HDR in Area Light

[iacdxb]

Hi,

- HDR using in Texture slot... render comes very noisy and 2-3x render time. HDR fit auto in light size.
- HDR used in Distribution slot... it fast and much clean render but pain to set hdr with transform in light size...!
- Emission / backbody emission also very noisy.

For best to use HDR in Area light with auto fix in light size...?

Thanks.

[frankmci]

If the point is that you want to SEE the image of the spot light lens and barn doors, but still optimize render time, I suggest this:

Put the image of the light that you want to be visible on its own object. Use the Emission channel, not the Diffuse channel. Turn off Visible in Diffuse and Visible in Specular in the Texture Emission node. Set Diffuse to black.

Now you've got the "visible" part of your light, even in total darkness, but it doesn't actually illuminate the scene, and it costs almost nothing to render.

Make that object a child of an area light, so that its position always makes sense with the actual source. In the area light, turn off Camera Visibility and Shadow Visibility.

Now you have a render efficient light source that can look any way you want it to.

[frankmci]

Great suggestion, frankmci. Actually realized it's a demonstration joining my third dot.
...
PS: the diffuse of all Octane lights should always be set to pure black ("disabled"), a section here shows & explains why (and would ideally be as such in the native/vanilla Octane, core & plugins).

Yes, it's a simple example of your third point, that in my experience covers the majority of needs. You can certainly get fancier with them using the same idea, if the situation calls for it.

To avoid some possible confusion, the diffuse specifically on mesh based lights using a fully emissive material should generally be set to black. Technically the normal Octane area lights are also meshes, but they don't possess the usual material properties, such as a diffuse channel. In some situations making sure the diffuse channel is set to black on the luminous surfaces of meshes won't make a noticeable difference, but it certainly can, so it's a good habit to form, or at least be aware of how it can influence the render. There are always exceptions, such as luminous surfaces that also need to receive illumination from other light sources, where the diffuse channel needs to be something other than black.