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Guest tennet

How To Render An Object From A Scene With An Alpha Mask?

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Guest tennet

Hi again!

I have this scene - http://www.c4dcafe.com/ipb/index.php?act=a...st&id=44216 - and I want to render it so that I get an alpha mask just for the toy in the picture (to make it easy to create cut-out masks in Photoshop later). How do I do that?

Also, is it possible to render just a part of the scene as a cut-out (maybe just a part of the toy..) BUT still with the same camera setup, lighting and reflections as in the scene? This would be useful if I, for example, had to change a material on the toy and I dont want to (or need to) render the whole scene again.

Is it possible?

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Yep!

Get rid of any kind of background in your scene just leaving the toy.

Then in your export settings, you need to make sure you export the alpha. If it's for a still image I would use multipass rendering for even more control.

Morph

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Guest tennet

Hi and thanks for the answer! I'm just playing around right now, learning, but I will probably do mostly still images.

Will this technique include the reflections and light settings that I've used in the scene? For example, if I'd like to render just the head or one of the wheels, they still must look like they do in the completed scene (same colors, ligths, reflections a s o).

Do I need to render the complete scene first - without the alpha setting - and then render the toy by itself WITH the alpha setting to get an alpha mask for Photoshop (i e two different renders)? Or is it possible to do ONE render for the whole scene and still get the alpha channel for the toy at the same time?!

When using the multipass render option.. is there any option/channel that includes an object in a separate layer (instead of exporting alpha channels)?

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If you look at the multipass rendering options there are a whole bunch of different layers you can use. I'm pretty sure one can be objects.

If you still want the reflections etc for this particluar render, you can put a compositing tag on all the background objects you don't want to see and uncheck them for seen by camera. This way you will still get all the reflections and everything on your toy.

Morph

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Guest dataflow

to keep the reflection.

instead of get rid of any kind of background its better to put a composite tag on the objects and untick "seen by camera"

Edit: morpheus you posted it faster then me :)

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Guest samwich

I recently did this on one of my scenes.

If your object is in separate pieces group them in a null and add a compositing tag to the object you want an alpha channel for. Go to the object buffer attribute and check off object buffer 1. Go back into the render settings -> multipass rendering and click the channel arrow in the upper right. Select the object buffer, and then select object buffer 1. Give it a filename, and when you render out your scene a .psd with the alpha channel will be created along with your image.

hope this helped

(edit) I guess if you don't give it a path in the multi pass settings (i.e. just leave it blank), and render your image to a .psd with the alpha channel option checked in the save settings, the .psd will include your image and an alpha channel

-Sam

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Guest tennet

Great help everyone! The compositing tag will do perfect and the multipass/object buffer setting sounds like the solution for my other queston. Thanks!

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does anyone get the black halo of doom in PS when using the object buffer? cause i do thats why i render against a black background instead so that photoshop reads the alpha as premultiplied. Anyone know if i can get the same clean alpha with OB multipass?

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Guest dataflow

turn on "shadow correction" in multipass

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