Question on lights

1.When setting an object to act as a light, why does this significantly increase the time it takes to render? Or is there something we need to make sure we do that will help this?

2. When rendering, some lights may not come on (shine light) until after a number of passes. So (without re-rendering a bunch, or rendering a lot of unecessary passes) how do we know how many passes to render to make sure things are displayed correctly?

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Permalink Reply by Render Plus on November 11, 2011 at 2:44pm

Some objects, when turned into a light create too many lights because of problems in the geometry. While rendering right-click on the rendering window and select statistics and it will tell how many lights you have, If you have too many, you may want to place a point light in your object, rather than treating it as a light.

For instance I just placed this tree component, and marked it as a light. I wound up creating 121 separate lights - because the trunk and leaves caused us to start new light entities as the material changed.

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Permalink Reply by Render Plus on November 11, 2011 at 3:02pm

Lights Per Pass

We don't process all the lights on every pass, because usually the overall rendering time is faster if you only process 1/3rd of 1/5th of the lights per pass.

Added to that is the fact that the light may be processed for the first time in pass 3, but its indirect light (bouncing) effect is not processed until a later pass. So you need 10 passes or so to start to see the direct and indirect effects of each light.

You can override this on the render Settings tab to process all lights on every pass, but our experience is that it is better to leave it at 33% and allow for more passes.

Sampling Points per lighting pass

Related to this is that just a single, large, light may need several passes because we only process light for a lamp one (random) point at a time.

In this model only one point is sampled from the large yellow light on the back wall after 1 rendering pass, so if creates a single crisp shadow. (As if there were a small, single light on the back wall)

After 4 passes, 4 separate, random, points have been sampled, so there are 4 shadows.

Even after 20 passes the individual shadows are still visible.

This will require hundreds of passes to converge. Here is the result after 100 passes.

(Note: This is an extreme example, with a single, large light source. With more lights - e.g. ceiling lights, and with indirect lighting , which I turned off for this example, this will converge better and faster)

Although we can often get good results in 20 to 40 passes. The results just keep improving as you add more passes, and many users let their rendering "cook" overnight for hundreds or thousands of passes to get a much more stunning result.