I’m sure this Cycles noise reduction technique is not new, but for those like myself who didn’t know it: baking light textures to plane(s) aligned close to floor(s), and using those textures as emission sources dramatically decrease render noise in scenes lit by small light sources.
Longer story: I’ve been trying to render an exterior archiviz scene where only light sources are small spot lights (night time). As is well known, this causes a lot of render noise, which in my case is practically never eliminated by increasing samples. However, using this light baking technique, I was able to get a tolerable result (5000 samples):
Then bake diffuse pass into it, edit texture (blur to make it smooth), make the plane an emission source and invisible to camera, and then render.
Admittedly, some of the photorealism is lost in this example (the foreground seems artificially lit), and it is easy to get light artefacts from texture. In future I’ll try to make separate lighting planes for the ground and see if that helps. But as it is, I’m quite happy with the result already now. Please reply if you know a better way around this lighting noise issue!
To re-gain some photorealism you could set your lamps to 0 bounces with branched path tracing, or by using lightpath node > ray depth. Then you should be able to see direct light effects: light on the ground around the house and all those shadows from the porch
Another critical realization is: there is a difference between the actual(!) lighting of the scene, and the so-called “practical lights.”
A “practical light” is a thing, like a torch or a porch-light, that we know from human experience is “a source of light.” Many people innocently try to actually light the scene with them, and this is a mistake.
Consider what you would see if you were on the set as Alfred Hitchcock filmed some dark and forbidding scene. What would you find, just off-camera? Lots and lots of studio lights! And then, oh yes, the candle that the heroine just lit as she stared fearfully at the ominous shape outside her bedroom window. As the candle is lit, her face appears to be warmly lit by its glow. But, “appearances.” That candle is not the actual source of the light. (Neither is the moon the source of the shadow outside the window.)
“First, light your scene.” Then, add practical lights to it. Then, add some source for the lighting effects that are to be “apparently produced by” those practical lights: these are by-now very slight increments to the actual lighting of the scene. It is usually most convenient if the practical light is not flagged as a source of light at all.
You can do the same thing with shadows. A “shadow-only spotlight” is a wonderful invention, as is the ability to extract the shadow-layer from a MultiLayer OpenEXR file. You can do well to light your scene with shadow-calculations turned off(!), which saves very-considerable time, then to use compositing to add shadows … and, only those shadows which are most-important to your scene.
Layer-specific lights are another useful dodge: if you know that an object doesn’t need to be considered by a (shadow-only) shadow or lighting calculation, exclude it from the calculation.
You are aware that Cycles does not have shadow-only or layer-only lights, right?
For the noise, 2.79 will come with a state-of-the-art denoiser that should be able to clean up mild to moderately noisy scenes (don’t forget to use the Sobol sampler for best results).
you wrote a lot good recommendations, but not always an user wants to cheat and go for the ‘artistic way’. If someone is looking for photorealism, a shot from an Hitchcock flick might look wrong
thanks for your comments and suggestions! In this case I am primarily interested in getting some rough idea how the exterior would look like if it really were lit only by those four spot lamps (and is it enough or not), so I try not to use extra lights.
I tried zero bounces with BPT (branched path tracing), and the result quite nicely illustrates the problem: direct light illuminates only very small area. The scene is extremely dependent on indirect lighting:
I then made two renders: one for the house lit by the floor baked light as emission source (500 AA samples), and second for the floor and ground lit by the walls-ceiling bake (1000 AA samples). To be continued in next post because of max 3 attachments are allower per post…
Here are the two separate renders and the final composite image. Note: I applied heavy indirect clamping (0.1) to reduce noise from spot lamps. That was not a problem, because of the large area of the baked light sources, which now provided illumination!
