[karma mechanic]

I posted some images yesterday where several fisheye images were stitched to give a mind-bendingly wide stereographic view, so today I'll give some details of how it is done. This is not an intensive guide, just an overview of what I did in case anyone else wants to bend their mind in the same way. I was using my K7, which only works in IR but the general principles will be just as applicable to normal cameras.

The images were posted in the 'Post your photos' section just here.

First, the hardware.
It is perfectly possible to do panos freehand if the subjects are distant, but with very wide angle lenses any small change of position can cause large apparent shifts in perspective between images. This results in great difficulty in stitching the result. So a tripod is necessary. Unfortunately just using a normal tripod head won't work very well, since the camera/lens needs to rotate around the nodal point of the lens in order to avoid perspective changes between near/far objects. Consider the nodal point to be the point in the lens where all the light rays cross on their way to the sensor.

I used the Samyang 8mm lens, and the nodal point on that is 21mm in from the front of the glass. That's where the gold ring on the lens is situated so that's a handy guide. This lens offers a diagonal field of view of 180 degrees.

In order to rotate this lens round the nodal point the gold ring needs to be located in line with the axis of the rotation. So an imaginary line drawn from the tripod head upwards needs to cross the gold ring. Likewise for the lens to pivot up/down around the nodal point the point of pivoting needs to be in this same line.

To achieve all this a pano head is necessary, where the camera is adjustable to put the nodal point in the right place. Here's what it looks like:



You can see from the above that everything is in line, so the camera can be rotated and tilted while keeping the nodal point in the same place. The head by the way is an early version of the Panosaurus head. This one is made of MDF - subsequent ones use steel and are consequently rather more expensive...

The individual images that make up the pano are taken in two rows. The first row is with the camera pointing downwards. Being a fisheye the field of view is getting the bubble of the spirit level in at the base of the picture. Normally only four or five images would give a full 360 degrees of pan, but in this case the pano head is actually obscuring half the lower part of the picture. Taking a few more images ensures that those parts that are obscured are always visible in the next image.

Then tilting the camera upwards some more images are taken as it is rotated round. Strictly speaking just one upwards image would probably suffice, but I tend to take several rotating around and let Hugin sort it out.

This is what the down and up-tilted positions look like:



In the left picture the bubble is at the bottom of the frame. In the right picture the zenith is near the top of the frame.

The actual sequence of images doesn't matter, but I always put my hand in the frame to indicate where each sequence stops.

So, now there are a set of images to work with. They don't look all that interesting at this stage:



The images are converted to TIFF files and then loaded into Hugin in order to do the next step.

In Hugin the default for images arranged like this is an equirectangular projection. Loading the images and setting the lens to 'stereographic' (the Samyang isn't a true fisheye, it uses a stereographic projection which is a different shape) Hugin then shows something like this:



The projection is equirectangular, the field of view is 360 by 174.5 degrees, and you'll notice that the top area of the image is a bit of a mess. That's simply because this projection can't easily show what's beyond the zenith, so it has limited the vertical FOV. Don't worry - we aren't going to use this projection. Also note that this is the fast preview page in Hugin so it uses a very downsampled version of the image.

Changing the projection type to stereographic in Hugin now gives a somewhat different image in the preview:



Don't worry - we can now zoom in and start to make sense of it.



Here you'll see that there is a rather bendy version of the stitched images. In order to get the right view we now need to change to the Move/drag tab, select a point in the image and move it. This is very sensitive, especially as you approach the two nodal points. For example, picking up the tripod head and moving it to the exact centre of the preview gives the following:



There are some pieces of the pano head still there because the preview isn't clever enough to fill them in with background. This is a 345-degree view with the tripod at the centre. But that's not what we want here, we need to carefully pick the centre of the sky and move it to the exact centre point of the preview window. Like so:



Again bear in mind that this is a crude preview, a lot will get sorted out when we finally hit the stitch button. At this point you'd use the crop view to move the edges to where you want them.

Output of the above preview when stitched is like this:



You will note that there is a chunk of the scale that surrounds the spirit-level bubble still showing. In Hugin it is possible to mask off pieces of individual images so that might well have fixed that. In most cases it appears to be clever enough to replace objects that are fixed in the frame (i.e. that moved with the camera) with the proper view of the subject.

These techniques are normally used to make interactive 360-degree panos where any individual view is quite normal. In this case the stereographic projection is being used to make a flat image with an insane field of view. Whether it is useful or not is another matter!

[Kerrowdown]

I posted a comment when I saw the couple of images the other day.

"Aye, your right enough, but they do hold your attention as a viewer, cos your trying to work out what's going on. Thanks for scrambling what little I have left".

You didn't need to go to this level of trouble, I scramble dead easy.

[Sagitta]

Thats really cool. I've messed around a bit doing kind of the opposite (lots of sky, itty bitty ground). I may try your technique out that way to see if the results are any better for me.

[altopiet]

After seeing this, I need a stiff drink just to get my mind straight again!!

[Kerrowdown]

I need a stiff drink just to get my mind straight again!!

I tried that, after several drams I was worse off than when I started.

Maybe it's just a Scottish thing.

[kh1234567890]

You are making it far too complicated

If you use decent software, such as Kolor Autopano, then you can get away with handheld shots and in 99% of cases the automated equirectangular stitch will be good enough to require only minor tweaking and gets done in less than a minute. Autopano is also very good at handling multiple viewpoints, i.e. not spinning exactly about the lens nodal point. Of course, you can set your projection centre point to anywhere or select your projection type. In practice, with the Samyang 8mm, you only need four shots plus zenith and nadir - six in all.

For example : York Minster - in Shockwave viewer or York Minster - in Flash viewer or York Minster on 360 Cities (right click for projection type selection with this one).

[karma mechanic]

I did try Kolor Autopano a couple of years ago. It didn't seem to have any features over Hugin, fewer projections, and of course it costs money. It does look neater, but as far as I know the control point detection is the same engine.

Nevertheless I tried it again today. It doesn't seem to have a stereographic projection, but the 'Little planet' one appears to achieve the same thing. Picking up the sky and putting it to the centre gives exactly the same result as with Hugin, and is just as sensitive to small wobbles. It does seem a little dumbed-down. Regarding speed, I didn't notice much difference for autostitching and final rendering - and they both chew up a lot of resources. Autopano did seem to use more memory though, around 8 Gigs when stitching the full 'vortex' image.

Regarding the freehand panos, it mostly depends on how much near/far content there is in the pano. Some of mine with near and far trees are very hard to stitch, simply because a very small movement away from the 'perfect' position gives large differences in the relationship between different objects at different distances. I couldn't see any difference between the two programs in those circumstances. Using a tripod fixes that and makes all the subsequent processing much easier.

I fully agree that with the 8mm it is possible to get away with a lot fewer images. However, I hate it when I come back with what looks like a good set of images and then find that one or two are shaky or otherwise afflicted with flare spots (in IR flare can be much worse than normal light). So I tend to take a few more than necessary to ensure a few 'spares' and some good overlap. This also means that the very edges of the frame tend not to be used, giving a higher sharpness overall. Printing large is much more demanding of overall even sharpness than the interactive tour type of viewer where you see small portions of the image at any one time...

So yes I am making the image capture more complicated than it might need to be, but I can't necessarily agree with Autopano being 'decent software' if that implies that Hugin isn't

[kh1234567890]

So yes I am making the image capture more complicated than it might need to be, but I can't necessarily agree with Autopano being 'decent software' if that implies that Hugin isn't

Hugin suffers from the open source syndrome - too many cooks and all that. It does everything, and quite well, but there are far too many options and possibilities and the user interface is not all that slick. I could never quite get up the learning curve. As a freebie, it is definitely the best there is though.