Posts tagged ‘equirectangular’

This article is the second in a series of articles first posted on 360 Cities site in 2008 by David Martin, Jeffrey Martin and the 360 Cities team. The first article was about what the spherical panorama is and how it can be used. Today let’s show you how to create one! The information is still interesting and relevant even after 2 years so I’ve left most of the text the same. I updated only some parts (marked in italics) where the information was not accurate anymore. -Jan

To create a high quality spherical panoramic image, you need some special photographic equipment and some special software. Typical photographic equipment includes a digital SLR, a fisheye lens such as the Sigma 8mm F3.5 EX DG or the Samyang 8mm f3.5, a tripod, and a panoramic tripod head. Because no lens can capture the entire image sphere in one shot, one needs to cover the sphere with several shots. The special tripod head is optional but it helps to rotate the camera without changing the viewpoint. If the lens nodal point moves between shots, then parallax will make the next step of stitching the photos together very hard. With a fisheye lens, one covers the sphere with 2-6 shots, depending on the equipment, circumstances, and desired quality.

Stitching the images together

The next step is to combine the individual images into a spherical panoramic image in equirectangular format. The process of doing this is far beyond the scope of this article. There is a good free and open source tool called Hugin that many photographers use for this purpose. There are also commercial programs available like PtGUI or Autopano. The process is currently semi-automatic, becoming more automatic as time goes on. (Update: The situation is now much better than it was in 2008. If you know how to take the images properly the stitching process is a very straightforward these days. When you want to erase your shadow or tripod from the image it can still be a bit of work, though.)

Publishing the panorama to 360 Cities

Once you have created a panoramic image in the 2:1 aspect equirectangular format, it is a simple matter to publish it on the 360 Cities platform, which will make the image available not only on the web but on Google Earth as well (if the image is approved for Google Earth). Once you have joined 360 cities, you can immediately upload your image. We recommend a minimum resolution of 5000×2500 6000×3000 pixels. You can upload any size, but the practical maximum resolution is currently 16384×8192 65536 x 32768 pixels. The image will then appear in your pending images list while our system processes it (see below). When processing is complete, the image will move to your unpublished images list. Before publishing your image, you should edit the image metadata to set at least the title, description, geo location, and heading. Within a few minutes, your uploaded equirect will be available for viewing on the web and in Google Earth. During those couple minutes, our system converts the image into a variety of formats that enable efficient viewing at full resolution on various platforms.

Viewing the panorama

Once uploaded and converted, you may view the image both on the web and in Google Earth. Note that the very nice fullscreen mode. Click and drag on the image to pan; press the shift and control keys to zoom in and out.

The birth of Benjamin Martin in Czech Republic

Old Town Square Christmas Market in Prague

You can view an image as an PhotoOverlay on Google Earth (birth and market). You may have to double click on the downloaded KML file in order to launch it in Google Earth.) The Google Earth panoramic image viewer is not as slick as krpano, but it does the job. Click and drag to pan the image; there is also a pan and zoom control in the top right corner. If you exit the photo, the overlay is rendered as a floating sphere! Double-click on the sphere to fly into it.

In either format, as you pan around or zoom in and out, you’ll see tiles loaded as needed, just as happens in Google Maps. Krpano and Google Earth load only the tiles from the multiresolution pyramid that are needed to render the current view. The tiled pyramids provide a representation that makes these resolution-independent viewers possible.

Getting Started

Getting Started With Panoramic Photography Guide

Useful links

PanoTools Wiki – information hub for panoramic photography
PanoTools – Panorama Tools
PTgui – GUI for Panorama Tools
hugin – panorama photo stitcher
autopano – automatic panorama stitching
enblend/enfuse – image blending and HDR fusion tools
Panorama Tutorials – more links

This article was first posted on 360 Cities site in 2008 by David Martin, Jeffrey Martin and the 360 Cities team. The information is still interesting and relevant so let me repost it here. I’ve left most of the text exactly the same but I changed some of the links and inserted comments in italics where the information was not accurate anymore. -Jan


You are probably already familiar with medium of 360 degree panoramic images from either Google Street View or KML PhotoOverlays in Google Earth (from the GigapxlGigapan, or 360 Cities layers). Tools for authoring panoramic images are getting very good, but the process of preparing and presenting such images on the web is still difficult. That is part of the motivation behind the creation of 360cities.net: panoramic photographers use our site to replace the otherwise difficult and time-consuming process of publishing their geotagged panoramas to the web.

You can turn on the 360 Cities gallery layer in Google Earth to view panoramas from our system (Update: 360 Cities is now a part of the default layer in Google Earth). The screenshots below show how these panoramas appear in Google Earth. From top left to bottom right, these images show (1) the 360 Cities icons marking the locations of panoramas; (2) the info window that opens when you click on one of those icons; (3) the floating sphere that appears when you get close to one of the images, and (4) the view from inside the spherical image. In this article we will demonstrate our technique of creating these spherical panoramic PhotoOverlays for Google Earth.

Google Earth Screenshots (click to view larger images)

What Is a Spherical Panoramic Image?

A panoramic image is the ultimate wide angle image. A normal photo shows you what the world looks like in an instant in time from a single viewpoint over some limited field of view. The widest wide angle lens, a fisheye lens, can show you up to a 180 degree field of view, which covers half the viewing sphere (Update: there are now lenses that can cover more than 180 degrees). A spherical panoramic image, on the other hand, covers the entire viewing sphere, so that you can look in any direction. A spherical panoramic image is not a 3D image: It is still a single viewpoint image, but its field of view is not limited. (Update: There are now so-called one shot cameras that cover the whole 360 degrees. They are great for taking spherical photos without any knowledge of stitching, but their image quality is still mediocre).

Because a panoramic image shows you what the world looks like from a single viewpoint, it is shaped like a sphere. Computers aren’t so good at representing spheres, however, so panoramic images are stored as regular rectangular images. One could use any cartographic mapping to unwrap the sphere onto a rectangle. The most common representation used for panoramic images is the equirectangular projection, which looks like this:


The Birth of Benjamin Martin by Jeffrey Martin

In an equirectangular projection, the equator and all lines of longitude are undistorted. You can picture the projection as follows: Prick a hole in the north and south poles of the sphere, and cut the date line. Now unwrap the sphere onto a flat surface by stretching at the poles without stretching the lines of longitude so that the image forms a rectangle. In an equirectangular projection, the lines of longitude and latitude form a grid of equal sized squares. The equator is the central horizontal line; the north pole is all along the top, and the south pole all along the bottom. Distortion is minimal at the equator, and infinite at the poles. The aspect ratio of an equirectangular image is 2:1, because the equator is twice the length of each line of longitude.

One does not typically view the equirectangular image, or “equirect”, directly because of the severe distortions in the polar regions. The equirect is simply a means of storing the underlying spherical image. Special software can render an equirectangular image into an immersive panoramic experience. Click on the image above or below to view it in the 360 Cities system. Make sure to click and drag on the image at 360 Cities to pan; shift and control keys zoom in and out.


Old Town Square Christmas Market by Jeffrey Martin

Conclusion

Panoramic images provide a compelling immersive experience that cannot be matched by traditional imagery. There exist good tools for creating and rendering these images, but managing and publishing them yourself is difficult and tedious. 360 Cities is a platform that fills that crucial gap in the tool chain. Not only does 360 Cities make embedding high resolution panoramic images on the web a snap, the image may be viewed in Google Earth as well. Now you have an excuse to buy that cool fisheye lens…