Chromatic aberration

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Chromatic aberration is an issue that arises from the inability of a lens to focus all colors in the same place. This happens because the focal length (the distance between the lens and the image sensor when the subject is in focus) depends on a property called refraction, which is the bending of a wave when it enters a medium where its speed is different. Since blue and red light have different indexes of refraction, their focal length is also slightly different. Chromatic aberration is clearly visible on photographs or video as the color channels are not perfectly aligned.

Chromatic Aberration in Virtual Reality

When chromatic aberration occurs in virtual reality, a viewer faces two issues. On one hand, the simple presence of this optical distortion can break the suspension of disbelieve as it serves as a constant reminder of the artificial nature of the virtual environment. On the other hand, it can lead to discomfort and headaches that could make longer VR sessions impossible.[1]

Software Correction for Chromatic Aberration

Fixing chromatic aberration in software is usually done by applying the classic "Brown's model" distortion correction formula to each color channel independently, using slightly different distortion coefficients for each color channel, thus changing the red, green and blue color channel to a different scale.

The drawback of these methods is that they lead to a loss in image quality. For increased accuracy, a subpixel offset has to be taken into account for the different color channels.[2] A correct use of subpixel interpolation enables developers to effectively reduce chromatic aberration below noticeable levels.

Hardware Correction for Chromatic Aberration

Chromatic aberration can be minimized by using an achromatic lens. This type of lens is usually composed of two individual lenses made from glasses with different amounts of dispersion. The use of a strong positive lens made from a low dispersion glass like crown glass coupled with a weaker high dispersion glass like flint glass can correct the chromatic aberration for two colors, e.g., red and blue.[3]

References

  1. http://blog.qwrt.de/improved-pre-warping-for-wide-angle-hmds/
  2. https://forums.oculus.com/viewtopic.php?t=32
  3. http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/aber2.html

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