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How to save the alpha channel?

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Monaco Felice

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How to save the alpha channel?

PostWed Feb 25, 2015 9:22 am

Please tell me, how in Fusion I can save Alpha-channel of the selected layer or object?
For example, if the final composition is created using multiple layers, then how can I do the final render Alpha-channel background image? (for example is Sky ... the Sky is been created a separated layer in the composition. But at the same time, in the final composition, the Sky layer overlaps the other layers and need to get an Alpha-channel of the Sky from the final composition).
Or if in the scene is 3D objects, how to save Alpha-channel of the details, which specify a particular material? (eg the scene is 3D model of the car. How can separately display (or create) the Alpha channel for the tires of the car?)
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Rony Soussan

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Re: How to save the alpha channel?

PostWed Feb 25, 2015 5:21 pm

Your' looking for matte channels, and this can be done by object ID or material ID. This will generate the 'ID" that 2D tools can use as a 'mask'
That beings said, there is no substitute for render passes, even in top 3D applciations, you will never get a better result in isolating the tire then rendering it out as a pass on its own.

Try building a couple of simple objects, give each their own Ojbect ID (use override) and within the object, create multiple material IDs (this can be on a 'group' of tools, not just a single)

The 3D render will now have 2 levels of selection for the pixels in the render.

As far as the alpha channel issue, I am a bit confused as to what your asking, but it sounds like you want to copy an alpha from one part of your comp to other tools somewhere else.

You can do that using either the 'matte' node with copy alpha, or boolean node to copy alpha from one to another.
I would be curious to see an example of is the real issue using a few simple tools.
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Monaco Felice

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Re: How to save the alpha channel?

PostWed Feb 25, 2015 7:38 pm

Thank you for the answer!
I really do not yet fully understand how to use the material ID and object ID for the Alpha channel. I am a novice in Fusion. But I'll be experimenting.

As for my question to obtain the Alpha channel of the final composition - attached example in the picture.
There are here such an image in which of background made as node DaySky and is on a separate 3D Plane. The buildings are made in 3D and connected with node Merge and to the same node connected Light sources and Volume Fog (by which was created a clouds).
I am interested, how I can take Alpha channels for Sky, Clouds and Buildings, from final composition?
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Rony Soussan

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Re: How to save the alpha channel?

PostWed Feb 25, 2015 7:46 pm

Aha, you want to render all the passes..

I would keep the single merge for your own review of the scene, but branch each object out to
and individual merge and 3D renderer, then you can comp them together like any other render pass you would get with alpha.

While its nice to use the 3D as a single render, and at times it's exactly what you need, you will get more control if you use 2D to merge the layers. You then can do lightwraps, individual color corrections, gamma adjustments on the edges etc..

It's why 3D departments don't render a single scene as well (that, and complexity of scene)

I'll try to get you a simple example of object ID as well.

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