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starbase1
12-07-2003, 10:35 AM
Hi All,
I have a problem that really ought to be simple, but it's doing my head in. I have an image I want to map onto a hemisphere, but I cannot get it right.

The image is from halosim, and is of the whole sky above the horizon.

The central point is the zenith, the horizon is at the edge, and the distance towards the centre is proportional to the altitude up the sky. So 45 degrees elevation is halfway in, 60 degrees elevation is 2/3rds of the waqy in and so forth.

So what projection do I use to make this map correctly?

If I get it right, the dark area at the bottom of the image will appear as a perfect disk.

Small version of the image attached...

Nick

danielkaiser
12-07-2003, 12:07 PM
I it looks like an image probe, try using Image World under the scene>backdrop>add environment tabs.

Dan

starbase1
12-07-2003, 01:19 PM
Hmm... That doesn't look right to me having tried it.

Also I really want to apply it to a sky dome type object, so I can apply it with additive transparency, brightening a background to make halos in the sky.

So how do I map it to an object?!?!

Nick

toby
12-07-2003, 02:47 PM
what about planar on the Y?

starbase1
12-07-2003, 04:26 PM
Nope, that won't work.

For a planar from above to work the 45 degree mark would have to be cosine 45 degrees out, not halfway.

If there is not a mapping that will work onto a sphere, I keep wondering if applying it to a different shape would work...

With the right profile, presumable a Y planar would work, even if it's not a hemisphere.

Nick

toby
12-07-2003, 04:34 PM
of course -

how about a UV, that would allow you to map it as 'half' a spherical map

It's hard for me to figure this out since I don't know what it should look like - the way one ring goes almost all the way around just above the horizpn but then 'stops' on both sides, at a star?

starbase1
12-09-2003, 02:10 PM
Sorry - been away for a couple of days.

It's actually a map of a complex halo aound the sun./ Sun position is near the bottom and centre. Around that the most obvious feature is a ring around the sun, (with a slightly darker interior). This is 22 degrees radius.

There is a fainter 46 degree arc outside this. On either side of this are two bright pathes, (sundogs or parhelia), and these then extend out around the sky at constant altitude.

If you are at all interested take a look at this site:

http://www.sundog.clara.co.uk/halo/halosim.htm

where they have much more info and free software to simulate halos. This is the source of the map I am trying to use...

See
http://www.sundog.clara.co.uk/halo/bishop.htm

for an astonishing photo of a sount pole display.

toby
12-09-2003, 10:09 PM
very cool! thanks for the link - I dig that kind of stuff

still looks like a hemi-spherical map though, that's very much like the results I got, and if you were to snap photos of the whole sky, it would also be hemi-spherical.

Hervé
12-10-2003, 12:31 AM
I have sphrical sky maps here.... you want to map it using cylindrical Y, and everything is fine....

starbase1
12-10-2003, 05:05 AM
Hi Herve,
Surely a cylindrical mapping will use the whole image, howver I apply it? And if I get the mapping right, then the white bits / corners will not appear in the sky?

Or am I missing something?

Feels like I get dumber with every post!

Nick

Hervé
12-10-2003, 09:50 AM
well I looked in HDRShop,and this image looks like a light probe, so you'll have first to convert it to Lat/Long, and then use it in mapping to a half sphere with polys inverted (f) and then put your image with cylinder facing Y, click auto size...

Hervé
12-10-2003, 09:51 AM
'll try with the image you provided...

starbase1
12-10-2003, 12:10 PM
Thanks to all who chipped in, I have sussed it.

The way I did it was to create a shape to untangle it. If you do an overhead projection of an image where even steps in altitude are evenly spaced, this will view correctly in Lightwave.

The formula is a bit messy, but if anyone wants the object, I've made it available for download here.

http://www.starbase1.co.uk/downloads/halo_map_holder.lwo

It's not compressed, but its only 33k!

I'm using the map as an additive transparent shell in front of a simple gradient backdrop.

If you want to see the VR Movie, I've put that here:

http://www.starbase1.co.uk/downloads/halo.mov

Not that it starts opposite the Sun, so the most interesting views require panning around a bit.

The thing is still a bit rough, but know I can view it in the correct way, I can work on producing cleaner halo maps.

And here's a thumbnail.

Hervé
12-10-2003, 11:26 PM
Ok OK I see now, very special map...

starbase1
12-11-2003, 12:44 AM
The odd thing is that evenly spaced lines of equal ltitude ought to be fairly simple, I would have thought... I still suspect I'l doing it the hard way.

But what the heck, it works!

Again, thanks to all who chipped in with ideas.

riki
12-11-2003, 04:01 AM
The QTVR looks great, it warps a bit as you pan, but I guess that's to be expected.

starbase1
12-11-2003, 05:07 AM
Yes, they all warp like that. It uses a cylindrical projection. The distortion is MUCH less pronounced if you zoom in a bit - the extreme wide angle default view causes the worst effects.

I've now done another one, with more extended view, and some proper ground added.

350k.

http://www.starbase1.co.uk/downloads/halo3.mov

I produced these by having a VERY wide angle view, and rendering an all round sequence of tall strips, with 50% overlap between each frame.

I then fed the results into the excellent (and free to try) panorama factory, available from

http://www.panoramafactory.com

This will then stitch bits together fully automatically. No reason you couldn't use the output as a cylindrical map to save render times on complex backgrounds and skies of all kins.

Cheers,
Nick