Many artists out there are not aware of the wonders of working with layers – also called multipass rendering. Picture this scenario. We´re doing a render with 20 lights and several scene elements. Lets say the final image takes 1 hour to render. Now, if something doesn´t look good, and needs re-adjusting, we need to re-render the entire image. Another hours goes by. Now, we notice that something else is wrong. And we spend another hour re-rendering.
Now, let´s look at this with layering in mind. We split the render into several separate renders. For instance, each of the renders can contain a single light. For instance, some background elements maybe can br on a separate layer. Now, rendering all the separate will take approx 1 hour. We then bring our images into a compositing application (Photoshop or Sony Vegas Pro). And here´s the beatuy. If we don´t like the effect of one of the lights, we can re-render that layer only. Let´s say that takes 5 minutes. We then re-insert that layer into our composition and save tons of time.
Not only that. ANY layers can be re-adjusted live in the compositing software without re-rendering. You can adjust the intensity, color an apply other effects. Now, see the power? This is especially powerful when dealing with 1000´s of freames long animations that take days or even weeks to render.