetically for it...these folk here are proper helpful...its wonderful


Mr Rid
06-22-2009, 06:42 PM
You may get acceptable results by placing the alpha in the transparency channel. But that does not actually pre multiply a key the same way a compositing app does for clean edges and semi-transparent subjects. I never figured out how to recreate the equivalent key in LW transparency. I first noticed the problem after rendering some hazy, smokey voxels and trying to then map the sequence on a card in LW. It would not key well at all in LW using the alpha as a transparency map, but looked fine in Fusion. I tried a different ways of multiplying the image but never got the same clean key.

Also, I usually find it better to seperate the alpha into it's own image sequence. When you load an image in the image editor with an embedded alpha and with 'alpha enabled', than if you map the image in the color channel the color will be affected by the embedded alpha.

06-26-2009, 04:31 PM
thats interesting, i'm gonna try that now, why dont they put this stuff in the manual? i've noticed mucky edges on transparency flats this way, but usually just shifting a few select points in by a few pixels in modeler it did seem to do the trick...but i agree, its a chore you dont need...now to try it your way...:thumbsup:

06-26-2009, 04:59 PM
sorry, mr rid, i dont seem to be getting this- i've made a 32 bit tga file in photoshop, so embedded alpha, right?

opened this file in image editor, mapped it to a flat poly in the colour channel, but no alpha seems forthcoming...i've checked render and opengl transparency settings, and all that seems in order...so i dont see what i'm missing...i do understand what you mean about the differences between LW and compositing packages...i use AE mostly, but i'm used to being able to import files with embedded alpha and have them work straight away...so any idea what i'm missing? do i still have to load the alpha in a seperate manner?

john not so grumpy really

06-26-2009, 05:10 PM
Embedded alpha makes the image map transparent, not the geoemtry. You have to use the alpha in the transparency channel to affect geometry.

Mr Rid
06-27-2009, 02:26 AM
To key your foreground RGB image over a background in LW, you need only the RGB to go in the color channel of your geometry, and only the alpha to go into the transparency channel. But if you apply a 32 bit image with embedded alpha in the color channel it has nothing to do with the key and it will affect how color layers combine which you may not want. An alpha present in the color channel will only serve as a mask between color layers. Transparency looks at grey values only to make the key and does not need the RGB info.

To keep them separated, you may duplicate/instance the 32-bit image in the Image Editor and set one to 'alpha enabled' and the other to 'alpha only.' Apply the first in the color, and the second in the transparency.

06-27-2009, 02:46 AM
Depending on the effect you are after, I've noticed that Clip-mapping is way more effective. I only use transparency if I need to create soft edges, since transparency works between 0 and 255, while clipmaps either cuts or do not cut at all. As Mr.Rid says, it is much better to have the transparency image/image sequence as a separate entity in LW.

06-27-2009, 03:54 AM
I usually use both clipmapping and transparency (mostly for leafs and whatnot)
This way you clip the absolute transparent parts (thus decrease render times), but you keep your nice alpha anti aliased edges.

Also, i think the last time i did this i pre-divided my footage (in Fusion) and used the method described by Mr. Rid.