## Abstract

Wide-angle images gained a huge popularity in the last years due to the development of computational photography and imaging technological advances. They present the information of a scene in a way which is more natural for the human eye but, on the other hand, they introduce artifacts such as bent lines. These artifacts become more and more unnatural as the field of view increases. In this work, we present a technique aimed to improve the perceptual quality of panorama visualization. The main ingredients of our approach are, on one hand, considering the viewing sphere as a Riemann sphere, what makes natural the application of Möbius (complex) transformations to the input image, and, on the other hand, a projection scheme which changes in function of the field of view used. We also introduce an implementation of our method, compare it against images produced with other methods and show that the transformations can be done in real time, which makes our technique very appealing for new settings, as well as for existing interactive panorama applications.

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## Notes

- 1.
For us,

*real time*means that the computations are done quicker than the rendering of a frame in the application. Even if the other methods are fast, they are not suitable to be implemented in real time. - 2.
Recall that, in our method, the viewing sphere is rotated to make coincide the north pole with the center of stretching, and rotated back after the stretching. For simplicity, we will assume here that the shrinking occurs with the north pole as center, without considering rotations.

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## Acknowledgments

The authors thank the Flickr and Wikimedia Commons users who made available their equirectangular images under the Creative Commons license, used to obtain some figures of the paper: Gadl (Figs. 6, 10, 11, 12), Luca Biada (Fig. 7), DXR (Fig. 8) and HamburgerJung (Fig. 9). L. Sacht acknowledges the doctoral scholarship from CNPq. L. Peñaranda acknowledges financial support from IMPA during years 2012 to 2014.

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## Appendix: Möbius transformations

### Appendix: Möbius transformations

We recall in this appendix some properties of Möbius transformations. The information here should be enough to understand the paper. We refer to the book by Needham [15] for proofs and details.

A Möbius transformation is a mapping of the form

where \(a\), \(b\), \(c\) and \(d\) are complex constants. Multiplying these coefficients by a constant yields the same mapping, thus what matter are the ratios of the coefficients (usually the coefficients are normalized by scaling them to satisfy \(ad-bc=1\)). This fact shows that only three complex numbers are sufficient to determine uniquely a mapping. It can be shown that there exist a unique Möbius transformation which sends any three points to any other three points.

Möbius transformations have fixed points, computed through the equation \(z=M(z)\). Since this equation is quadratic, it has at most two solutions (except for the identity mapping). When \(c\ne 0\), both fixed points lie in the finite plane. When \(c=0\), at least one of the fixed points lies at infinity (as it can be deduced from Fig. 13, infinity is represented by the north pole \(N\) of the Riemann sphere). In this case, the transformation is simplified, taking the form \(M(z)=Az+B\). We can write \(A=\rho e^{i\alpha }\) to view \(M(z)\) as the composition of a rotation of \(\alpha \) centered in the origin, plus an expansion by \(\rho \) and a translation of \(B\). This interpretation lets us view geometrically the Möbius transformation, as depicted in Fig. 13.

When \(\alpha >0\), \(\rho =1\) and \(B=0\), \(M(z)\) is a rotation of the complex plane (i.e., a rotation of the sphere), as shown if Fig. 13a. The fixed points of this transformation are the two poles of the sphere, which correspond, in the complex plane, to the origin and infinity. This is called an *elliptic* Möbius transformation.

Figure 13b illustrates the transformation when \(\alpha =0\), \(\rho >1\) and \(B=0\). \(M(z)\) is, in this case, an expansion centered in the origin. The two fixed points are the same as in the previous case. If \(\alpha =0\), \(\rho <1\) and \(B=0\), it is an origin-centered contraction. These Möbius transformations are called *hyperbolic*.

When \(\alpha \ne 0\), \(\rho \ne 1\) and \(B=0\), the resulting transformation is a combination of the two former cases. This is called a *loxodromic* Möbius transform and it is illustrated in Fig. 13c. The two fixed points of this transform are the same as the above cases.

The last case study is the translation, occurring when \(A=0\) and \(B\ne 0\). The only fixed point of this transformation is infinity, represented on the sphere by the north pole. This transformation, the *parabolic* Möbius transformation, is depicted in Fig. 13d.

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Peñaranda, L., Velho, L. & Sacht, L. Real-time correction of panoramic images using hyperbolic Möbius transformations.
*J Real-Time Image Proc* **15, **725–738 (2018). https://doi.org/10.1007/s11554-015-0502-x

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### Keywords

- Panorama
- Perspective projection
- Möbius transformation
- Real time
- Implementation