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Render Passes/Layers (zDepth and 2D Motion Vector)

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Spencer MacPherson
Render Passes/Layers (zDepth and 2D Motion Vector)
on Apr 15, 2013 at 6:18:58 am


I have a few questions about render passes, render layers and the workflow of them. I also have a problem with 2D Motion Vectors.

Let's start with the 2D motion vectors. On my pass for the 2D motion vectors, I get this weird lines/grid on it. The following image show's what I mean. How would I go about on fixing this?

Secondly, I'm trying to export a zDepth pass to add dof in After Effects, however it produces an unwanted result of a halo around the edges, shown on the lower image. Is there an easy way to fix this?
I though that if I put the scene elements on different render layers it would remedy my problem, however I don't know the workflow on render layers. I don't know how to render to objects out as separate passes, but still have the interact with other objects, such as casting and receiving shadows. If rendering out the objects is the best way to fix my dof problem, can someone explain the best workflow for it?


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Steve Sayer
Re: Render Passes/Layers (zDepth and 2D Motion Vector)
on Apr 19, 2013 at 2:30:43 pm

Hi Spencer. I'm not sure if I've got the answers to your problems, but I can take a few guesses.

For those lines/grids that you see in the motion vector pass... it is possible that what you're seeing is the correct result. Since monitors can only display colours from 0 (black) to 1 (white), but the speed of an object can be much more than 1, you might be seeing that colour pattern appear as values increase well beyond 1; the display would be 'looping' back to 0 every time it crosses that maximum, so a surface that had a velocity of 45 on one edge and 49 on the other, say, would have 4 visible 'bands' like you're seeing.

Again, this is just a guess, since I don't know what renderer or shader you're using, etc.

As for your second question, yes, render layers might help with this. A fundamental concept when working with render layers is that objects can be present on a layer without being directly visible on that layer. Here are some relevant links from the Maya documentation:

Render Layer Overview
Work with attribute overrides

So in general if you need object A to cast shadows on object B without being rendered together, simply include object A in the same render layer as object B, but open its render attributes and uncheck 'Primary visibility' while leaving 'Casts shadows' checked. When the name of the attribute goes orange, you know it has created an override which is specific to that layer. You can still include object A on its own render layer and it will be visible as normal.

Hope that helps.

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