DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Pixel value, 8-bit scale
0 50 100 150 200 250
Figure 1.17 Grayscale ramp on a CRT display is generated by writing successive integer values 0
through 255 into the columns of a framebuffer. When processed by a digital-to-analog converter
(DAC), and presented to a CRT display, a perceptually uniform sweep of lightness results. A naive
experimenter might conclude – mistakenly! – that code values are proportional to intensity.
Pixel value, 8-bit scale
0 50 100 150 200 250
Luminance, relative 0 0.02 0.05 0.1 0.2 0.4 0.6 0.8 1
CIE Lightness, L* 0 10 20 40 60 80 100
Figure 1.18 Grayscale ramp augmented with CIE lightness (L*, on the middle scale), and CIE
relative luminance (Y, proportional to intensity, on the bottom scale). The point midway across
the screen has lightness value midway between black and white. There is a near-linear relationship
between code value and lightness. However, luminance at the midway point is only about
18% of white! Luminance produced by a CRT is approximately proportional to the 2.5-power
of code value. Lightness is roughly proportional to the 0.4-power of luminance. Amazingly, these
relationships are near inverses. Their near-perfect cancellation has led many workers in video,
computer graphics, and digital image processing to misinterpret the term intensity, and to
underestimate the importance of nonlinear transfer functions.
14 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES