VR系列——Oculus Rift 开发者指南：五、色差

色差

色差是通过透镜观察产生的一种视觉假象。

这种现象会导致在物体周围产生彩色条纹，并且越接近透镜边缘越明显。这是由不同波长的光通过透镜时的折射率不同导致的（波长较短的蓝光折射小于波长较长的红光）。Rift的图像由红、绿、蓝像素组成，很容易受到色差的影响。图像的红、绿、蓝像素点会呈放射状向外扩展，并且数量也会不同。受影响的效果取决于图像内容以及用户对边缘区域的专注程度。

色差

幸运的是可编程GPU能够大大减少可见色差，但会造成额外的GPU开销。

要做到这一点，需要对图像进行预变换，使其受色差影响后更接近普通图像。这与我们先扭曲图像来抵消透镜造成的扭曲的处理类似。

子通道畸变

即使通过失真校正能够减少假象，但是在LCD液晶面板上这种影响是没办法完全消除的。这是由于每一个颜色通道道都是由可见光组成，而不同波长的可见光通过透镜的折射率不同。因此，虽然我们能够通过失真处理让每个通道的图像的峰值频率保持立体对齐，但是这不可能消除颜色通道内部产生的畸变。典型地，在设计光学系统的时候，会通过谨慎地组合特殊的光学元素来管理相当大波段的可见光产生的色差。（要了解更多，可以在其他文章查找“消色差双合透镜”）

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原文如下

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, 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”).