In this tutorial I will try to explain the various render passes that I found useful in Maya and Mental Ray workflow during the production of my latest character. This tutorial will also cover some workarounds for mental ray shaders that won’t render properly with render layers.
At the end of the production on my Asian Lightning mage I decided to make a few very large renders off the character to show off all the detail. These renders where all about 8k Tall this resolution was needed for high fidelity printing on poster sizes.
Rendering on such high resolution can be very time consuming, tweaking small things can be really expensive because you have to wait the entire render time again to see the changes. To negate this problem a lot of render passes can be made, each with it’s own information. They can then be composited in a compositing package or Photoshop.
Before starting to work with render passes, a batch of all available passes was rendered to see which ones are useful for the creation of the final image. Below a list of all the useable outputs. For maximum flexibility during the compositing stage all passes are rendered out as 16bit uncompressed tiff images to avoid banding during compositing. Below I would like to share my findings.
2. Beauty/ Master Beauty
This is the image you normally get when rendering. Without the use of render passes you would be stuck with this image, only being able to do simple adjustments in Photoshop.
When working with render passes you can either neglect this pass since you can recreate with the other passes. Or use it as a starting point and comparison image for you final image.
3. Diffuse/ Diffuse Material Color/ Diffuse no Shadow
These passes only render the diffuse component, giving a very matte look. Diffuse Material Color is an unlit version that can be multiplied with lighting. However some problems occur with rendering some of the mental ray shaders. The Architectural Mia Shader and any SSS shader will appear black in the Diffuse Material Color and Diffuse no Shadow pass.
There is a workaround for this by placing an ambient light with a value of 1 in your scene. But the Diffuse Pass renders correctly, this might be enough in most cases.
4. Specular/ Specular no Shadow/ Reflection
The Specular and Reflection Components can also be rendered out separately just like the diffuse component. The way to add these in post is by using either Screen, Color Dodge or Linear Dodge. However some pretty dramatic reflections can be created by using Overlay, Soft Light or Hard Light.
5. Shadow/ ShadowRaw
The Shadow Passes need to be used with Multiply, Color Burn or Linear Burn. The ShadowRaw pas serves as a mask. But when inverted it can also serve as a shadow pass when using the same blending modes as with the shadow pass.
6. DirectIrradiance/ DirectIrradiance RGB Masks/ Indirect
These Passes are the lighting passes, they are great for tweaking the balance between the direct and indirect illumination and the intensity of specific lights. The directIrradiance and Indirect pass are created straight via the render passes. The RGB Masks variation is a custom made pass. The 3 Most important lights are picked and given red, green and blue as color respectively. These can then be extracted via the individual color channel to get a greyscale map. The only reason why red green and blue are used is because it allows you to extract the information of 3 lights at ones. You could also render each light individually with a white color but that would be less efficient.
The Depth Pass is a real workhorse, it takes just seconds to render and can fake an effect that is one of the most expensive to render; depth of field. Next to that you can also use it to create fog.
8. Ambient-Occlusion/ Ambient-Occlusion-Max-Distance/ Reflective-Occlusion
Ambient-Occlusion is a great way to quickly add realism to your render. It is lot faster to render then global illumination or final gather. The only drawback is that it doesn’t create color bleeding since it is a greyscale effect. The maximum distance has a huge impact on the look of the effect so it can be practical to render more then one Ambient-Occlusion pass.
The Ambient-Occlusion pass uses an infinite max distance and the Ambient-Occlusion-Max-Distance pass uses a finite max distance. The fact that these passes are rendered at 16 bit helps a lot with compositing. It even means that you could fake a max distance render by blowing out al lot of detail on the Ambient-Occlusion pass.
The Reflective-Occlusion is a special pass that is meant to be used to occlude specular and reflection passes. Just as the Ambient-Occlusion pass is actually meant to be used only to occlude diffuse lighting. When compositing the specular component can be dulled out by the Occlusion passes. If that happens you need to add the specular after Occlusion.
9. IncidenceCN/ IncidenceCNMat
Incidence is a mask for a Fresnel effect. Meaning that it visualises the facing ratio of the geometry. Black is directly facing the camera, white is facing 90 degrees away from the camera.
CN and CNMat are different variations of this effect. CN does not take the normal map into account but renders mental ray materials with normal maps. CNMat does take the normal maps into consideration but is unable to render the special mental ray materials such the architectural MIA shader.
You could also use the blue channel of the NormalCam pass, when inverted it gives an incidence pass that does render both types of materials but only the mental ray materials with normal maps.
10. NormalCam/ NormalWorld
The Normal Map passes generate normal maps depending on the coordinate space. Camera space for the Cam variation and world space for the world map. Especially the NormalCam pass can be useful since it contains an incidence pass in the blue channel and a vertical and horizontal light pass in the other two channels. Which can give nice effects in overlay mode.
Some compositing packages can use the NormalCam pass to fake lighting, this way you can light your image interactively in post production.
The Incandescence pass separates all the self illuminating parts from the rest of the image, this is great if you want to add a glow around light sources during compositing. In this example you can only see the light dome in the distance, since the model does not contain any light sources.
12. RGB Masks
The RGB Masks pass is a custom pass in which you can define up to 3 different material types that you want adjust separately from each other in post. Each color channel contains one mask that can be used by the compositing package. By rendering out more RGB masks you could potentially create a mask for each object.
13. Before and after
Below an example of what compositing can do to an image. Compared to the composited image, the original render looks a bit dry and dull. Compositing is a good place to tweak the colors and contrast and can either make or prevent your image from looking CG.
In compositing we take all the layers created during the rendering phase and combine them into the final image. At this point all the decision are already made about the look of the final image, but compositing is the stage where the images either remains to look like a CG image or become something striking and believable.
Besides the technical considerations and the way you need to blend all the specific layers ( which is explained in detail above) most of the compositing has to do with evoking a feel to the spectator and playing around with colors. There is no real guideline for this because it varies from image to image. The color corrections done for portrait render for example do not apply to the full body render.