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VR panorama on the iPhone

On Monday is the deadline for third party applications to be considered by  Apple for the officially opening of the iPhone App Store. Brian Greenstone is working to complete PangeaVR for the iPhone in time.

After Marco Giorgini and Geoffrey Morelle’s CubeWorld, this is the second VR viewer known to me on the iPhone.

Apple’s own QuickTimeVR is still missing in action.

So how do these panorama viewers compare?

For authors, PangeaVR takes a single equirectangular input, while CubeWorld takes six cubefaces. The cubefaces format requires slightly more authoring work (which can be automated), but in return it is easier on the rendering engine and should play better.

PangeaVR limits the size of the equirectangular image to 2048×1024, while CubeWorld works with cubefaces up to 1024×1024. However the image’s megapixel count (2 Mpx and 6 Mpx) can’t be immediately compared because of the different angular resolution. To compare, the dimensions must be normalized. The conversion formula is:

Which roughly means that an equirectangular image of 2048px long is equivalent to a cubeface of 591px.

Does this matter? given the iPhone’s screen resolution of 480x320px and that 90% of the audience does not zoom, a 512px cubeface is enough (and saves the users a bandwidth intensive and potentially expensive download).

How does it look in practice? I don’t know because I don’t own an iPhone and I do not intend to buy one for the following five reasons not to buy a 3G iPhone.

5 Responses

  1. yuv,
    would u kindly explain how u arrived at the “conversion formula” for cube face size? i’m used to seeing the ideal cube face size as equi-length over pi — 6/sqrt(3) is kinda close to pi, but why this specific irrational number?

  2. @Pedro: the conversion formula above achieves the same pixel count.
    1024 * 2048 = 2097152
    2048 * sqrt(3)/6 = 591.2066757
    (2048 * sqrt(3)/6)^2*6=2097152

    other formulas, e.g. those matching the surface of the sphere against the surface of the cube, result in an even smaller cubeface dimension. This specific formula is rather conservative – I prefer larger cubefaces than a loss in resolution.

  3. thanx yuv, now i understand your goal, ergo the formula.
    i’m not sure it’s a worthwhile goal (do we really need equi-length pixels to represent the zenith and nadir without loss of resolution? in equirectangular we have no choice, but after conversion to cubic i’d rather save some bytes), but i’ll let you have it ;-)

  4. @Pedro: what do you mean with the term “equi-length pixels”?

  5. yuv, using your example, i mean 2048 pixels.

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