Control light intensity decay over distance.
- [Narrator] Arnold obeys physical laws, but also gives us the freedom to art direct the look of our lighting, using physical parameters, just like a real cinematographer or photographer would. Also, to a considerable extent we are permitted to bend or break physical laws to achieve our desired production goals. In lighting, one of the primary concerns is control over attenuation or light decay. In physics, this is described by the so called inverse square law.
It states that a radiant energy source will diminish in intensity, according to the inverse square of the distance. In Maya terms this is known as a Quadratic Decay. In Arnold version five, point spot area and mesh lights all use Quadratic Decay. The decay rate setting that you may be familiar with in the Maya light attributes is ignored. Intensity curves do work correctly, but the effect of the intensity curve is combined with the Arnold decay.
For physically accurate illumination with omni directional light sources then we need Quadratic Decay. However, many lighting instruments are constructed with reflectors or lenses to focus the light into a beam, which travels farther and decays less than the inverse square law. Although there's no obvious setting to disable decay in Arnold 5, we can still reduce or eliminate decay. As we saw in a previous movie, we can focus light energy into a collimated beam, using a spotlight with a very low cone angle and a non zero lens radius.
Another way to focus light energy and reduce decay is to use an Arnold area light with a low spread value. Create the Arnold area light from the menus. Arnold lights area light and once again it's very small at the origin, deselect it in the view port. And reselect it to get access to the transforms in the channel locks. Set scale X, Y, Z to 20.
Click and drag across the three X, Y, and Z attribute fields. Release the mouse, type in 20, and press enter. Now let's see what this looks like in the camera view. Right click in the camera view port to activate it. And in the menus, choose Arnold render. The Arnold render view opens. But we get a black screen. We need to adjust the attributes for the selected Arnold area light. Use Control + A to open the attribute editor.
And, let's first of all, disable normalize. And now we see some light in the Arnold render view. Adjust the exposure a little bit. Bring it up by one stop to a value of one. Now let's move the light back a little bit. I use the move tool, in the top view. Just click and drag to move that back. And now if we go into the area light transform note. We have exact values for X, Y, and Z translate.
I'm going to put in a couple of values that I've already established. For translate Y, we want a value of 30. Press the tab key to go to translate C and give that a value of 300 and press enter. And now the light has moved up slightly to be level with the table. And move back farther into the room. To control the decay of the area light, let's go back to the area light shape note and look for the spread attribute.
Bring that down and as you approach a value of about two or three, you start to see a spotlight type effect. And if we bring it all the way down to its' minimum of zero, we get an, actually, pretty clearly defined spotlight. We can't get a hard edge on this. And the suffusion or softness of the light is dependent on distance. But the decay has been completely eliminated for all practical purposes. The foreground and background are the same brightness.
We can also play with the roundness. Bring that up to its' maximum of one. And now we've got a perfectly circular pool of light. The final destination for this light is up in the ceiling. I've got some values for that, once again. Go into the area light transform note. For the translate Y value, enter in 250 and press tab. Or translate Z, 500. Press tab again.
And in rotate X, we want a value of negative 23. And now the light is positioned and rotated so that it's shining down onto the flowers. Let's further direct the look of this. Currently we've got no decay. And it's a spotlight type of effect. Let's say we want a floodlight. We'll go back into the area light shape attributes. And increase the spread, ever so slightly to a value of zero point zero seven. And that caused the light to spread out quite a lot on the set.
The brightness has diminished. And we'll need to compensate for that. Go into the exposure attribute. Give that a value of three point five. Now we have a floodlight effect with no decay. And the foreground or flowers, is fully illuminated at the same brightness as the background. And if we want to direct viewer attention. Or simply achieve an aesthetic effect, we'll want to employ some non physical effects here. And in this case we'll use an AI decay filter.
So let's add that to our selected area light. Scroll down in the attribute editor, to the light filters and then click on the add button. In the dialogue that pops up, click light decay and click add. And now we have an AI light decay node attached to the current area light. We can select that node tab in the attribute editor. And now we can enable, use far attenuation. And this will give us distances where we can determine where the attenuation starts and stops.
Enable that and once again we get a black screen. But that's because we have not yet adjusted these values. Increase the far end. And as you get up into the range of about five or six hundred, you'll start to see some light. So this is the distance at which the light will diminish to intensity of zero. And the far start is the distance at which it will begin attenuation. We want the subject to be fully illuminated. And so we'll set the far start to be approximately the distance between the subject and the light.
Increase the far start up. By controlling the far start and far end attributes, we can determine how much light will appear on the foreground and background. I've got some precise values for that. Set far start to 550. And set far end to 670. We can test this by disabling and enabling use far attenuation. And observing the results in the Arnold render view.
Here's the before and after rendering. First we see the area light with no decay. And then with far attenuation enabled. Although the rendering, with decay is not physically accurate, it looks more aesthetically pleasing, because the background is dimmer. This serves the purpose of bringing the viewer's attention to the foreground. Next, how to reduce the decay of an Arnold area light with it's physical spread attribute. And how to introduce non physical distance attenuation with the AI light decay filter.
And that concludes the chapter on studio lighting with Arnold 5 in Maya.
- Arnold rendering concepts
- Lighting with Maya and Arnold lights
- Controlling exposure
- Filtering light with Gobo
- Light attenuation with Decay
- Image-based lighting with Skydome
- Exterior daylight with Physical Sky
- Arnold Standard Surface material attributes
- Mapping material attributes
- Rendering refractions
- Mesh subdivision and displacement at render time
- Shading effects such as ambient occlusion and vertex color
- Camera effects such as fisheye and depth of field
- Animation image sequence rendering