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I have a render layer. I need to be able to access the alpha of this render layer in the compositor. I know I can access the alpha by enabling the Combined pass, but I don't need to have a combined render, I just need shadows and alpha. Is there any way to do this? Or some workaround that lets me get the areas where nothing is in the compositor?

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    $\begingroup$ Yo do need to render something with F12 for the compositor to have information. The alpha is always available on the Alpha socket on the render layers. If you are using Cycles remember to enable Film->Transparency $\endgroup$ – cegaton Dec 7 '15 at 4:22
  • $\begingroup$ @cegaton when I disable the combined pass there's no alpha socket. film->transparent is enabled. Even after rendering: i.imgur.com/0YquQ5J.png $\endgroup$ – markasoftware Dec 7 '15 at 4:29
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    $\begingroup$ Without the combined pass there is no access to the alpha. Why are you disabling it? If you don't want to use it just don't plug anything to the the image socket, but you'll have the alpha channel available. If you absolutely do not want to have a combined pass for whatever reason, then you can use object indexes... in my opinion a lot more complicated... $\endgroup$ – cegaton Dec 7 '15 at 5:15
  • $\begingroup$ @cegaton I have often wanted to optimise animations by skipping the combined pass as I am only interested in saving the alpha channel. Even an optimised combined pass takes time to render, skipping it would be useful. $\endgroup$ – Ray Mairlot Dec 7 '15 at 10:26
  • $\begingroup$ @cegaton the only reason I'm trying to skip it is because I assume that it would slow down the render a lot $\endgroup$ – markasoftware Dec 7 '15 at 17:29
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In case the reason behind this is saving time, I was researching the same thing and found "accidentally" a semi-solution. Combined pass still renders, but I assigned a material in the Render Layers, this overrides the objects assigned materials, assigning a simple diffuse shader this way, will make the Combined pass render much faster, specially when you have lots of reflections, refraction and light emitting objects in the scene, and it's a non destructive process, since you simply need to click the X icon next to the assigned material in the Render Layers tab to go back to your original scene materials. I know this is not the perfect solution, but at least it will save render time in most cases.

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