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Greg Hughes avatar image
Greg Hughes asked ·

How to shift Physical Sky black horizon?

The Physical sky shows a black horizon depending on the angle of the camera, e.g if tilted up. Is there a way to lower the horizon of the Physical Sky to shift the black horizon line down?

Otherwise you have to keep the camera angles perpendicular to the ground plane or tilt the geometry plane you add for the ground surface upward to hide it. Not an ideal solution.

Update: The best method I have found is to make the ground plane geometry very large (e.g. 150000 on the Z axis) and slightly rotate the Skydome.

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Mads Drøschler avatar image
Mads Drøschler answered ·

We can also just bend the rays in the particular area and extend them. So and we just affect a suitable range to cover a visibility ramp and use a different curve for the IOR.
We just tell a pixel to cover a smaller area than an untampered pixel and the result is expansion in the direction we want to bias.

Then all it requires is tapping a single float spinner at couple of times and the black bar is gone.
This works right out of the box with a straight ground which is, like in this case, pritty small.

You may have seen Lee Griggs refractive portraits and this is just a single axis control instead of introducing various distortions to the IOR.
So and this does not change light or shadow angles as well and you utilize just 1 single sky map for everything.


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Thiago Ize avatar image
Thiago Ize answered ·

I know it's been a while, but thought you might like to know that if you try with the just released Arnold 6, the sky is now automatically extended all the way down, so it's possible this might solve your issue.

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Aaron Ross avatar image
Aaron Ross answered ·

This is only really solvable through modeling the terrain to be generally concave. The camera needs to sit in a depression in the center of the terrain. This is, of course, not at all physically accurate, because the surface of the earth is convex. I guess no one at Autodesk or at Solid Angle ever considered the fact that the earth is not flat.

The VUE developers were smart enough to include an infinite groundplane in the layout toolset, but I've never seen this in any Autodesk product.

Of course, the Autodesk developers could fix this situation by retrofitting the Physical Sky, but somehow I doubt that is ever going to happen.

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Wait, the earth isn't flat!? :) Arnold does have an infinite plane, see "kick -info plane". I don't know how this is exposed in the various plugins.

We have actually already done some work on extending the physical sky below the horizon. The reason we haven't yet released it is that we want to make sure it doesn't make things slower or more complicated for everyone. After some optimizations in Arnold 5.3, some of the overhead we'd incur with extending below the horizon was minimized. I've made a note in our internal ticketing system that there's more demand for this feature.

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mehul shah avatar image
mehul shah answered ·

im sorry increasing the ground size doesnt work that much. i still have to change the environment tilts which makes the shadows also shift. the story board doesnt allow me to change the angle. can u please help me lower the horrizon lines to lower than eye level

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You can always render the map out with a spherical camera and use it as a texture, in texture space you can shift it down. If you need to animate, export a sqeuence of frames you load to the new texture you wrap.

Use the original shader for actual light and use the baked map for background, noone can detect the slightly offset angle on the y axis of the map, you will even have hard time to proff it by measuring in scene since the offset is tiny. I need to offset the map 0.015 to get rid of the black line.

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Lee Griggs avatar image
Lee Griggs answered ·

Maybe have two skydomes with Physical sky. One just visible to the camera and rotate that down?

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I put a note to replicate a skydome in OSL with the standard scatter methods, with a small user feature that simply just slides the skymap a nodge down if needed.

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Some more details on this approach:

  • create a regular skydome light for the top
  • duplicate skydome with a negative Y scaling factor
  • tie azimuth and elevation for the bottom skydome
  • disable sun for the bottom skydome
  • disable bottom contribution for all rays except camera
  • adjust scaling factor for bottom skydome as needed
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