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Chromatic Aberration

Chromatic aberration is a visual artifact seen when viewing images through lenses.

The phenomenon causes colored fringes to be visible around objects, and is increasingly more apparent as our view shifts away from the center of the lens. The effect is due to the refractive index of the lens varying for different wavelengths of light (shorter wavelengths towards the blue end of the spectrum are refracted less than longer wavelengths towards the red end). Since the image displayed on the Rift is composed of individual red, green, and blue pixels,2 it is susceptible to the unwanted effects of chromatic aberration. The manifestation, when looking through the Rift, is that the red, green, and blue components of the image appear to be scaled out radially, and by differing amounts. Exactly how apparent the effect is depends on the image content and to what degree users are concentrating on the periphery of the image versus the center.

Chromatic Aberration

Fortunately, programmable GPUs enable you to significantly reduce the degree of visible chromatic aberration, albeit at some additional GPU expense.

To do this, pre-transform the image so that the chromatic aberration of the lens will result in a more normal looking image. This is analogous to the way in which we pre-distort the image to cancel out the distortion effects generated by the lens.

Sub-Channel Aberration

Although we can reduce the artifacts through the use of distortion correction, we cannot completely remove them for an LCD display panel.

This is due to the fact that each color channel is actually comprised of a range of visible wavelengths, each of which is refracted by a different amount when viewed through the lens. As a result, although we are able to distort the image for each channel to bring the peak frequencies back into spatial alignment, it is not possible to compensate for the aberration that occurs within a color channel. Typically, when designing optical systems, chromatic aberration across a wide range of wavelengths is managed by carefully combining specific optical elements (in other texts, for example, look for “achromatic doublets”).