4

Panoramic Image Projections

This is Part 4 of the ‘How To Make A 360° Panorama?‘ tutorial series. Here are the links to Part 1Part 2 and Part 3.

If you want to get started in panoramic imagery it is essential that you get at least a rough idea of the different panoramic image projections that exist and how they affect the images.

  • Rectilinear Projection

Admitting that the environment around you takes the shape of a sphere, the rectilinear projection is used when you take photos of only a portion of that sphere. This portion of the sphere is then projected on a flat surface. That’s the rectilinear projection. It is often used to stitch a rather small number of images: it is well adapted to partial panoramas and mosaic pictures.

  • Cylindrical Projection

This one will be used when the panorama does not have any data on the zenith or nadir (the poles of the sphere) – typically a single row (sometimes two rows) of images. It is the projection used for cylindrical panoramas and some larger partial panoramas that look too distorted in a rectilinear projection. Again, it applies this portion of the sphere on a flat surface.

Often, when creating a partial panorama, my software would automatically choose a rectilinear projection but I found out that it would look nicer in a cylindrical projection. Don’t hesitate to play with the projections in your software to understand how they affect the image and choose the best one for your panorama! That said, most of the time the software will automatically detect the right projection for your panorama.

  • Spherical/Equirectangular Projection

This projection is used to project the whole sphere on a flat surface. The result of that is an equirectangular image, with a 2:1 ratio. The width is exactly twice the height. This is logical, since it covers 360 degrees horizontally and 180° vertically. You will see that in this projection, the top and bottom of the image appear very distorted and this is perfectly normal. This is the projection you will use as a source for a full spherical panorama.

Equirectangular

  • Mercator Projection

This projection can be seen as a derivative of the equirectangular projection, with less distortion at the zenith and nadir. The world maps are in this projection. For panoramas, it can be useful if you want to print the image of a spherical panorama for example. I personally never use it.

Mercator

  • Cubic Projection

This one can also be used as the base of a spherical panorama. The principle is that the whole sphere is turned into the 6 faces of a cube – front, right, back, left, zenith and nadir. The 6 square images have a rectilinear projection.

This projection is very useful when you want to edit the image of a spherical panorama. Indeed, the rectilinear projection is the one with the less distortion. It is not always easy to edit the warped images on an equirectangular image, and it is impossible to edit the completely distorted zenith and nadir.

You can turn this image into the 6 faces of a cube that will be easy to edit. That way, you can also very easily add a logo on the nadir image, for example. When you are done editing, you just have to convert it back to an equirectangular image and you are done!

Cube Faces
There are some other projections that exist, but I believe you won’t have to deal with them in your panorama creation.

Basically, just try to remember that:

  • You stitch a few images and get a partial panorama using a rectilinear projection
  • Your stitch a larger partial panorama or a cylindrical panorama using the cylindrical projection
  • You stitch a spherical panorama using the spherical or equirectangular projection
  • You can edit an equirectangular image more easily by first converting it to a cubic projection, and then convert it back to an equirectangular image

Now let’s see what editing you must apply to your panorama to make it look good before you hit the Rendering button! Click on the link below to continue.

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