Remove the render noise from your LuxRender scenes

New users of LuxRender and Reality sometimes find themselves in a situation where an incredible amount of noise is still present in the scene even after thousands of samples have been computed. Something like this might happen:

A very noisy scene
A very noisy scene 

After waiting even more time for the scene to render things seems to clear up a bit, but not miuch. This is where the sinking feeling in the stomach hits you. Will all scenes be like this? The answer is a resounding “no”! What happened in the scene above is simply a lighting mistake caused by inexperience.

When you work with Lux you must remember that it uses real-world physics. If you had been shooting that scene with a photo camera and with the same lighting conditions, you would end up with a similar result: lots of noise.

So, here is the secret on how to avoid that from happening to you:

when lighting a scene you must light for the camera, not for your eyes.

Repeat that to yourself five times, until you memorize it. What that means is that, what your eyes see is not what a camera “sees”. In fact cameras have a much more limited range than our eyes and when we push a camera too far it shows characteristic noise in the image. Surely you have seen grainy pictures made with either film or digital cameras. That noise is the desperate attempt of the camera to see in the darkness.

You know why Hollywood movies don’t show the same problem? Because they are lit by professionals!

Those professionals, called cinematographers, know cameras and film stocks and they know what works and what doesn’t. That scene in “Lord of the rings” that seems lit by candles? No candles were use to light it! It had tons of other lighting appliances that you don’t see. The candle is just a prop to make you think that the scene is lit by the candle. So, today I’ll show you a “secret” of the trade and in the process we will get rid of the noise forever.

Lux needs light to paint what is on the screen. If an object is visible by the camera than it needs to be painted, but only light allows Lux to paint the object. If not enough light hits the object then you end up with noise.

Here is a typical example of a scene that will not work:

Scene setup
Scene setup

There is only one light in this scene: a sun pointed from outside to shine through that opening at the top. You can see by the position of the camera that we look at the inside of the crypt. Specifically, there is a spot on the floor, in front of the camera, that has a splash of sun light. See the first picture for the example of the render. The rationale of the aspiring artist is that situations like this happen in real life, and that the sun is usually providing enough light to show the inside of the environment.

That is correct if you base your evaluation on the human eye but it’s a grave mistake to do so. Remember, we are rendering with the software equivalent of a digital camera and that camera has not enough latitude to replicate the scene as planned.

When rendering a scene like that we are asking Lux to paint an impossible image. This is demonstrated by doing a test render and setting the exposure correctly:

Single sun light correctly exposed
Single sun light correctly exposed

Notice that the splash of light shows the texture for the floor in full detail. That is how it should be. Unfortunately the rest of the scene is almost pitch black. This is a function of the camera’s latitude, the ability of a camera to render both bright and dark areas of the frame. Every camera has a very defined range of light that it can represent. This dynamic range, aka as latitude, is the range from the brightest spot to the darkest spot of the image that can be rendered by the camera . Very good, very expensive cameras can have a latitude of 11 stops (f/stops). This is the range of pro-level film stock or $100,000 digital cameras like the Sony F900 (without lens). Prosumer cameras might have around 9 stops of latitude. Point and shoot digital cameras might have much less than that. I didn’t measure Lux’s latitude (aka dynamic range) but that doesn’t matter. It has a fixed latitude and we have to deal with it as legions of photographers had to deal with it in more than 100 years of practicing the trade. Here is were all the nonsense about “digital photography not needing lighting” falls apart. The way you light a scene today has not changed in decades.

If you look at the “Single sun light correctly exposed” image and you try to adjust the exposure to “see” in the dark areas you end up with this image:

Incorrectly exposed scene
Incorrectly exposed scene

The dark areas are all noisy because Lux didn’t have enough samples and, most importantly, the floor is badly overexposed. You simply can’t have that, it looks ugly and unprofessional.

So, what’s the solution? To answer that question you have to first formulate the problem correctly. The problem is that the contrast ratio between the dark areas and the light ones is too high. The contrast is so high that it’s beyond the capabilities of our camera. So, the solution is to re-balance the contrast ratio. This is done by adding lights!

Ask yourself: what do we want to obtain from this scene that is not there?

The answer is simple: we want to keep the sun light on the floor but also see the inside of the crypt. Therefore we need more light. So, we need to add lights to illuminate the areas of the crypt that are in darkness. Lastly, we need to do that in a subtle way so that it looks natural, as if the sun light is providing the bounced illumination. To achieve this we need to avoid casting unnatural shadows from the additional lights. This rules out spots and point lights. Point lights are especially hard to control, and spot lights cast a light that is harsh with visible shadows. So, we need a couple of large mesh lights.

A couple because that distributes evenly the light, and we need them large because the harshness of light depends on the size of the light source. The presence of shadows does not depend on the brightness of the light, it depends on the size of the light. That’s how it works in the real world and thats’ how it works in Lux.

So, here is the new setup:

Crypt with fill lights
Crypt with fill lights

Notice the position of the lights, behind the camera.
Since we had a good exposure in Lux for the initial scene, with only the sun light, I copy the exposure settings to Reality to make sure that we keep the right exposure throughout our tests. This step is crucial. We want to change the contrast ratio, not the exposure.

Next, I assign a custom group to each mesh light so that I can adjust their intensity in Lux independently. By having this level of control we can effectively paint with light.

Reality Light configuration
Reality Light configuration

Now if we render the scene again we obtain this:

Crypt with sun and fill light
Crypt with sun and fill light

After just twenty two minutes of rendering, instead of almost three hours, the scene looks nice, with a good fill that shows the inside of the crypt, the splash of sun light still perfectly exposed and not much render noise at all. Just letting it render for a few more minutes will take care of all the remaining noise.

So, the lesson here is simple:

  • Light for the camera, not your eye
  • Be sure to expose correctly every part of the scene
  • Don’t be afraid to add lights to compensate for the camera’s limitations
  • Take advantage of Lux’s live light adjustment by using Custom Light Groups. See the Reality User’s Guide for how to create those.

Here is the final render, created with a couple of more “tweaks”:

  • I changed the Sensitivity to 64
  • The Film Response is set to “Agfacolor optimaII 200 CD”
  • The gamma has been set to 2.80 to stretch the ability of the camera to see more dark tones.

The sun spot is almost on the verge of being overexposed but that is perfectly fine. A bit of overexposure can add to the realism, when you have it under control.

Final crypt scene
Final crypt scene

And as a bonus I also tried adding the bloom effect in the Lux’s “Lens effects” panel:

Crypt scene with bloom
Crypt scene with bloom

I hope this was useful, have fun with Reality.

Pin on PinterestShare on FacebookTweet about this on TwitterShare on Google+