### Comparative resolution of 360 video capture devices

Paul Bourke
October 2014

Introduction

360 degree video capture has a long history but with a renewed interest in immersive displays and in particular commodity head mounted displays such as the Oculus, the question is often asked how does one capture video at sufficient resolution. Indeed, what is sufficient resolution? The following will provide one way to evaluate the resolution of various 360 degree video capture devices. It should be pointed out that while an idealised capture system will be considered, that is in practice far from the reality. Some of the factors that further degrade the actual capture quality are listed at the end of this document.

One way to evaluate what resolution one needs is to consider the size of a pixel on the final 360 video frame and what area that covers in the scene. In what follows only resolution in the horizontal axis will be considered, that is, cylindrical projections. Similar arguments can be extended vertically for cameras that capture spherical projections such as the LadyBug range.

Imagine a camera that captures cylindrical or spherical projections with N pixels across, these pixels are spread around the 360 degrees and therefore a single pixels subtends 360/N degrees. One can then project this angle into the world and compute the size of the pixel at different ranges R. The width of a pixel W at range R is then simply:

W = 2 pi R / N

This then provides an upper limit on the resolving ability for a camera that results in N pixel across 360 degrees. This assumes perfect optics, no sensor noise, no image compression, no multiple camera blending/stitching and a raft of other effect that will limit the effective resolution.

Examples

In the following table the pixel width at 2m, 5m and 20m are presented for various 360 camera solutions. Please note that the optimal resolution of some of these devices is a "best guess" or based upon manufacturers specifications (often dubious).

 Estimated resolution Pixel width W at Device N 2m 5m 20m Comments LadyBug-3 5400 2 mm 6 mm 23 mm 16 fps at this resolution.Spherical. LadyBug-5 8000 1.5 mm 4 mm 16 mm 10 fps at this resolution.Spherical. H3Pro7HD (Kolor) 8000 1.5 mm 4 mm 16 mm 7 GoPro cameras mounted in portrait.Cylindrical. H3Pro7 (Kolor) 6000 2 mm 5 mm 21 mm 7 GoPro cameras.Spherical. Lucy (Kogeto) 2000 6 mm 16 mm 63 mm Example of camera based upon 1920 video.Cylindrical. Joey (Kogeto) 4000 3 mm 8 mm 31 mm Not yet available at time of writing.Cylindrical. Bublcam 2500 5 mm 13mm 50 Not yet available at time of writing.4 x 720p camera at 30fps.Spherical. Bublcam 3600 3.5 mm 8 mm 35 mm Not yet available at time of writing.4 x 1920p camera at 15fps.Spherical. Centrcam 6900 1.8 mm 4.5 mm 18 mm Not yet available at time of writing.Maximum 30fps.Cylindrical.

Confounding factors

In the above it has been assumed that if a device can capture N pixels horizontally, then those pixels are "pure", this is never the case. Further degradation can occur and some sources are listed below.

• Many of the camera solution do not result in a uniform resolution across the cylindrical or spherical projection. This is particularly true of single camera solutions based upon conical mirrors.

• In the case of many commodity devices, while they may contain the same sensor as more expensive versions, there can be a poorer quality lens in front of the sensor. This generally means that pixels are no longer independent (they receive light over a wider region of the world) and generally an increase in chromatic error. This is particularly true of many of the mobile phone based options.

• Different sensors have different levels of random noise, especially in low light.

• Digital video capture is often limited by the bandwidth to the recording device. To maintain a recording resolution and frame rate the video is generally compressed with corresponding artefacts. The slower the recording device, the more compression and video degradation that occurs. Particularly so with SD storage medium in the GoPro, and others.

• While the only scalable means of increasing the resolution is to use multiple cameras, this introduces potential resolution degradation across the overlap and blending zone. While there are multiple camera configurations, that with the use of mirrors, can create a single projection point, the majority have an offset projection point. For very fundamental reasons this means that a perfect stitch/blend across all depths is impossible.