Alpha at zero


I’m trying to find a way to be able to generate mattes right in the scene. I mean that I want an objet to render black and hide another without that objet having an alpha channel.

I was used to be able to do that in softimage 3.9, it was in the material that I loaded a material shader called alpha and I would set the alpha value at zero.

I don’t seem to be able to find the equivalent in XSI 2.0.2

Thank you


What you are looking for is called OGL Alpha Transparency. It can be found under Get>Material>OGL Alpha Transparency.


Yes, that’s exactly it! Thank you mr. Harriss!


could you plz post some pics or explain to me plz what exactly it’s doing and perhaps exampls of the use of that OGL alpha node? i didn’t understand the first msg but this kinda sound like something i was looking for or will have a use of later.


Well, hmm, I’ll try and keep this simple.

It can be used in alot of ways. One use might be to create a matte. Lets say that you have an object that you can see trough and in the angle of camera you are positionned you see a part of that same objet trough itself and that you do not want this part in the render.

Since it’s all the same objet it’s difficult to easely hide a peace of it. (I know there are ways but for the sake of the explication…) A way to fix this is using ogl alpha as called in XSI. So example you place a constant colored grid between the part you want to see and the part you want to hide and turn the object alpha value to 0.

So you now see the part you wanted plus the constant colored object that is hidding the part you didn’t want and since the constant object is not in the alpha channel, it’s like you have a single object composed of what you wanted.

Follow me? Think of it as a black hole…

Hope this helps.


thanks marc. but i guess what i wanted to hear more is, why would you even bother or need to do that for? why whould you have an object and then want to hide it. is it like for compositing with life footage? if not then what for? perhaps some real everyday use examples?

thanks again.

it’s just i’m total dummy in combining life with 3D and for that i wanna take a course later, after i finish my animation diploma. i’m just curious how it works. but if it’s not for compositing 3d and life footage then i’m even more curious about any use for that otherwise. i mean there are layers and other stuff for this kind of task… hmm


Well taking my past example I could aswer that everything depends on your shot. Making layers and stuff just for one angle of camera can take more time then just dropping an ogl alpha object to hide what you don’t wanna see and forget about it.

It can be used in compositing, yes. For say you have a sphere and 2 cylinders comme out from behind of that sphere on a background plate. You want to treat the sphere and the cylinders seperately from temselves and the background in after effects.

Well one method witch is most used is to render the shot strait and then make a black and white mask of the render (spheres white and cylinders black.) So you can use the luma of this map to isolate the spheres and you have to process another one to isolate de cylinders. With the ogl alpha method you also do 3 renders:

1- Spheres shaded and all the rest in ogl alpha.
2- Cylinders shaded and all the rest in ogl alpha.
3- Background shaded and all the rest in ogl alpha.

So both ways you have 3 renders total. But the ogl alpha way you get the objects alone (witch makes it easyer for the compositers) and if an object goes infront of another it hides it but doesn’t apear in the alpha channel witch is super praticle cause you can put your layers in more combinations of layering then if you’d see a black object pass in front (as with the b/w matte method) that you have to hide with the corresponding render that you put on top.

Hope you followed me.

:slight_smile: :beer:


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