One problem that commonly plagues QTVR artists is the problem of finding the correct lens parameter for a given camera or lens used for shooting a QTVR panorama. This is the stitcher "Vertical Field of View" or VFOV, the angle that has its apex the center of the Panorama cylinder and the lines of sight to the top and bottom of the cylinder defining the sides of the angle.
Both PanoWorx and Nodester have a large number of templates for 35 mm camera lenses and digital cameras that include the VFOV value. QTVRAS has only a few lens settings preconfigured. But new digital cameras with different lens parameters keep coming out. Ken Turkowski's Field of View Calculator works well for finding the VFOV for 35 mm camera lenses, but is problematical for digital cameras that don't use a 35 mm X 24 mm negative. Then there is the situation when you have taken a pano on a zoom lens with the lens zoomed to some unknown position. Also, you may use a wide angle adapter on your camera, and need to use Helmut Dersch's PanoTools to compensate for the barrel distortion that is introduced, making finding the VFOV even more problematical. Here I offer an alternate approach to finding the VFOV of a given set of pictures that get stitched for a pano. It works well if you have taken a pano shoot using a precision pan head like the Kaidan or Peace River heads that give you consistent angle of rotation between shots.
I used to think I could get this VFOV from the dimensions of a stitched PICT using simple trigonometry. The width of the final stitched PICT for a QTVR pano movie would be the circumference of the cylinder and the height of the PICT would be the height of the cylinder. The VFOV could be calculated with the formula:
VFOV = 2 * arctan(h / (2 * r))
where h is the height and r is the radius of the cylinder.
Of course r is gotten from r = C / (2 * PI)
where C is the circumference of the cylinder--the width of the stitched PICT.
This simple-minded formula is not quite correct in the creation of a QTVR panorama. According the Helmut Dersch, this is because rectilinear lenses have a greater VFOV in the center of the frame. To compensate for this lens design artifact the correct formula for the stitcher VFOV is:
Stitcher VFOV = 2 * arctan(h/(2 * r * cos(arctan(w/(2 * r)))))
where w is the seam-to-seam distance of the blended PICTs that make up the stitched panorama PICT, also known as the offset, because it is the distance the stitcher offsets each PICT to match detail and blend.
The method requires estimating the final dimensions of the stitched PICT as closely as possible. To do this, you need to find the offset in pixels between each shot and multiply that number by the number of shots needed for a 360-degree panorama to get the Circumference of the cylinder. The height of the cylinder is the same as the height of the individual pictures in the pano shoot.
To find the offset open two adjacent pictures from your pano shoot in a graphics program that will let you measure pixel dimensions of a selection. Rotate them so that they are in their proper portrait orientation and next to each other in the order they were taken. Above, we see what this would look like in GraphicConverter. Find detail on the left edge of the right picture that matches a detail in left picture.
Use the graphic program's rectangle selection tool to make a selection that goes from the matching detail in the left picture to its left edge. GraphicConverter reports the dimensions of a selection you are making on the right side of the picture's title bar. The width of the selection will be the offset you use to estimate the final Circumference. Make this measurement as carefully as you can, since any error on the offset will be multiplied by the number of frames in your pano shoot. Once you have gotten this average offset, quit the graphics program without saving the pictures.
Alternatively, if you have a graphics program that has layering, such as Photoshop or Paint Shop Pro, you can copy and paste the right picture into a new layer on the left picture. By adjusting the transparency and blend method of the pasted layer, you can make it easy to position the right picture so it matches the detail on the left accurately. The picture above shows how this looks in Photoshop using a 50% transparency and the "Multiply" blend method. Use the rectangular selection tool to find the offset from the edge of the top layer to the left side of the bottom layer and read the offset distance from the Info window. Be sure you quit the graphics program without saving after you have gotten the average offset value.
Enter the offset you have found using the method above, the width and height of the individual shots, and the number of shots in the JavaScript calculator below. Click on "Calculate VFOV Parameter" to get the stitcher VFOV for this lens arrangement. This value should be close enough to give you successful results in QTVRAS, Widgetizer, or PanoWorx. (My example used full-size Nikon CoolPix 950 pictures reduced by 1/4 in each dimension - 300 X 400 pixels. 18 shots were used to take a 360-degree pano.)
Here is the result from the example. Photo-JPEG at Low, 116 K.
