How to…….. 2.2: Panorama images 2

Taking pictures – the right way

What can you do to improve your chances for a good match? Before we delve into the many little things to keep in mind, let’s get one big thing out of the way. What type of final image are we talking about? People always mix up two different things: true panoramas and mosaic images. For each type, you need to take the photos in a different manner, and mixing them does not really make the stitching task any easier.

Panoramas

A true panorama consists of a view from one point. This means that the photos from which it is calculated are taken from the same point, but that the lens is aimed in a different direction for each photo. It is the digital equivalent of standing still and looking around.

Now, does that mean that you can simply stick your camera on a tripod and snap away? Theoretically, for an ideal panorama – NO! You need a special adapter so that the camera doesn’t rotate around the attachment point to the tripod, but around the point where the light enters it (which shifts with lens length). However, if what you’re photographing is not too close to the camera, the minor error doesn’t really matter. And even hand-held will usually deliver good results. That’s where good stitching programs do a so-much-better job than you could with simple cutting-to-fit of individual images.

Mosaic images

A mosaic is created by stitching (planar) photos together where each picture is taken from a different location, with the camera pointing in different directions. A special, easier case is stitching flat scans (or images where the camera moves in relation to the object, but has the same distance and points in the same direction). This method can be used to remove objects in the foreground, by applying masks or cutting out parts of the pictures. If distortion is removed from each image singly before they are combined, this is also wonderful for creating high-res images of objects, or for getting totals of objects you can’t step far enough away from to take one photo (AMNH Styracosaurus comes to mind).

But what about skeletal mounts? That’s what us dinosaur palaeontologists are into, after all. For them, the mosaic technique is not that well suited, because objects at different distances will experience parallax shifts between images. And this makes it very hard to create a good complete view, because you’d have to split each image into several, based on how far what part of it is from the camera. For aerial surveys of quarries, however…. a nice juicy bonebed….

OK, let’s move on to what you can do to improve your chances for a good stitch:

The rules

1. Take many photos.
2. Take photos with generous overlap.
3. Take photos with limited distortion.
4. Take photos at identical angles and distance from the object.
5. Take photos with identical camera settings.
6. Use a tripod.

1. Take many photos.
Well, this is a no-brainer: more pictures mean that each picture needs to show a smaller area, thus the resolution of the final image can be larger. Additionally, screwed up pics can be omitted, with overlapping neighbors delivering the data. Furthermore, you will often end up with unexpected problems, such as someone walking into the picture. How nice if you have a near-identical second picture you can use instead.

2. Take photos with generous overlap.
Again, this is a no-brainer: the more photos overlap, the easier can you (or a program) find corresponding points. But there is a second advantage. You can cut away frames from the pictures, removing the most distorted areas, and still retain overlap enough to stitch.

3. Take photos with limited distortion.
There is a simple secret: the right lense makes the good photo. It is always true, and especially so for panorama images. Long lenses tend to suffer less from barrel-distortion (and vignetting, btw), good ones show practically none. So if you have a 300 mm lense and a fisheye, don’t use the fisheye. Duh!
Realistically, you’re not going to have fixed-length lenses. You will, as any person with limited funds, have a zoom lens. Two, if you’re rich or into photography. Again, usually, the longer one is better. And please, please, use it in the middle of its range! Zooming out to the bitter end and zooming in will maximize problems! All the way out means barrel distortion, all the way in means you need way more light or a longer shutter time. Because your panorama image will be less crisp anyways, taking well-lit and perfectly sharp images is a good idea, so stay away from the 300 mm if you can.

4. Take photos at identical angles and distance from the object.
When moving the camera between pictures, make sure to move it around one axis only (usually the vertical). If you plan to stitch two or more rows of pictures, take one row from left to right, without pitching up or down or rotation around the lens axis, then pitch, then take the next row. Don’t move around if you want to create a true panorama.
For mosaic images, move parallel to the object’s front and make your life easy by having the lens axis parallel between pictures.

5. Take photos with identical camera settings.
Many programs can correct for exposure differences, but why make this step necessary in the first place? With a tiny hand-held you’re likely screwed, but with a good DSRL camera you can select a manual mode, and fix the white balance, exposure time, etc. Oh, and keep your grubby fingers away from that zoom lens, too!

6. Use a tripod.
This is not a necessity, but it does help a lot! Ideally, you should get a special head that allows panoramic photos, but it’s not really that important. As long as you manage to follow the other rules, a tiny shift between camera positions won’t ruin your  final image. But a tripod makes following the other rules a lot easier.

Now off to your computer and let the stitching programs handle your pictures. Usually, you’ll get results that are between atrocious and excellent. Play around with the options, get a feel for what works and what doesn’t.

Hardcore panorama creation

Finally, there is the hard way: dig deep into specialized software that allows you full manual editing. Hugin seems to be able to do a lot! Other (expensive) programs do, too. Get to know how to use them, measure out your camera lens, fix your pics the hard way. If needs be, set up aids when you take the photos! For example, when taking pictures of a mounted skeleton, pictures that will have insufficient vertical and horizontal lines, place big cardboard sheets with painted-on grids in next to the skeleton, make sure they are in the pics but do not hide the skeleton, then fix your image using these lines. Try fixing the individual images first, and combining them second, or the other way round. Some programs can do both in one go.

For mosaic images, aids are almost a must! Draw a fixed-distance line on the floor, mark of regular intervals from where to take pictures, check the overlap, and have a second line with corresponding marks or some other method of keeping your lens pointed in the identical direction for each photo.

Same difference – a quick&dirty comparison of programs

Just for fun, I threw a bunch of pics at autostitch, its commercial version Serif PanoramaPlus X4, and at Hugin. I used automatic control point determination in Hugin, to level the playing field. Here’s what I got after cropping and editing (all scaled down to roughly same width, then all to 50% because of excessive size) – top to bottom:

• manual
• autostitch
• Serif PanoramaPlus X4
• Hugin (default stitcher, automatic control points, no masks)
• Hugin (default stitcher, additional control points, some masks, some playing with view field and angle)

and here they are again, with problems circles in red:

The manual one, aside from the lack of exposure adjustment, has obvious jumps in lines where pictures overlap. Autostitch produces a pretty good image, but people and objects in the front are very blurry and there are “ghosts”. The same is true for Serif PanoramaPlus X4 (no surprise, because it does not use masks or anything). One thing I noticed that is a bit annoying is that Hugin works by stitching pairs of images (which inevitably distorts them), then stitched these pairs into bigger picures. In this case, this results in a “step” in, e.g., the lamp post and the ship, despite a perfect match of the real photos on these areas. Ugh! I tried adding control points right around that place where I noticed the errors. No joy. However, using masks allowed me to have some of these problem areas to be based on one picture only, so that the lamp post is fine in the final picture.

So this is for a set of pictures of good quality. I snapped them in quick succession with the intent of stitching them, making sure I didn’t move too much, had generous overlap, and  had a lot of free space in the foreground. Also, I made sure not to pitch the camera up and down very much. If you use badly taken pictures, the advantages of spending a bit of time and manually fixing stuff in Hugin gets bigger and bigger. My recommendation thus is: give a stitch ad quick fling in autostitch or a similar program. if the result isn’t good enough, turn to Hugin.

Panorama fun

Before I forget: you can have crazy fun with these stitching programs, too. Simply throw them pics that have matching parts, but don’t go together as a panorama. Here’s an example:
Cervus perlucidus, or Thermopolis Ghost Deer!