Compositing 3D Renders

This month I completed a project in compositing 3D rendered images into photographic images. This helped me to better understand how composite 3D render layers and passes. I believe that the term ‘Compositing’ has a few different meanings and is sometimes misunderstood, so I want to try to explain it here.

In most of the popular 3D applications, the user has an option to render in different layers or elements. For instance, if there is a scene with a boy and his dog standing in a park-like area with a tree, then the tree and the rest of the environment could exist on one layer, and the boy and the dog could be rendered out in another with proper alpha channels to preserve transparency. These layers could then be broken down further into diffuse, specular, shadow, and reflection/refraction, translucency, ambient occlusion, indirect lighting, and others as needed. This allows for maximum control over every part of the image while post processing.

To explain why render layers and passes are so important, there should be an understanding of post processing. Post processing is manipulating the image or video after an image has been shot in-camera or rendered on a computer. So what’s wrong with just using a raw image? Doesn’t the renderer do a good enough job? It is ‘good enough,’ sometimes, depending on what you need. But it could always look better with a little bit of color correction, some levels adjustments, and so on.

Render passes allow a high level of control in this area by allowing adjustments to be made on individual aspects of the render. Shadows can be lightened or darkened as needed; transparency can be decreased on ambient occlusion or reflections that are too strong. This doesn’t have to be to make a final animation look as ‘correct’ as possible. Creative effects are possible, too, such as changing the color of a shadow or using ‘color grading’ to make a less color accurate version, but more expressive of an aesthetic vision. Perhaps the artist is told by his supervisor that a rendered car model needs to be a different shade than the original. If the model was rendered on its own layer, then there is no need to go back into the 3D application and re-render the entire scene. Another possible use is to render out the background of a scene only after camera layout is complete, so that when final animation is done, less render time will be required. It could also be helpful to use multiple layers for especially complex scenes that require too much memory to render all at once.

To give a more personal example, I am working on a short animated film involving superheroes. At the compositing and color correction stage, I intend to grade the film to give it more of an old cartoon feel, similar to the original Superman series. The different layers and passes will assist in matching shadow density while keeping it isolated from the overall color, adjusting the overall color of the environment based on the Superman cels, etc.

If this is still hard to grasp, I created a flowchart to show what this might look like.

I would also really recommend reading “The Importance of Compositing at http://greyscalegorilla.com/blog/2010/10/the-importance-of-compositing-a-layer-by-layer-breakdown-in-after-effects/.

Another good visual resource would be the video below, which is a breakdown of the different elements in compositing.

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