Render layers (or passes...whatever you prefer:)
fxphd.com video tutorial on render passes
Jeremy Birn "Multi-Pass Rendering Overview"
Nice compositing tutorial from Simon Reeves
So now that you know a little bit about how the render layers work let's get to work! (I'm trying to motivate myself as well here)
There are different reasons for rendering in layers, the main points being, it gives better creative control and you can save a lot of render time by using multiple computers.
If your scene includes multiple cg elements you may want to render these separately. You can even render different parts of the same objects to split the render up into finer pieces. For this project I decided to divide my object into the main body, wings, windows and ground.
Let's set up which render layers we need for the different parts
Main Body:
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Wings:
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Windows:
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Ground:
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In addition we'll include:
- Masking pass (or RGB pass)
Choose one(!) frame of your shot to make a test render and see how it will come out after compositing. This is a big time saver instead of rendering out everything praying that it will come out perfectly, oh and remember to save your images as .tif or .tga to store the alpha channel.
To be able to control our render layers we need an understanding about what the different layers do so that it is possible to adjust and fix the things that can go wrong. You can make use of Mayas' render presets by right-clicking the render layer and use the presets menu but I will go thorough how to effectively modify or create your own.
Maya render layer setup
Compositing in Maya and different render settings
Maya lets you at any time render out you render layers separately. As in any compositing program the render layers can be mathematically brought together to create an arbitrary composite although with very limited tools. By navigation to 'options'->'render all layers' options you will see that maya can 'composite layers', composite and keep' or just 'keep' layers. To composite the layers the render layer must be renderable (marked with R) and have a red recycle icon (green icon will not render the layer and composit it as is). By clicking on the 'normal' tag reveals the mathematical compositing tools available (lighten, darken, multiply, screen, overlay) which can be assigned to the layers. The sphere icon indicates if there are different materials added in the layer and the flag indicates 'member override' in the layer settings.
The mighty 'Render Layer Override'
One incredibly handy feature in the Maya render layers is the 'Render Layer Override'. This function enables the user to right click on any feature while working in a render layer to change the values and attributes. When the attribute is marked as 'render layer override' it will turn orange and all the changes done will only affect that render layer...pretty cool and extremely handy if you ask me...
Before we start make sure to disable the image plane from your render camera. We do not want to include the background in our renders to keep the apha-channel information. I usually do this simply by offsetting the footage by, say 1000, using the 'Frame Offset' in the image sequence settings of the image Plane. There are situations where you need the image plane information and this will be covered later.
Main body renders:
Diffuse
If you use the 'diffuse' preset you will end up with a flat shaded diffuse because what Mayas preset does is to turn off ray tracing altogether. We want a diffuse pass with ray trace shadows so create a new render layer with all the geometry and lights. Do the following in each of the lights' attribute:
- Right-click on 'emmit specular',
- choose 'Create Layer Override'
- uncheck the 'emmit specular tab'.
It will now turn orange to indicate the override. As ray tracing also effects the reflection you should select the reflective materials and override it not to show any reflection (set reflection to 0).
Diffuse pass with final gather
A little anecdote about self shadow
I'd like to go through how to hide certain parts of your object and still keep some of the needed information. In this case I chose to include the self shadowing from the wings in the diffuse layer. As we see here the wings cast a shadow on to the main body so how can we hide the wings in the render without loosing the shadow information?
We could go in to each part of geometry and hide these individually in it's 'render stats' but this is not very effective when you have a lot of pieces making up the part. The best solution, as I see it, is to group the different pieces together and use the 'mental ray' tab in the 'groups' attributes to control the viability through (again) a 'render layer override'. This keeps all the shadow information while removing everything in the group.
Now we can control which objects will cast shadow by grouping and hiding with override. Sweet
Reflection
Maya does not have a reflection preset and so this one is a must to create. Thankfully it's not that bad. All you have to do is to make sure to create a new render and include everything you want. Then go into the reflective materials and use override to set the material to black, turn the eccentricity way down and set the reflectivity. You can also create a new black reflective material and assign this. If you are working in a render layer assigning new materials will only affect the layer. Remember to include the environment sphere from the lighting and reflection section in the scene and that it is being lit up somehow (increasing incandescence usually works).
Ray traced reflection pass
Specularity
Specular is pretty straight forward. Either use the preset or disable the 'emit diffuse' with the override and turn off ray tracing. Again, hide objects as needed.
Specularity pass
Occlusion
Occlusion is a shader creating soft contact shadows between objects and brings out the little details. When selecting the occlusion preset a surface shader with an ambient occlusion connection gets assign to all the objects in your scene. This creates a nice soft shadow pass to multiply or darken over you composition but we need to include our CG elements and the ground to get the occlusion information we need.
Note that the occlusion pass is pretty slow to render and requires the most process power to finish so start this early if possible.
Occlusion render
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