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?
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.