TABLE OF CONTENTS Nonlinear coding of luminance is essential to maximize the perceptual performance of an image coding system. This chapter introduces luminance and lightness, or what is loosely called brightness.
| Note | In Color science for video, on page 233, I will describe how spectral power distributions (SPDs) in the range 400 nm to 700 nm are related to colors. |
Luminance, denoted Y, is what I call a linear-light quantity; it is directly proportional to physical intensity weighted by the spectral sensitivity of human vision. Luminance involves light having wavelengths in the range of about 400 nm to 700 nm; luminance can be computed as a properly weighted sum of linear-light red, green, and blue tristimulus components, according to the principles and standards of the CIE.
Lightness, denoted L*, is defined by the CIE as a nonlinear transfer function of luminance that approximates the perception of brightness.
| Note | The term luminance is often carelessly and incorrectly used to refer to luma. See Relative luminance, on page 206, and Appendix A, YUV and luminance considered harmful, on page 595. |
In video, we do not compute the linear-light luminance of color science; nor do we compute lightness. Instead, we compute an approximation of lightness, luma (denoted Y?) as a weighted sum of nonlinear (gamma-corrected) R', G', and B ? components. Luma is only loosely related to true (CIE) luminance. In Constant luminance, on page 75, I explained why video systems...
