getting correct alpha with opacity mapped trees?

[vermu]

Hi,

I wanted to use 3d pine trees in my latest scene but run into following problem. I am using hdri skymap in domelight and I am rendering separate alpha pass by applying white 100% self illum material to all my objects and rendering agains black bg. This is how I did it with brazil 2 always. The problem is to get the fine details of the opacity mapped needles of the pine trees. They got this horrible halo around them. Is it even possible to get this kind of fine detail into alpha pass. Or is my only way to post process the final image and then put it in screen mode as environment bg and put matte material on my scene. And the trees would be the only objects I render in this pass. I did something like this with brazil 2 way back but can't remember the correct workflow anymore. How would you solve this kind of problem? Now it just popped into my mind that I haven't tried vray object or render id. Can they be used with opacity mapped stuff?

[Morbid Angel]

why cant you use regular alpha that's generated by the renderer from the beauty?

[Deflaminis]

Generally speaking this is because you have filtering enabled on your mask. Go into the map slot and disable filtering, should fix it right up in most cases. (If you are going to use alpha)

Otherwise on your currently setup, you might try to render against your actual bg instead of black.

[vermu]

why cant you use regular alpha that's generated by the renderer from the beauty?

It seems that if I use domelight with hdri map it blocks the alpha channel. If I save the rendered image as tiff it won't give me the alpha channel. If I remove domelight it works ok.

I'll try the filtering option. If that doesn't work I'll render against the final bg with matted scene or use 2d trees in final comp.

[Morbid Angel]

dome will only block alpha channel if its set to be visible to camera. Unless you are rendering everything in one pass, there is no reason to have dome light visible.

[vermu]

I am rendering everything in one pass.

[Morbid Angel]

so if you are why do you need to worry about the alpha? You see if you render against the background, the alpha is premultiplied against an arbitrary color, you wont be able to set a proper matte to it anyway.

[vermu]

Maybe my workflow ain't correct then. I am using domelight + hdri skymap to get the lightning and reflections. After render in post I remove the hdri bg cause I get nice lightning out of it but I don't like the actual bg. That's when I need the alpha. I am just assuming that the hdri works better in domelight than in environment slot?

[Morbid Angel]

the dome light has an option - invisible, which makes it still light and be visible to reflections, but not visible to camera. You check that on, render you stuff against black with proper alpha and then comp anything you want there no problem.

[vermu]

Just testing and working. Thanks alot. It's always just one tick away from success