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Decay rate

From: Lighting and Rendering with mental ray in Maya

Video: Decay rate

This movie reviews the basic different types of decay rate for direct lighting in the Maya scene. Decay rate refers to how fast the intensity of a light decays as it leaves a light source. So in this scene, I have added two point lights to represent candle flames above the candle geometry. I am going to select this light here, and we can see I have positioned it just above this candle here.

Decay rate

This movie reviews the basic different types of decay rate for direct lighting in the Maya scene. Decay rate refers to how fast the intensity of a light decays as it leaves a light source. So in this scene, I have added two point lights to represent candle flames above the candle geometry. I am going to select this light here, and we can see I have positioned it just above this candle here.

Now, if I am going to position these lights and then do a quick render, what we'll see is something sort of similar to a nuclear blast. Certainly, no candle that I can think of creates this much lighting in the scene. So this is a typical problem when you first add a light to a scene, especially noticeable with things like point lights where they shed light in all directions. So what I need to do in order to fix this problem is turn on the Decay Rate for the light.

So I am just going to hide my first candle here, and just look at this light right here. Now that the light is selected, I can open up the Attribute Editor and start adjusting some of its attributes. I am clicking on the candleShape2 tab right here, so I can get to the actual lighting settings. And as you can see that we have the Light Type is set to Point, the Color is White and our Intensity is set to a value of 1. These settings ensure that it actually sheds light in the scene, and it emits both specular and diffuse lighting.

The Decay Rate is set to No Decay, which means that no matter how far the light travels from the light source, it's never going to lose intensity. It's essentially infinitely bright, and that's why when we render with just a single candle flame, we get what looks like a nuclear blast. You can see how it affects the scene. So I am going to store this image, and I am going to turn on the Decay Rate. So we have three basic types: Linear, Quadratic, and Cubic.

Linear Decay Rate essentially means that as light leaves the light source, it goes down in intensity, equal to the distance as it travels from the light source. So it's just got a Linear drop-off and if I render using Linear, I'll lose my nuclear blast, but you will see that this room is much darker now, because by the time the light actually hits this table surface, it's decayed so much that it's not bright anymore. So in order to fix this, all I need to do is increase the Intensity.

And generally speaking, this is something I do when I'm just establishing the basic lighting for the scene. So I am going to pump this us up to 5, do a render, and see how it affects the lighting in the scene. Now, we are getting something closer to candle light. I don't have any shadows turned on for this light; it's just basically what we are seeing is the falloff, the decay of the light as it hits these other surfaces. So I can store that and compare it with my previous render. So this is with an Intensity of 1, and this is an Intensity of 5.

If I set this to Quadratic, Quadratic is actually the most realistic type of decay, and what it means is that the light intensity drops off based on the square of the distance from the light source. This actually represents how light in the real world works. So this is the most realistic type of lighting, but you'll notice if I leave this at an Intensity of 5, and do a Render, we are going to get back to a situation where it's almost completely dark; you can just barely see a little bit of lighting there. So I am going to store this, and now I am going to pump this us up to, let's try 15 and see what happens.

I am trying to get a little bit more lighting, so I am going to go up by 25, and I usually do this in a very experimental way. I'm just trying to see what looks good. Since I am not a physicist or mathematician, I don't actually think of the formula right offhand. I just basically pump up the light until it starts to look good. So I am going to set it this to 50 and do another render, and we should start to see something that looks a little bit more like a candle flame.

I'll compare this to, this is a linear drop-off; this is quadratic. We see a pool of light here that really drops off in more of a softer kind of way, so it leaves the light source. If I set this to Cubic, now the intensity of light drops off based on the cube of the distance from the light source. This means, as you can probably guess, that we really start to have to pump this light Intensity up.

Well, let me do a render with it set to 50, so it matches what I had for Quadratic. And what we are basically going to get is a dark room. The light falls off so quickly that it doesn't even reach the surface before it's at 0. So, I am going to pump this us up to 200. This is an experiment, and we'll see how this looks. A very small pool of light, it is actually lighting the scene. Let me set this up to 500. There we see it's bright here in the candlestick, but even by the time it reaches the surface, it barely reaches the chair. It's very dim.

So if I compare this to Quadratic, and this is Linear. Now, which type of Decay Rate you use depends on the kind of lighting that you are creating, and how realistic you want that light to be. Generally speaking, for most basic lighting, you can get away with using Linear because it's one of the easiest ones to work with, or Quadratic. Very rarely I'll actually use Cubic because Cubic actually the light is falling off even faster than it would in the real world. But every once in a while, when you just can't get the right intensity for your light, you might want to experiment with one of these settings while you are establishing the basic lighting with the scene and see which one works best for you.

I am going to set this back to Linear with an Intensity of 20, and do a quick render and see what we get. A little bit bright, but you get the basic idea.

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This video is part of

Image for Lighting and Rendering with mental ray in Maya
Lighting and Rendering with mental ray in Maya

59 video lessons · 7890 viewers

Eric Keller
Author

 
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  1. 3m 46s
    1. Welcome
      1m 32s
    2. Using the exercise files
      2m 14s
  2. 19m 8s
    1. What is a CG light?
      1m 22s
    2. Types of CG lights
      10m 55s
    3. Direct lighting
      4m 48s
    4. Indirect lighting
      2m 3s
  3. 53m 20s
    1. Decay rate
      6m 30s
    2. Previewing lighting and shadows
      2m 37s
    3. Creating depth map shadows
      1m 57s
    4. Troubleshooting depth map shadows
      2m 38s
    5. Shadow map overrides
      5m 30s
    6. Using the shadow map camera
      5m 31s
    7. Saving and reusing shadow maps
      2m 48s
    8. Creating raytraced shadows
      1m 56s
    9. Adding softness to raytraced shadows
      3m 42s
    10. Creating area light shadows
      5m 11s
    11. Sample: mental ray area light
      4m 23s
    12. Setting area light visibility
      8m 7s
    13. Creating soft shadows with spot lights
      2m 30s
  4. 43m 35s
    1. Setting global illumination for interiors
      2m 33s
    2. Tuning global illumination
      5m 56s
    3. Global illumination photons
      1m 12s
    4. Activating caustic light effects
      3m 28s
    5. Tuning caustic settings
      3m 35s
    6. Setting caustic light effects on metal
      2m 35s
    7. Using final gathering for indirect lighting
      2m 9s
    8. Tuning final gathering
      4m 2s
    9. Reusing final gathering maps
      3m 21s
    10. Adding light with shaders
      5m 27s
    11. Creating final gathering maps for animation
      4m 26s
    12. Combining final gathering with global illumination
      4m 51s
  5. 1h 2m
    1. Activating the Physical Sun and Sky network
      2m 33s
    2. Tuning the Physical Sun and Sky settings
      7m 18s
    3. Applying physical light shaders
      8m 54s
    4. Applying image-based lighting
      8m 57s
    5. Tone mapping
      6m 23s
    6. Applying portal light shaders
      7m 45s
    7. Creating light beams with participating media
      10m 9s
    8. Adding depth of field with the Bokeh lens shader
      10m 39s
  6. 48m 21s
    1. Introducing render layers
      6m 13s
    2. Creating render layers
      4m 28s
    3. Splitting a scene into render layers
      15m 36s
    4. Applying render layer presets
      7m 47s
    5. Setting render layer overrides
      7m 7s
    6. Creating render layer composites
      3m 52s
    7. Organizing renders with tokens
      3m 18s
  7. 42m 24s
    1. Introducing render passes
      2m 56s
    2. Comparing render passes and render layers
      6m 44s
    3. Editing render passes
      10m 41s
    4. Using appropriate materials
      5m 51s
    5. Batch-rendering passes
      5m 56s
    6. Compositing in After Effects
      6m 41s
    7. Rendering the EXR image format
      3m 35s
  8. 23m 3s
    1. Anti-Aliasing Quality
      6m 44s
    2. Setting color profiles
      2m 53s
    3. Diagnosing raytracing
      5m 7s
    4. Adjusting motion blur
      6m 57s
    5. Finding mental ray help
      1m 22s
  9. 21s
    1. Goodbye
      21s

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