Panoramic/Stitched Images

5. Perspective adjustment

Something you have to understand and be aware of is that stitched images will exhibit perspective in just the same way as a single shot covering the same degree of view. In other words shoot upwards at a building and converging verticals will exist, or assemble a cylindrical stitch greater than 120° and the resulting perspectives will be the same as those found when a fish-eye lens, corrected not circular, is used.

Here's a 360° cylindrical stitch Panorama. The two cars on the right, the silver and blue, look to be parked at 90° to each other, yet in actual fact they were in line, although not quite bumper to bumper. They were directly opposite the 4x4's on the left which would seem to be on another road altogether. The odd branches at the extreme left top hand corner are those from the tree seen at the right hand edge. 

The horizontal perspective encountered in images such as these, the bending of straight lines and edges, cannot be altered, but vertical perspective can, depending on the degree of vertical view involved, because the perspective in each part of the image is in relation to that around it and so it has to be treated as a whole.

Here's a 180° cylindrical stitch Panorama which probably illustrates more graphically the bending and blending of the different perspectives involved over such a wide field of view. The road used for this stitch is dead straight, which is why we chose it, yet it would appear here as if it takes a 90° right angled turn right in front of the camera.

Unfortunately on the only decent day weather-wise we had to take the shot, a very cold winters day, it was extremely bright with a low sun and high contrast. The software made a fair stab at blending the images, all 28 of them, in two rows taken vertically, but another of the problems that arise with images like this is the variation of colour - banding - that can occur in the sky. We've left it untouched here so you can see the result, which would benefit from some tweaking.

Here's the initial pre-adjustment merge the software presented for comparison. The initial perspective results from the lower row being taken horizontally, and the upper row with the camera angled upwards to capture the top of the church at the centre, which would otherwise have been cut off.

As you can see we adjusted the perspective before rendering by dragging the stitch downwards. In PTGui you can move the complete stitch anywhere you like, up, down, sideways etc. As it is moved the perspective is re-drawn allowing you to alter stitches until you are happy with the resulting perspective.

It's not only multiple stitches covering a wide view that need and can benefit from perspective alteration though, it can apply equally to simple ones too. Here's an initial merge of two ordinary shots of the old town fort at Riva Del Garda. This is situated on the northern shore of Lake Garda in Italy.

The merge is okay basically but the forts perspective looks a bit odd. There's too much 'leaning inwards' - converging verticals - involved. So we moved it about a little bit to correct this. Here's where it ended up. We didn't move it much. Sometimes only a slight movement can have the desired effect.

And here's the finished shot, nothing special, but a bit better than the individual images.

The only advise we would give regarding perspective adjustment with stitch images is that we have found it to be better to do this in the stitching software, before rendering the finished stitch, than afterwards in a photo editor such as Photoshop. The reason is simple. Firstly, in the case of PTGui at least, the stitching software works with thumbnails so it's quicker and easier to do. Secondly, and probably more importantly, when you re-draw the image in stitch software, the perspective in each individual image is re-drawn to match each other and the horizontal and vertical perspective relationship is maintained. This does not happen with a finished image opened in a photo editor, the relationship between horizontal and vertical perspective is not always maintained and it can result in a lot of hard work to get the same final result.

We hope the information we've provided here has been of use and interest to you, and has given you an idea as to the possibilities that exist with Panoramic and stitch images