Area Lights 101
As computers and algorithms become faster and smarter, area lights can be used more often without much performance penalty.
Why would you use an area light?
In the real world, lights of all types are represented in 3-dimensional space, meaning all lights have area from which they emit light.
A tungsten light bulb has a tiny filament, the tube fluorescent light has a cylinder, and the sun has a disc. This means several things for representing light from the real world inside the computer.
- Light from an area source of appropriate size looks more natural. This is because our brains perceive light strength and size based on reflections of the actual light and the relative softness (spread) of the shadow.
- Recreating real-world lighting should be more natural because you should think in terms of the literal source from the set.
- Scenes in correct scale along with their lights should need less tweaking for the correct “quality” of light.
This tutorial will go over the basics of the mental ray area light settings inside Maya. We’ll look at how to control the quality of the light and settings that work well for Brute Force Unified Sampling rendering seen in several places on the blog, starting here: Unified Sampling: Visually for the Artist
In the legacy days of Maya and mental ray, you had to create a light such as a spotlight and ‘turn it into’ or convert it to an area light. This is no longer necessary and is in fact, deprecated. Don’t let anyone see you doing it! Instead you should create an actual area light and use the mental ray rollout to create an area light.
Some things to notice under the attributes:
1. Color: Obviously the color of the light. Sometimes replaced with a utility such as the mib_blackbody. Keep in mind from previous posts that for correct linear workflow you need a Gamma node here if you are simply going to choose a color from the color picker; correct it using gamma 0.4545 since Maya colors are sRGB. This corrects to linear color workflow: sRGB -> Linear Color: Linear Color Workflows in Maya: Part 1
2. Intensity: This is the strength of the light. Incidentally this does not match anything like watts, etc.
3. Decay Rate: To maintain a physically correct light source this should be set to Quadratic. In doing this you will find that your intensity will have to be increased very much depending on your scene scale. High values are perfectly fine.
Lastly, time to turn “on” the mental ray area light shape. Under the mental ray rollout there is: Area Light -> Use Light Shape. Tick this “on”.
Place this in a scene with the quintessential “sphere on a plane” setup and hit render current frame.
It probably looks atrocious (unless you’re savvy enough to have already set up your scene for Unified Sampling, but even now we can probably improve your result.)
Area lights can introduce grain into your render. Why?
In order to correctly see an area light, the point being shaded needs to sample it. In doing this the shader will send rays back to the area light to try and see as much as possible. These points are spread across the surface of the light to avoid a regular pattern in exchange for noise that is more pleasing. Such a pattern might look very similar to that used for QMC sampling.
You can control this locally for the light.
How do you make sample decisions based on the light and the scene?
First lets do a few different things to the light.
Turn on shadows in the light. I cannot for the life of me understand why the default for Maya lights is still no shadow. It is the year 2012, do not fear shadows. Maya 2014 has thankfully changed the defaults to those you see here for shadows. See the post on what has changed here: mental ray changes for Maya 2014
In this section you will see:
1. Color: Leave it black. In the past you would change this to “fake” an indirect light by giving it some color to mimic. We will assume you are using modern illumination techniques like Final Gather in your scene. Leave it black.
(You see I have collapsed the section for Depth Map Shadow Attributes. They are used less often now that raytracing is relatively less expensive. They will be covered later.)
2. Shadow rays: The Autodesk light shader allows you to resolve grain in a shadow by adding more local samples in different lights from the area light to point, spot, etc. We will use a different control for this, leave it at 1 (Simplify your life by reducing the places you go to for settings.)
3. Ray Depth Limit: This is a bit more tricky and also relies on the global raytracing settings found here:
This setting along with the global setting above restricts how may times a ray may bounce for a reflection or refraction and still generate shadow samples (to make them visible in a reflection or refraction.)
Zap explained these settings here: Maya’s Default Shadow Settings
For simplicity I will restate them here with his images and update them a bit.
In order for a shadow to be seen in a reflection or refraction you must allow the shader to call the shadow after the ray has been reflected or refracted. mental ray will count down the number of times this happens and eventually tell it not to sample for shadows (cast shadow rays) You see below this affects even transparent (colored) shadows and can make your scene look incorrect. Notice the red transparent rectangle and the view behind it.
This is a useful optimization because shadow rays can be very expensive to propagate everywhere, especially from area lights. The defaults he mentions (2 for ray depth) are generally visibly acceptable for many scenes. Especially those with blurry reflections where such an effect isn’t noticed at all. However, a depth of 3 may provide you with the best quality if you can afford a little extra time. You will notice that the Final Gather preprocess phase will see the shadows at a depth of 3 (this is a Maya specific bug, 3 is also now the default in Maya 2014).
Ok, so how do you make the light and shadows look good?
Area lights have a section under the mental ray controls to provide samples. So let’s look at the settings you maybe have typically seen before Unified Sampling appeared.
I have seen this section abused time to time.
1. High Samples: this is the amount of samples to shoot (draw) towards the light when an eye ray strikes an object. This means primarily visible. You want this to be your most important level of quality.
1a. The larger and closer the area light is, the more samples you may need
1b. Inversely, the smaller and further away it is, the fewer samples you need
2. High Sample Limit: Once this number of combined reflections/refractions is exhausted, the sample can draw fewer samples as defined by the Low Samples setting.
3. Low Samples: this is the amount of samples to draw for a sample taken after the number of reflections/refractions in the High Sample Limit have been exhausted.
4. Visible: Will the area light be visible in the render. In the case of the Portal Light shader it must be on to work correctly. The mia_material will also skip generating a specular highlight for a visible area light by default. This is desired because a spec is a fake for a direct reflection of a light with no area. A light with actual area should genuinely reflect in the object. Doing both doubles the energy incorrectly.
In many cases I see the High Sample Limit set to 16 or 32 without any understanding of what it really does. In this case up to a combination of 16 or 32 reflections/refractions will still draw 32 samples. In a scene with a lot of raytracing effects and depth, that’s murder on render time. Or similarly I see the Low Samples set to something obscene like 64!
These examples were rendered with fixed pixel samples of 4 so only the effect of the area light samples is taken into account.
Notice that changing the Area Light Samples locally reduced grain in not only the shadows, but the highlights and directly lit areas as well. This is also why low Quality or samples for the Native (builtin) IBL can show grain on highlights, etc. It is a similar effect. So for your overall quality you can use one set of controls and then allow Unified Sampling to choose more when necessary. Also keep in mind that multiple overlapping lights on the same area can get away with fewer samples individually as these will add up on the area being sampled and show less grain (assuming the lights aren’t creating a high contrast color difference.)
Using Unified Sampling and changing the size of the area light:
Be careful with scaling an area light when you have a custom shader attached. Some shaders will scale the intensity of the light based on size. In many cases this is correct and desired for the shader, but it is not the default behavior.
What about Unified Sampling and Brute Force?
In testing scenes with large and multiple area lights (10+) as well as special area lights like the Native (builtin) IBL, we found low but not single samples are best.
Generally speaking, a range from 4 to 8 is good. And in fact we have set the samples to (High, High Limit, Low) 4 1 4 or 6 1 4 and variations with good results.
Area Light Samples 4 generates more eye rays from Unified Sampling. This means it’s good for Depth of Field or Motion Blur where more eye rays are already useful for the overall effect and multiplying these is less expensive. Area Light Samples 8 produces fewer eye rays but at the cost of more shadow rays; this might be useful for a still frame. Area Light Samples 6 seems to be a good middle ground when used with Brute Force Unified Sampling. (Best of both worlds)
Quick metric: In an unreleased still (hopefully to be added later) I can render a car interior full frame at 6000 x 3376 with 11 area lights and brute force Unified Sampling in 2.5 hours. These area lights were set to 4 1 1 because the majority of reflections were very blurry/soft for leather and cloth.
- Some versions of Maya have a bug in mental ray where the Shadow Limits for area lights always reach 3. So setting a lower limit will have no effect. More recent updates may have introduced a fix for the bug. (I am not on SP1 here.)
- Autodesk uses their own way of making light shaders to mimic legacy lighting. In some situations this is not desirable (in the case pointed out by Jeff Patton; where the center of an area light may be brighter on a surface. Although very subtle, it can be annoying.)
- Further optimize your scene by selectively choosing what object may cast or receive shadows. For example: a car window may not need to generate shadow samples or even receive them to look good.
- Understand that “clear” and “colorless” for shadow objects are not the same concept. Windex is clear, but it’s blue and should cast a blue shadow. Clean water is clear and colorless.
- When you have a lot of art directed imagery with lots of lights, you can reduce indirect illumination quality without image quality loss. This is especially helpful with lots of area lights.
- The Native IBL is a giant area light. When using this on exteriors and other images you can greatly reduce Final Gather settings since it will only return secondary lighting information.
- Area Lights generate multiple samples per eye ray sample. When you naively layer shaders this will increase the number of rays linearly. For example: plugging in a shader to the additional color of a mia_material and then assigning it will double the number of rays shot (For this example 2 shaders are run for the light loop: 2 * total lights * samples = a lot of rays) Try to avoid this by keeping networks simple or using mib_interpolate to use importance and weight to run a shader layer.
- Use further optimizing like the threshold for the physical light: Optimizations: Lighting and Thresholds
- In the render settings you will see an option for Sample Lock underneath Jitter. Sample Lock keeps similar sampling patterns across frames. In the case of Area Lights you may see a static noise pattern slide over your animation frame to frame. Disable this feature to randomize the pattern and generate noise which may be acceptable when seen in motion.
- I didn’t use depth map shadows. Mostly because I am using Unified Sampling and want a fast and accurate setup. If I were using the rasterizer, need lots of soft shadows, and want motion blur, then I would possibly use Detail Shadow Maps. Detail Shadow Maps can generate very slowly at first but motion blurring them is inexpensive. I can also save a detail shadow map for certain parts of a scene (or a whole scene) and reuse them from disk at significant time savings. But for now we’re focused on raytracing and simplicity.
Optimizations: Lighting and Thresholds
We will be discussing different ways to optimize scenes for rendering.
For this one I will visit an old attribute of the Physical Light shader. You can connect the Physical Light Shader in the area light (or many light types) under the mental ray rollout and under the Custom Shaders rollout (here just attached to Light Shader, I am not emitting photons)
Here is the default setup for the Physical Light Shader:
This attribute is called the Threshold. According to the docs it is described:
[threshold] is for optimization: if the illumination is less than threshold, the illumination can be discarded and no shadow rays need to be cast. The default is 0.
This means that anything receiving less than the described threshold of light will neither cast shadow rays or add the light from it.
You might think this is handled by the regular falloff of the light where objects outside its influence will not attempt to run the light shader. This isn’t true. Your renderer is blind to this information and will try the calculation anyway only to return no illumination. (The renderer never knows unless it tries.) So this parameter is a hint that it shouldn’t even try beyond a certain level of ‘darkness’.
How will this help?
In VFX work you may have expansive scenes as opposed to cozy interiors. In many cases the lights are only going to affect objects nearby (this is true of other scenes than just VFX but means most in these scenarios). If I have 20 practical lights in my scene then my light loop will try to run the list and figure out their contribution. In a case where the lights are expensive (like an area light) this can mean large amounts of rays are traced for lights that are too far away to make a visual difference.
In the below example I have magnified the result so you can see how it helps cut back on expense.
A close up of this simple scene and a large area light with the physical light attached has a sample limit of 128. This image renders with the following statistics:
RC 0.3 info : rendering statistics
RC 0.3 info : type number per eye ray
RC 0.3 info : eye rays 946945 1.00
RC 0.3 info : shadow rays 105251572 111.15
RC 0.3 info : probe rays 3479348 3.67
RC 0.3 info : fg points interpolated 946945 1.00
RC 0.3 info : wallclock 0:01:12.35 for rendering
You can see here that I am casting 111.15 shadow rays (samples for the area light) on average for each eye ray.
So let’s zoom way out where the light won’t be able to illuminate the scene.
Now look at the statistics:
RC 0.4 info : rendering statistics
RC 0.4 info : type number per eye ray
RC 0.4 info : eye rays 474331 1.00
RC 0.4 info : shadow rays 54898447 115.74
RC 0.4 info : probe rays 298992 0.63
RC 0.4 info : fg points interpolated 474331 1.00
RC 0.3 info : wallclock 0:00:29.78 for rendering
Ouch, still a lot of rays per eye ray even though there’s nothing out there. Just a long expanse of flat dark plane.
The renderer knows the area light is out there somewhere. . .and it’s trying to sample it.
So now let’s look at the parameter. Generally speaking a good binary search is good for testing settings. Get the frame where you want it and jump halfway to say 0.5 The next test can be 0.25
If you need to go up, go to 0.375; if down go to .125, etc. Each time taking a halfway point to get it where you want visually.
So let’s say I am eventually going to zoom into the closeup with the cubes. So that is my measure of what I want it to look like at its best. (Assuming I don’t animate the parameter.) So from the original setting of 0.000 let’s try 0.5 first and see what makes the most sense.
It looks like a threshold of .125 looks pretty identical to the first image. The stats for threshold 0.125 are:
RC 0.3 info : rendering statistics
RC 0.3 info : type number per eye ray original
RC 0.3 info : eye rays 946837 1.00 1.00
RC 0.3 info : shadow rays 105195167 111.10 111.15
RC 0.3 info : probe rays 3478268 3.67 3.67
RC 0.3 info : fg points interpolated 946837 1.00 1.00
RC 0.3 info : wallclock 0:01:06.41 for rendering
Ok, there’s not a huge difference there, but I didn’t expect one because the images are the same. So now let’s zoom out again.
RC 0.3 info : rendering statistics
RC 0.3 info : type number per eye ray original (zoomed out)
RC 0.3 info : eye rays 514853 1.00 1.00
RC 0.3 info : shadow rays 11272699 21.89 115.74
RC 0.3 info : probe rays 312064 0.61 0.63
RC 0.3 info : fg points interpolated 514853 1.00 1.00
RC 0.3 info : wallclock 0:00:13.38 for rendering
We’ve gone from over 100 shadows rays to just 22 per eye ray. And the render time was cut in half!
Even though this is a trivial scene, you can see that multiple lights spread out across the scene will introduce overhead for areas they don’t even illuminate. For example, this image below if rendered would be very expensive. But lights down the bridge don’t contribute to the foreground and vice versa.
So spending a few minutes using this setting along with Brute Force-like Sampling and Progressive Rendering can help you quickly find a setting you can live with and render much faster than before.