This is the overview. I am breaking up the parts into individual answers that we can link to from other questions. This is a community wiki, so feel free to add ones that need to be addressed completely in another answer. If you decide to add another answer, please follow the pattern that I used with the first individual answer. This will make it easy to ...
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.)
You can see ...
Those two images you're trying to composite together have "correlated" alphas. The Alpha over operation isn't the appropriate operation for this situation.
The solution is simple: you just have to add those images together, then add their alphas together and assign the resulting alpha to the result of the first addition.
Why alpha-over doesn't ...
Yes, this is possible using the following node setup (click for larger image):
As you can see, the coordinates are the same as the channels (click for larger image):
In the screenshot above the RGB values are slightly different from the coordinates displayed in the Properties panel because those coordinates are measured at the object origin and the RGB ...
Render OpenExr Multilayer for Blender, Nuke and Natron. Go to the Scene Tab, enable all passes which key for compositing and set the Output Format in the Render Tab to OpenExr Multilayer.
Blender automatically writes all enabled passes and layers as single channels to the EXR as expected.
For After Effects and Fusion I'd suggest render single EXR's. For ...
There is nothing specifically designed into Blender to tell it to ignore the transparent areas currently.
That being said, there are some things we can do to help speed up the render somewhat.
Transparent Background - Choose transparent in the Film section of the Render properties panel. This will render faster than a coloured background (whether full ...
I think what happened here was just a misclick in your render layer settings. It looks like you accidentally added a layer as a mask layer.
What is a mask layer
A mask layer is a layer that is used as a mask to turn parts of a renderlayer transparent depending on if anything (that appears in the render) is infront of something from that renderlayer.
After reading Leon Cheung's answer, I've redone my solution:
Say you have this setup
and you want to make the plane into an invisibility cloak that affects only the red monkey.
First, make the background transparent (see here for help).
For Blender Render, select the plane, and under the Materials tab of Properties window, enable Transparency, select ...
Another alternative is to render out to separate image sequences. This is handy if you want to output to different render formats for each Render Pass. As well it gives you the flexibility to only re-render some passes if need be.
You can do this by piping the Render Pass outputs into a File Output node. You can add additional inputs to the FO node and make ...
Your own solution seems nice enough. For Cycles, I just want to share with a simple way, which needs just two renderlayers and less nodes:
For the cloak object, you can use Holdout shader.
For cases related to Z, you can simply add a Z Combine node (as you did in your own answer). Furthermore, Alpha Over brings more control on Alpha. Anyway, it could be one ...
Full disclosure, the add-on I'm about to recommend is a paid for add-on created by me, but I think it does what you need as it was built partially for the very task of skipping transparent parts of an image.
If you were rendering a still image you could use the 'render border' feature of blender to skip areas outside of the chosen area. For animations you ...
You can't switch between CPU and GPU if the render layers are in the same scene, but you can have multiple scenes that each use a different rendering device (CPU/GPU). You can then work in the compositor with the render-layer from the GPU rendered scene and the render-layer from the CPU rendered scene.
So, you need to recreate the render layer you want to ...
The Denoiser in Blender version 2.79 can be activated per Render Layer, and as such you can mix denoised and noisy footage as you like. If you ever wondered why the Blender Devs have put that Denoiser into the Render Layers context of the Properties Panel, that's why.
All you need to do is separate your scene into Render Layers with the objects in question ...
Old "Render Layers" are now here, in the top right corner:
In the context menu (RMB) in outliner you can set to any collection to be Excluded, masked (now holdout) or hided (now Indirect Only).
View layer -> Set Exclude is old Exclude function
View layer -> Set Holdout is old Mask function
View layer -> Set Indirect Only is like you Hide layer, but not ...
Various aspects of a Freestyle Line Style can be given modifiers, including the Alpha of the stroke.
The modifiers available to Alpha include Distance From Object. (In this example, the blue empty):
Which, with settings like this:
Can get a result like this:
Set the renderlayer with the reflective object to Exclude layer one:
Now objects on layer 1 will not have any affect on layer 2.
In BI this is more complicated, because it appears to be impossible to separate reflections into separate layers with BI.
In other words, you can either reflect every object in the ...
Found the solution.
The trick is to enable the desired render layer (ie "vector") and then save the render output as OpenEXR file. That format will save all the desired render layers (in this case, motion information in 4 channels) in a single file, which could then be accessed through various interfaces like Python and Matlab.
To enable compositing go to Post processing in the Render tab of the Properties Panel and:
check the Compositing option
uncheck the Sequencer option
this will process the render result through the compositing pipeline if the compositing nodes are enabled.
The source of the problem is that Freestyle view map has (can see) the smoke domain geometry. This is a typical problem. Transparent mesh like glasses can be solved with the same method.
Put torus, cone and sphere on a new render layer (mesh on layer 1). This render layer will be rendered with freestyle.
Isolate the smoke domain cube to another render layer ...
It could be that you are not actually providing the compositor with any transparency. The alpha output socket from the scene node will just be a solid white unless you check the following properties:
Blender Internal: Turn on Shading > alpha > transparent, in render properties.
Cycles: Turn on film > transparent, in render properties.
The way this works is indeed quite confusing.
TL;DR: M moves objects between layers, which are independent of the many layer visibility settings.
There are 20 layers and several independent places where they can be visible or hidden:
Scene layer visibility
These are the layers which are visible in all 3D views with Lock Camera and Layers ...
2. Render layer not showing in the compositor
There are actually a few different ways to run into this kind of problem. I will divide this up into different cases. If the Use Nodes button is not checked (in the node editor), you will not have any of these problems.
Render layer order doesn't actually effect the render (aside from maybe changing the order in which the layers are rendered).
However, for organisational purposes it can be nice to reorder them. For this reason I created a script, available on Github, which adds up and down arrows to reorder the render layers:
Simply set the output format to OpenEXR MultiLayer and save the render, it will store all render passes. Create another project for the compositing where you load the image(s) through an Image node. All render passes will be available as sockets, if you set the Layer in the Image node to (one of) the View Layer(s) you've rendered.
Example of multi-layer ...