Understanding Projecting Modes
Before we can effectively describe the various projection types, let’s go back to the notion of image assembling (i.e. image stitching).
The resulting texture on the sphere’s surface is the stitched image.
The resulting image can either cover the totality of the sphere’s surface in the case of a 360° x 180° panorama, or only a fraction of it.
We are assuming here that the stitching being performed is based on a model where the camera rotates around its nodal point as illustrated in the above image.
Other stitching models are possible and will soon be supported by Autopano Pro. The most popular is the Orthogonal, also called Orthographic stitching.
All the source images are shot orthogonally to the same plane. Instead of rotating around its center, the camera is following a linear motion path, always pointing in the same direction.
This is exactly what is being done when scanning a A3 size sheet of paper with a letter size scanner. We end up with 4 or 5 files assembled using this stitching model.
It is also what happens when taking pictures of building fronts in a street by walking down the street and taking a picture facing the buildings every 10 steps.
The projection modes are referring to what’s being done with the texture covering the base sphere.
If we project it on a plane, we will then have a Rectilinear or Planar projection; if we project it on a cylinder we are doing a Cylindrical projection; and if we use the texture as is we are talking about a Spherical projection.
All projections surfaces are not illustrated but principles stay identical: start from the stitched sphere, project this sphere on adapted 3D surface and unwrap this 3D surface on a plane to obtain a picture.
|Spherical or equirectangular projection: to see all field of view.||Cylindrical projection: to see all horizontal field of view with less vertical distortions.|
|Planar or rectilinear projection: like standard lens view||Fisheye: like fisheye lens view|
Variants of classical projections
|Mercator projection:||Hammer projection:||Pannini projection:|
Funny or artistic projections
|Mirror ball projection:||Orthographic projection:||Little planet projection:|
The projection modes can vary depending on the panorama’s orientation. This is the case for the Spherical and Cylindrical projections, not the Planar projection which is not sensitive to the panorama’s orientation.
When looking at the drawing illustrating the Cylindrical projection, we can see that we assumed that the cylinder’s axis was vertical as this is what we are looking for.
But multiple types of cylinders and axis can exist: we can imagine a cylinder with a vertical axis.
The visual aspect of the panorama will then be very different.
Note: Be careful to correctly set the point of view when working with this kind of very tall subject (in this example at the base of the tower: where the two grey lines intersect).
The Spherical mode, just like the Cylindrical mode, is dependent on the orientation of the panorama.
We will generally want for that type of projection that the verticals stay vertical in order to obtain nice views.
In some cases, using another axis than the horizontal can turn into a great creative tool.
The following three examples are Spherical views of the same 360° x 180° panorama with very different resulting looks.