Q: Alpha Channel in Raytrace

[AndersEgleus]

I assume your actor is behind the window from our point of view, right?

The alpha image doesn't store reflections, but you can output reflections and refractions as separate render elements.

If the window is semi-transparent, it should come out grey in the alpha. But there's a fool proof trick to create your own custom alpha images if you're compositing in another program. First render the scene with the window fully opaque. Then create a white metal shader, fully opaque, with 100% self illumination. Then copy it and make the copy black. Apply the white one to everything that should be opaque and the black one to the window and render. If the window is semi-transparent or transparency mapped, make the black material grey, or put the transparency map in the diffuse slot. Then render the blackwhite scene.

Of course, in this case it sounds easier to just use the footage as a background image (screen mapped), but maybe your composite is more complex than that.

[Notum]

make 2 spheres, one slightly behind the other, asign a raytrace material to the sphere in front. Set transparency to 50% and leave all the other settings as they are... now hit render and look at the alpha... it's totaly white... even though you would expect the front spheres alpha to be 50% gray.
This is such a basic thing... and a must for composits.... why doesn't it work?

Please look into this...

Thanks

[Notum]

I need help with this real bad

[skellybobbly]

Hi Notum,

One way to do it is to leave the transparency of the material but set the object visibility (object properties) at 0.5.

This will give you a 50% grey in the alpha channel.

Alternatively render with the material you've got but render again with the visiblity 0.5 and use the alpha from this render.

Cheers


Jon

[Notum]

I found what I think is the problem. If "Raytrace Refractions" is on in the Raytracer Controls for the material... the alpha becomes white... regardles of transparency. What if I need refraction and alpha... together? Strange if noone has noticed this before.

Any other thoughts?

[jason-slab]

yeah, as soon as u turn on the refrations u loose the alpha channel, pity. i've had this problem before with a bottle

only thing i can suggest u try is (havn't tried it), is make all the other objects in the scene non visible to the camera, then render out a refaction pass and overlay it in your comosite proggy

|jason

[Notum]

but it's still strange as hell though... it works fine in both maya and xsi... I don't see why it would be big deal thoug...

can anyone say if it's like this in Brazil and Final Render as well?

[jason-slab]

dunno bout brazil, but its the same in final render, think its just a 3dsmax thing

[ZeBoxx]

Heya,

In Brazil r/s there's an option to create a non-opaque alpha for raytraced transparent objects - mostly used for windows, of course.

Best regards,
Richard Annema

[Wout]

Just use the same opacity map for the raytrace material as for the window, works just fine.
I just found out about this, thought you'd appreciate me sharing this ..

Raytrace material/Basic Material Extensions/ Opacity map

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