When I click 'Render' or hit F12 and select IndexOB pass in the viewer I'm able to see a mask (white on black) as expected, but when I click 'Animation' to output a series of these masks as '.png' files. What I get is a normal color 'Combined' pass.
My blender file is set up to create a masks of several objects. Each object has a pass index of '1' and Object Index is enabled.
How do I output a series of '.png' files in order to get all the masks as seperate files?
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Result:
It's basically the same for 'Material ID passes'. Assign a unique 'Pass Index' value to each material and enable Material Index pass (Properties > Render Layers > Passes > Material Index):
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Unfortunately the above two answers (including my own answer) didn't actually do what I needed.
I want to use the mask in the Video Sequence Editor (VSE). The simplest way is to apply a strip modifier, but that doesn't seem to make use of masks with alpha. It seems to want a grey scale image as a mask.
Here is the set-up for a black and white mask:
Render Layer node connected to IDMask connected to Invert RGB... and there is a composite output node.
If a file output node is connected this produces TWO outputs, one going to the selected file address in the output node and the other going to the address in the render panel. (BUG?)
The background shows the mask as white on black. This works as a VSE strip modifier.
Nodes:
(Note: not using fileOuput)
VSE use of mask as strip modifier:
Object Image:
Mask:
Background:
Combined Image:
(The reason why I wanted to do this is based on ignorance. I had a low sample test render of the entire film. I liked the low res. grainy look for the background, but wanted the foreground to be done with more detail so I did new renders of just the foreground objects. I thought I was doing this with alpha, but forgot to click the RGBA button. So I had objects on black instead of objects on alpha. From there I had to find a way to fix that. First I tried color keying to turn the black to alpha, which partially worked, but some of the darker parts of the objects went transparent. So I read up on masking, but the info was about masking in compositing and that isn't the same as masking in video editing.
Another way to produce this effect, if it had been planned from the beginning would be to put the background on a different render layer and make that low sample while the objects on other layers were high sample.)
Creating a single mask of multiple objects.
For each object go to Properties>>Object>>Relations
set Pass Index to the same number (1) -arbitrary number, but same for all objects
go to Render Layers >> Passes
enable Object Index (can delete all others)
Change to COMPOSITING set-up. In node window Enable Use Nodes
The Render Layer node should appear with just one output INDEXOB
Add>>Converter>>IDMask
Connect RenderLayer IndexOb to IDMask
Add>>Output>>File Output
Connect IDMask to File Ouput
Set the folder (path address) in File Output to be where you want to send the masks.
Back at
PROPERTIES>>Camera(RenderSettings)>>Post Processing
enable COMPOSITING (so that when you click render or animation it uses the output from the compositor and not the 3d scene or video editor.)
Click ANIMATION in Properties>>Camera
The output is in openEXR (This can be loaded and viewed in Blender, but my Windows10 doesn't offer anything else capable of displaying it.)
