Illuminate an interior with a Skydome.
- [Narrator] Rendering photo metric daylight interiors in Arnold can be challenging. An effective workflow is actually simpler than it seems. All that's really necessary is a skydome light with medium to high render samples. We just need to be aware that our render times will increase accordingly. If you need fast renders then create a studio lighting setup as we saw in the previous chapter. The Arnold product documentation warns that skydome importance sampling is not optimized for interiors and that you should use light portals over the windows to remove noise or grain in the rendering.
You can find light portals in the Arnold menu under lights. Light portal. Let me explain to you precisely why you should not do what the manual says and don't use light portals. A light portal is a simple rectangle like a quad light. It's purpose is to focus computation by tracing rays from objects through the windows and out to the skydome light. You see in ray tracing the rays start from the camera hit the scene objects and then bounce off those objects generating secondary rays that can hit other objects and lights.
Without a light portal most of the secondary rays in an interior will hit other objects and won't make it out the window to hit the skydome. The light portal guides the secondary rays through the portal and out to the skydome. The result is that more rays are allocated to the direct illumination. The sampling is improved and the noise is reduced in the area that receives direct sunlight, but there's a very big downside to this method. Unless the windows are huge and direct sunlight is filling most of the frame the noise will actually become noticeably worse with light portals.
In most interior daylight renderings the light is predominantly from diffused reflections and the area in direct illumination is relatively small. In this case we'll have light coming through the window and hitting the table cloth and that direct illumination is pretty small, but the rest of the room will be illuminated by bounce light or indirect illumination. With a light portal the computation is focused to optimize direct illumination and unfortunately that means the bounce light becomes even more grainy than without the portal.
And there's another very big problem with light portals. The illumination from the skydome light is all diverted through the portal. If you have exterior geometry, such as this landscape then as soon as you add a portal the exterior geometry receives no light at all and renders as black. Now there's a setting in the skydome light to determine the behavior of the portals and you might think this would fix the issue, but in fact it does not. So let me demonstrate, let's create a skydome light, go to the Arnold menu and choose lights, skydome light.
Its shape node is selected and in the channel box we can set the sky radius to 70,000. Now although we don't have any light portals in the scene I just want to show you the portal mode of the AI skydome lights attributes. Open up the attribute editor with CONTROL + A and we have portal mode down here with three options off, interior only, and interior-exterior. The default mode is interior only and that blocks camera rays through the portal.
It makes exterior geometry such as landscape invisible to the camera. Interior-exterior mode does allow the camera to see through the portal, but it does not allow skydome rays to go anywhere except through a portal. In order to render the exterior geometry you have to be in interior exterior here and you have to create another very large portal and position it over the geometry to allow secondary rays to reach the skydome and illuminate that geometry.
So all of this mean that the light portals are actually more trouble then they are worth and I don't recommend using them. The legacy method for interior day lighting is to place area lights over windows and that works fine, but I do recommend placing the area lights a few tens of meters away from the windows rather than directly up against the windows. If the area lights are too close then you may have issues with the geometry near the windows being overexposed. Moving the lights farther away flattens out the decay of illumination so that it is not so dependent on distance and the shot is illuminated more evenly.
The simplest and most effective method for photometric interior daylight is to simply create a physical sky and increase the render samples. Then the diffused reflections won't be degraded by any light portals and you can predictably remove the noise in the shadows. Let's open up the Arnold render view, Arnold render. The skydome shape node is still selected and in its attributes let's increase the samples to five to remove the grain in the direct illumination area, especially here on the landscape.
Now we'll add a physical sky environment map. In the skydome light attributes in the color attribute click on create render node. In the create render node popup dialogue go to Arnold, open up texture and click environment and you have two possibilities here, AI physical sky and AI sky. AI sky is really for flat colors and under water shots. AI physical sky is the simulation of sun and sky that we want.
So click AI physical sky. The physical sky environment map is created and connected to the skydome color and its attributes are displayed. Let's go ahead and change these up first being the elevation and azimuth, which is the position of the sun in the sky. Bring the sun around to the side of the sky here by setting the azimuth to 250 degrees, and then let's adjust the position here vertically by setting the elevation or the height of the sun in the sky.
Set that to a value of 20 and now we have the pool of light that we want. Increase the intensity. Let's bring that up quite a lot to a value of 23, and turbidity is the haze bring that down to a value of one. The suns size controls the softness of shadows. Set that to a value of two to make the shadows a little softer. Then we can adjust the color of the sky and sun. Click the sky tint color sample and set the hue to 214, press TAB, set saturation to 0.7, press TAB, and value 0.5, and we're making the sky a much darker richer blue.
We can adjust the color of the sunlight as well. So go to sun tint and set it also to 214, because we want to push it a little bit towards blue. And we got the saturation here we can adjust the slider and see what that does. If we bring the saturation up above about 0.5 then it doesn't look quite right, so let's bring the sun tint back down to 0.3 for the saturation. Okay that looks pretty good. Now we just want to adjust the brightness of the sky itself.
Select the skydome though the window and in the skydomes attributes under visibility bring the camera visibility down to 0.5 and that'll make the sky half as bright as it was before.
- 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