Monday, November 19, 2007

St. Elizabeth's Cathedral and Urban's Tower, Košice, Slovakia

During my last visit to Slovakia I took hundreds of shots for virtual panoramas. Kosice, the second largest slovakian city really worth seeing especially because of its historical core which is the largest national cultural preserve in the whole country.

The Gothic St. Elizabeth's Cathedral in the historical district of Kosice is located right in the center of the Main Street, the central boulevard of the Kosice Old Town. It is actually the largest cathedral in all Slovakia.

Although the cathedral has been under construction for past 2-3 years, the interiors are open to the public. In the virtual panorama you can also see the Urban's Tower, a Gothic prismatic campanile which is associated with St. Elizabeth's Cathedral.

Monday, May 28, 2007

Virtual tour - Kasperk Castle

Kasperk Castle (Kašperský hrad), situated 3 km north of Kasperské Hory in Czech Republic is a ruin of a gothic castle build in 14th century for defense and observation purposes. The exhibition inside reveals the history of the castle as well as of the gold mining in the area.

Wednesday, May 23, 2007

Nodal Point confusion made clear

One of the important issues in panoramic photography is the proper setting of the vertical rotaton axis of the camera to what people often call the nodal point. Although "What is the nodal point?" is one of the most frequent questions when it comes to panorama creation the term of "nodal point" is being misinterpreted in this context in vaste majority of cases. As I will show you in this article the "real" nodal point is not what one should be concerned about when adjusting his photo equipment.

When turning a camera around or changing the point of view position you generally experience an apparent shifting of foreground objects relative to background objects which is called parallax. When taking the pictures for a virtual tour scene you want to rotate the camera around a line that runs through (or as close as possible to) a certain point such that the parallax error is completely eliminated. Failing to do so may result in problems with the stitching process and will require a lot more work with the possibe need for image retouching.

So first of all, in order to set the terminology to rights I will provide you with the proper definition of the nodal point. In fact, it consists of two points - the front and rear nodal point. In order to understand this issue let us first define:

  • The principal planes (front and rear) which have the property that a ray emerging from the lens appears to have crossed the rear principal plane at the same distance from the axis that that ray appeared to cross the front principal plane, as viewed from the front of the lens.
  • The principal points which are the points where the principal planes cross the optical axis.

If the medium on both sides of the optical system is the same (e.g. air), which is indeed our case, then the front and rear nodal points coincide with the front and rear principal planes, respectively (see the diagram). So there is nothing like a single nodal point to which so many pano tutorials are wrongly referring.

On the other hand, the most common explanation of nodal (or principal) point as being usually heard from pano photography folks says that it is the point inside a lens where light rays converge and flip over before being focused onto a film plane or a digital sensor. It should also be the place where the entrance pupil of the camera is situated. So far so good, this explains where the point to which one should set the center of rotation for shooting panorama is situated. However, using the term "nodal point" is absolutely improper here and one shoud rather think of a different term to avoid confusion. I personaly prefer naming it "pivot point". By the way, it is also worth noting that the nodal point is not the same as the film/sensor plane (sometimes marked at the bottom of cameras) as some photographers think.

In practice determining the pivot point of a lens is quite easy to do visually. The only thing you need is two vertical features that you can use as reference lines, for instance a flag or light pole, corner of a wall etc. Set one of them very close to the camera (1 meter or so) while the other one stays far away. Take two shots at different angles so that the vertical features you chose are close to the right or left edge of the frame, respectively. Most likely you will see the parallax shift of the objects. Adjust the corresponding rail on your panoramic head (the one that changes the vertical axis of camera rotation) so that you reduce the parallax. Eventually you should be able to eliminate it entirely which usually requires few iterations depending on how accurate your judgement was in each step. By checking the test images down to a pixel resolution a sub-milimeter accuracy in the setting of pivot point can be achieved.

Monday, May 21, 2007

Virtual tour - Silesian-Ostrava Castle

The Silesian-Ostrava Castle (original czech name "Slezskoostravský hrad") is one of the most historical and cultural monuments of the city of Ostrava. This originally gothic castle was built at the end of the 13th century. In mid-16th century it was rebuilt into a renaissance chateau. It was restored recently after many years of dilapidation, caused by coal mining under the castle. It is definitely one of the most important tourist attraction of the city that worth seeing. Our virtual tour brings you to this remarkable place.

Saturday, May 19, 2007

High Definition Viewer from Microsoft

Just a few days ago Microsoft released a Beta test version of their new utility called HD View. This viewer was developed by Microsoft Research's Interactive Visual Media group to aid in the display and interaction with very large images. Thus, it's not just a panoramic photography viewer but it is generally following the demand to visualise high resolution visuals over the web as well as the increasing resolutions of modern digital cameras. With this tool, displaying images of gigapixel resolution shouldn't be a big deal even in web-based applications.

Apart from the "on-the-fly" image loading capability (so that just the current view is loaded with the necessary level o details) for the panoramic photography branch the most unique and powerful feature of HD View is the zoom-dependent perspective transformation which is greatly reducing image distortions present with other current viewers.

As Microsoft research lab staff claims, it should:

  • allow smooth panning and zooming on large images,

  • only download enough data to create the current view (and possibly look ahead to the next), and

  • always display the current field of view with an appropriate projection. This means that when zoomed way in you should be presented with a standard perspective projection providing a sense of immersion, and when zoomed out you experience a curved projection so that get a full overview of the scene. In between the projection should smoothly transition.

  • Finally, it should be easy to create your own HD View content and present it to the world via the web.

The following video shows all the main features of this new viewer:

At the moment HD View is an ActiveX browser component, thus the first time you use it you will be asked to install it. It also only runs in Internet Explorer under the MS Windows. Hopefully there will be more support for other browsers and operating systems added in the future versions of HD View. Microsoft also announced that the tone adjustment functionality (very usefull for huge images that typically has very deep dynamic range) will be released in a new version soon.