Render passes

When a 3d programme such as Maya or 3D Studio Max renders an image, sometimes the results are not quite perfect. The lighting may be off, the colour may be overblown or something may need changing that would require the entire scene to be re-rendered.

There is a solution however in render passes. Essentially a render pass is used to separate the colourmap/shadows/light/specular/diffuse and other effects onto their own separate layer. Rather than the 3D programme combining these effects, tehy can be layered on top of each other in a compositing programme such as Nuke/After Effects/Premier/ and Sony Vegas. This allows for instance, control over the darkness or hue of shadows on their own layer. From here the shadow could be blurred, sharpened or have other effects that cannot be simulated in the 3D programme it was rendered in.

Hypothetically, a scene that has a bright red light in it was passed to the director and he did not like it. Rather than re-render the entire scene the layer that the light was applied to could be tweaked to a more suitable colour. The compositor already has the information and render pass applied in a sequence, so they can adjust it in real-time rather than waiting for it to be processed all over again.

Render passes always come down to control over several elements on layers, rather than one whole element.

An example of separated render passes, each with their own layer then processed and edited on a single image.