To watch 360 videos on normal 2D displays, we need to project the selected part of the 360 image onto the 2D display plane. In this paper, we propose a fully-automated framework for generating content-aware 2D normal-view perspective videos from 360 videos. Especially, we focus on the projection step preserving important image contents and reducing image distortion. Basically, our projection method is based on Pannini projection model. At first, the salient contents such as linear structures and salient regions in the image are preserved by optimizing the single Panini projection model. Then, the multiple Panini projection models at salient regions are interpolated to suppress image distortion globally. Finally, the temporal consistency for image projection is enforced for producing temporally stable normal-view videos. Our proposed projection method does not require any user-interaction and is much faster than previous content-preserving methods. It can be applied to not only images but also videos taking the temporal consistency of projection into account. Experiments on various 360 videos show the superiority of the proposed projection method quantitatively and qualitatively.
use a geometric model to project spherical panorama images onto an image plane. Rectilinear, Stereographic, and Pannini projection @cite_5 models belong to this category. Rectilinear projection is the perspective projection from the center point of the spherical image onto the image plane. This model preserves all lines, but the contents on the margin of a projected image can be extremely stretched and distorted when the FOV is large. On the other hand, stereographic projection is the perspective projection from the opposite point to the point of tangency as the center of projection. This model preserves a conformality of the contents, but not lines. Pannini projection based on the cylindrical projection keeps the vertical lines straight. Furthermore, the horizontal lines or radial lines are selectively preserved by the vertical compression. The methods mentioned above are very simple and fast, but have a common drawback --- they can not preserve all lines and salient objects simultaneously.
@cite_5
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The widely used rectilinear perspective projection cannot render realistic looking flat views with fields of view much wider than 70°. Yet 18th century artists known as 'view painters' depicted wider architectural scenes without visible perspective distortion.We have found no written records of how they did that, however, quantitative analysis of several works suggests that the key is a system for compressing horizontal angles while preserving certain straight lines important for the perspective illusion. We show that a simple double projection of the sphere to the plane, that we call the Pannini projection, can render images 150° or more wide with a natural appearance, reminiscent of vedutismo perspective. We give the mathematical formulas for realizing it numerically, in a general form that can be adjusted to suit a wide range of subject matter and field widths, and briefly compare it to other proposed alternatives to the rectilinear projection.