Horizon position with spherical sky

[flipya]

Hi all,

I've been doing some testing for the past days and there's something I can't seem to work out.

I once learned that if you're standing on groundlevel, let's say a desert at exactly 0 elevation, end there's an equally tall person (same eye-height) standing some distance away from you, his eyes should exactly line up with the horizon. But if you're standing on, for example, the roof of a 20-story building and tried the same, the eye-height and horizon would no longer line up, horizon would be much lower.

Now I'm doing a render from inside a building on the *nth floor, but using the spherical sky provided by Twilight (the Field one) makes it look like I'm at groundlevel. I tested the same model and viewpoint from the model at both 0 elevation and 100 metres up but there's no difference in the position of the horizon. How do I fix this so the sense of elevation is properly displayed?

Thanks in advance!

P.S. I'd also like to create my own sphericals, of course I'll share them, could anyone point me to a tutorial? I got the photography part down, but what's with the xml?

[Chris]

Unfortunately that's one of the basic limitations of spherical skies. It's not a render-specific issue; more a photography issue. You are limited to the exact location of the camera when the picture was taken. You can't move the "perspective" that has been captured by the spherical image.

As for tutorials on making sky libraries for KT, I don't have anything, but I'm sure others can make suggestions. (Using it in Twilight, you only need the image!)

[flipya]

Allright then, thanks for the prompt response. Guess I'll fire up photoshop and manually lower that horizon then...

I'll also start work on a few spherical skies of my own. If they pass the testing-stage I'll post a link for download.

[tomsdesk]

I'm confused: as the camera moves up in height, seems to me the horizon is staying at 0 elevation just as it should...what am I missing here?

[Ecuadorian]

This is when placing a huge circular plane comes into play. Add a few hills if needed.

[flipya]

I'm confused: as the camera moves up in height, seems to me the horizon is staying at 0 elevation just as it should...what am I missing here?

I'm a little confused myself as well to be honest. Let's look at it like this: When you lie on the ground and take a picture straight forward (centerline of your view being parallel to the ground you're lying on), the horizon is in the middle. Now when you go up to the 100th floor of a building and lie down there and take a picture straight forward (again, exactly parallel to the floor/ground the building is standing on) the horizon won't be in the center of your frame anymore. Right?
Well, using spherical skies this shift in perspective is (apparently) not corrected. Try it, make a cheap-and-dirty room with one window, copy it upwards by any big amount (600ft or 200m for example), put your camera inside the elevated room and render a quick prelim with spherical sky. Now when you look out the window (on the render) it still seems you're on the ground floor.

Did that clear things up a little?

[tomsdesk]

...Now when you go up to the 100th floor of a building and lie down there and take a picture straight forward (again, exactly parallel to the floor/ground the building is standing on) the horizon won't be in the center of your frame anymore. Right?

No, I think this is basically wrong(?)...true, you are actually 1000' in the air, but the horizon will still appear to be in the center of your field of vision (the way our head works you'd have to be up in a plane to really notice the elevation change). And since the world is flat in SU (and other programs) the center of the sky pic (the horizon line) is stuck to elevation 0...you look straight ahead, you center the horizon. You look up or down, you change the location of the horizon in your field of view. Right?

The difference in the second image from reality is that you would actually see more of the roofs and yards in the distance (anything included in the sky image), as the perspective is obviously off...but the horizon is as it would actually seem. Yes?

[tomsdesk]

on the ground looking up

ztest0-lookup.jpg (67.83 KiB) Viewed 14614 times

[flipya]

I hear what you're saying, I just think the difference is noticeable at a lower altitude than (as you said) being on a plane.

I did a little drawing, couldn't resist



See I only put the little communist-guy 2 floors up and already the difference is noticeable, or so I thought. Because in making the drawing I realised that the further away you define the horizon the closer it will approach the center of your frame. On the other hand, being at an elevated position you'd have the horizon shifting more to the lower part of your frame because of the curved surface of earth. So I guess we both have a point, though the question remains what the horizon-shift-to-elevation-ratio is. Best way to find that out would be to take a camera and bubblelevel to a high point and test it. Sadly, the highest puclicly-accessible building I know around here only has 14 floors. I'll see if I can give it a go sometime soon though. Unless of course there's someone who did pay attention in school and wants to do the math

P.S. That's a nice spherical btw, did you make it yourself?

[Fletch]

what Chris said earlier is correct, but perhaps the "key" point is simply this:
Twilight renders the spherical sky as centered on the camera. So no matter how you move that camera, the horizon will always align with it.

The "sphere" of the sky is a "virtual" sphere, and does not actually exist as geometry in the model... (hmm... that's a mind-bender right there, because technically the model is virtual and doesn't exist either... so that would make it a "virtually virtual sphere". whoa.)

Chris said clearly that you can't change the perspective of the photo itself... any more than you can rotate a photo in 3D inside of Photoshop... you can skew it, but you won't see any more info than is already presented. In Twilight, or any program AFAIK, the spherical image is centered on the camera for this reason. And therefore, as Flipya mentioned, if you want to move the horizon down, you have to slide it down in photoshop and re-save the image. The placement of the exact center of the spherical image is very important when creating sphericals. Because the sky represented by the image is huge, so even a few pixels up or down makes a big physical difference to the observer "on the ground" in the model.

Essentially: use a spherical sky image taken as close as possible from the eye-height of the camera in your rendering.

If rendering with Easy 1-8, you can actually put in a real sky dome mapped on the inside with the sky, and turn off shadows for that material, and tweak it using Ecuadorian's background image trick as outlined in the manual's reference section. This will "rotate" as you desire, with the model, because now it's a physical geometry.