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I am really confused with the different uses of the terms "Layers" and "Passes" in Blender.
Can someone please explain how they work in Blender. I know about Render Passes like AO, Diffuse and so on, but there seems to be a confusion with layers.
Also, how do the layer buttons work in the properties panel like in the image shown? I get it that you can assign objects onto certain layers, but whats the difference between "Scene" and "Layer" buttons in the image? How do they refer to rendering? How can you see what layer a certain object is on?
LayersScreenshot

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Layers


Layers are a way of splitting up and organizing the objects in your scene.

In the 3D viewport.

You can select the currently visible layers in the footer of the 3D view or under properties panel > render layers tab > layer rollout > scene layer selector. (With any layer selector you can hold down Shift to select multiple layers.)
enter image description here

You can see what layers have objects on them by the dots in the layers' boxes, layers containing selected objects will have an orange dot. You can move the slected objects to a new layer by pressing M and selecting the layers you want in the pop-up layer selector.

In the scene section of the properties Window, there are four sets of layer boxes that control which layers are computed in the render and how:

enter image description here

  • Scene: The scene layers determine two things: which layers are shown in the 3D view, and which layers are taken into account in the render. (As mentioned earlier this layer selector parallels the one in the footer of the 3D view.)
  • Layer: The layer layers determine which layers are visible to the camera in the render. This allows you to have certain layers counted into the render (by selecting them under scene), but not visible to the camera (by not selecting them here).
  • Exclude: Any layers selected under exclude will not be calculated in the render in any way.
  • Mask: Objects in layers selected under mask will be rendered as transparency to the camera. They will take part in all other calculations if selected under layer though.

Here is an example showing the functions of the 4 different layer selectors. The ground plane, light, and camera are on layer 1 (top left); and the red box is on layer 2.
enter image description here
Click to enlarge.

Render Passes


Render passes are simply different ways of rendering a scene, often used in the compositor for post processing effects. You can choose which passes you want to render under properties panel > render layers tab > passes rollout. Which ever passes you select will then appear as outputs on the render layers node in the compositor.
enter image description here

Render Layers


I think what may be confusing you is that there is also something called render layers. Render layers are a way of rendering the scene multiple times with different settings (including passes and visible layers).

You can add new render layers and select which ones should be rendered at the top of the render layers tab.
enter image description here

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    $\begingroup$ A little complexity that shows up when you start having reflections and transparency is that objects not in the current render layer can appear in reflections and refractions unless those objects are in an Excluded layer. $\endgroup$ – Mutant Bob Jul 13 '15 at 21:18
  • $\begingroup$ @MutantBob Exactly, that's the point of having a scene layer that isn't a layer layer. $\endgroup$ – PGmath Jul 13 '15 at 21:20

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