Panoramic/Stitched Images

4. Creating Panoramic and Stitched Images

Although all Panoramic software has it's own particular way of working there are a number of common factors. We will use images created using PTGui to illustrate some problems that can arise, practical limitations that exist, and the options that allow you to create different looking images. 

Panorama v Stitch

First of all though, what can be classed as a Panorama? Because the definition means different things depending on your viewpoint. Most would view the term as applying to wide vistas of countryside, a sweeping landscape or cityscape, but that is really a rather narrow definition and in general terms it's better to consider that it applies to any image that has a format ratio of about 2.5-1 or greater. However, there are many instances when images can be stitched together that don't fall into this general category, the format ratio being squarer in nature.

Panoramas and Perspective

There is no doubt that most digital camera users will probably say they would be quite happy just being able to stitch the odd Panorama together consisting of two or three images so long as it looked good. However even with a simple Panorama such as this, taken horizontally or vertically, options should be available.

There needs to be a choice between producing it as either a planar or cylindrical image because as the Panorama gets wider perspective distortions are created. Although a planar is classified as possible up to 180° in width and 120° in height, in practical terms the limit of view is around 100°-120°. Above this figure width ways perspective distortions can become obvious and un-realistic. Because planar images keep all straight lines straight the result is that from the centre outwards horizontal stretching of the Panorama occurs. This is not a linear effect however, but a progressive one, with the outcome being that as the Panorama gets wider the perspective at the edges becomes worse, with objects appearing to be 'squashed' as a result of being stretched sideways. It's particularly noticeable when it is people or animals that are involved.

Some programs, such as PTGui, won't stitch an image in this format if it is outside these figures and automatically changes to cylindrical. Cylindrical images have a practical limit of 360° horizontally and 120° vertically. Spherical images can be 360° in both directions.

Generally, a planar image is wider horizontally and narrower vertically than a cylindrical image when stitched using the same set of images. It doesn't matter whether the images used are horizontal or vertical, but particular problems can arise when horizontal images taken using a wide or very wide angle lens are used because even if there are only two or three shots involved the perspective view can exceed the planar limits even with a 50% overlap. 

For example a 35mm lens has a field of view of 63°. If you take just three horizontal images, one centre, one to each side, with a 50% overlap, the view when stitched will be around 126°. Using a 28mm lens the total view would be 150°.  Three vertical shots won't have the same field of view, because the field of view is actually across the diagonal of the image frame, whatever orientation is used. But the basic premise holds good. This is another reason why many photographers who take Panoramic stitch images prefer to use the camera in the portrait position. It provides a better view height wise without causing view problems width ways when assembling a horizontal panorama. You can of course also produce vertical panoramas, high rise buildings would be one reason, and here the use of horizontal shots would be better for the same basic reason.

It's important here to fully understand the relationship, and differences, between field of view - the angle covered - and the area of view covered, which are not the same. You can capture the same area from different distances by altering focal length. As focal length shortens the view in degrees increases and perspective alters, and these same perspective differences exist when you capture the same Panorama in the same way using different focal lengths.

Planar v Cylindrical

Here is a view of an un-cropped planar stitch involving just three horizontal shots. As you can see the individual image frame edges remain straight, but only the 'key' image, the centre one, remains square. Perspective is applied to the others and in this case the people and the carved stones at the edges look squashed, at least they do in the full size image, it's not quite so obvious at this size. Another problem is cropping. Crop this one as it is and top of the trees at the sides will be lost. So some additional work, cloning the foreground and sky is needed if this is to be avoided.

You can also see that as the shots were taken from a low ground level position with a wide focal length, the image has a perspective reminiscent of that taken with a ultra wide angle lens. The immediate foreground dominates.

Now compare this image with the next one, the same images used in a cylindrical stitch. The different Panorama that results is quite easy to see. A totally new perspective but still with a dominant foreground.

With a cylindrical stitch only a horizontal line running through the middle remains straight. Above and below this imaginary line all lines bend, the amount depending on the images used and the angle they were taken from as well as the distance they are from the centre line. The amount of bending increases the further away from the centre line the other lines are and the effect is basically the same as that occurring in lens barrel distortion or the effect of using an un-corrected fish-eye lens.

We find that each type of Panorama stitch has both advantages and disadvantages in terms of the resulting perspective and this alters depending on the width and height of the Panorama in degrees, the number of images involved, the distance that it was taken at, and the focal length of the lens used. As a result we usually produce both types from a set of images where the option exists and choose the best final image after cropping. Sometimes we crop the Panorama at the pre-rendering stage, at others we produce them un-cropped, as here, and crop to taste later. Generally, a planar stitch is better perspective wise until the format ratio exceeds about 3-1 when cylindrical is often better, but there is no hard and fast rule. The content featured, buildings, landscape etc, and the perspective the shot was taken from, all affect the final outcome.

You might like to save the un-cropped images here to your hard drive, open them in your image editor and crop them in different ways to see what the results can look like. Just because the original stitch is wide doesn't mean you have to leave them that way if a narrower view looks better. Many iconic images are crops of larger frames, the trick is getting to know what to discard.

The images used here are of the Palais De Chaillot in Paris. This is situated in an area known as the Trocadero and is on the bank of the River Seine opposite a rather well known Paris icon, the Eiffel Tower, which resides on the other bank. To be honest we should probably have gone up the Eiffel Tower to the first or second stage to take shots of it, the perspective looking down would have been better, but the tower is always so busy that the time taken to do this just wasn't thought worthwhile. You really have to allow a couple of hours if you want to go to the top. Loiter on the lower floors first and you can sometimes find you can't get to the top at all. Anyway, we digress.

Blending stitch shots

The Palais De Chaillot is a great viewing point from which to take shots of the Eiffel Tower, or we should say the square in between the two halves is. This leads through to the Trocadero. So here's two shots of the Eiffel Tower taken from this point, one daytime, one night, in portrait mode.

We also took some in landscape as well, but the problem we found that was that in either position the formats didn't capture as much as we would have ideally liked. The viewpoint overlooks and takes in the fountains and statues in front of the Palais De Chaillot but either they, or the tower, end up getting cropped. Later, back home, we discovered two consecutive landscape shots we thought we might be able to be merged in a stitch, even though they were not taken with this intention. 

Here they are. You can see that the one of the tower is much darker. This resulted from the sun being at about 45° to the shot, and nearly in the frame at the top right hand corner. Metering to expose for the ground would have bleached out the sky, as happened with the other, lower view, where there is no colour in the right hand portion.

Stitching the two together in PTGui as a planar produced the following result which is somewhere between a square format and a 4x3 ratio. On the left is the un-cropped render and on the right the cropped image.

You can see that the software has managed to blend the different exposures quite well. We tried altering the levels in the first image before merging, which many recommend you should do, but it actually produced a worse final stitch blend wise. We've actually tried deliberately taking stitch shots using auto-exposure with huge differences in exposure just to see how well PTGui coped with the blending, and have been surprised at the results. We wouldn't go as far as to suggest you can totally ignore exposure if you use this program, and just use auto-exposure all the time, but there's no doubt it can cope with wide variations in exposure very well, which is a great help.

If you compare the final cropped image with the portrait version above you can see that a much improved image results, it still isn't right, but it's better, and more in line with what we were trying to achieve. Next time we are in Paris we'll know what to do. Take more shots over a wider area to combine, and take the Palais from the Eiffel Tower. At the time these shots were taken we had yet to discover PTGui, and the odd few shots that were taken for stitching, like those of the Palais De Chaillot, were done more out of hope than expectation. Now we roughly know what to do and what type of shots combine best. 

Merging non-stitch shots

As we hope you now see, stitching has many more uses than just creating wide Panoramas. It can overcome the lack of wide angle view capabilities, and is in some ways better and more versatile. 

Of particular interest to us was that software such as PTGui can often stitch together shots not taken specifically for the purpose. Indeed we have now gone back through our archives to find the shots we took in the past when trying stitch programs and seeing what PTGui can make of them. In many cases a decent stitch now results even where it didn't previously, despite no EXIF data and only a rough guess as to the focal length and LMF factor involved. 

Here's an extreme example of what's possible. Four separate panoramic shots combined into one. It's a view over London and St Paul's. The shots were taken, through glass, from the restaurant on the top of the Tate Modern when trying out the 'panoramic mode' on a digicam some years ago, which was really just a top and bottom crop of the image frame. They were pretty dire as you can see, and we forgot about them until we went searching for old shots. We didn't really expect them merge with any degree of success, if at all, so we were a bit surprised at the result. 

We have left the stitch un-cropped to show the crop problems that can occur, as we explained on the first page, when shots are merged that don't align properly with regard to the frame edges. Cloning the sky in the left of the image would be the only way to avoid chopping off the spire on the dome of St Paul's and the skyscraper. Although the image is small it also shows the perspective problems that can arise with views covering a wide arc. The River Thames is straight at this point, but in the stitch it appears to bend. 

On the next page we will deal with image perspective and altering it