2.6 How many gray levels can the eye distinguish?
Several studies have converged on the same answer, roughly 450. Eight bit color (256) will produce banding artifacts if mapped over a wide brightness range. The situation is complicated by the inequality of step sizes across brightness levels. At the low end steps should be very small while the high end needs larger steps (Weber?s Law). It is unlikely that 8 bits really supply 256 gray levels, let alone 450. In general, the steps at the low end are too large, so each gray level jumps more than one JND while at the high end, the steps are too small, so successive gray levels appear identical.
But there is good news. CRT monitors have a defect, nonlinear gamma, that partly corrects this error by automatically decreasing step size at the low end and increasing the number of discriminable gray levels. Unfortunately, there is also bad news. Both the size of human brightness JND?s and the screen gamma change with viewing conditions, so the number of steps that is really available also changes and is somewhat unpredictable.
2.7 How can I get more than 256 gray levels out of my computer by using color? Even a 24 bit system only supplies a maximum 256 levels of gray since it uses 3 eight bit numbers to control intensity. However, there is a simple trick which can improve gray level resolution.
The relative intensity of the colors green-red-blue form a rough ratio of 4-2-1. The exact ratio will vary from monitor to monitor, but the order is always the same - green is most intense, then red, the blue. Now suppose that you have a gray at (red=50,green=50,blue=50). Normally, the next step would be (51,51,51). But you can create 6 intermediate steps between these two levels:
2.8 How many saturation levels can the eye distinguish?
Starting at white and going out to a pure spectral color, there are gradations in apparent saturation. However, some hues appear inherently more saturated than others. The most saturated yellow still appears pale compared to saturated red, saturated green and especially saturated blue. As a result, the number of distinguishable saturation levels is smaller in yellow than in the rest of the spectrum. One study concluded that there are only 10 saturation steps around yellow with the number gradually rising as wavelength increased or decreased. The lowest wavelengths, blue-violet had about 60 steps while the red reached about 50. Viewers will find small differences in blue, violet and red saturation highly discriminable while small differences in yellow saturation will be hard to detect. Green and orange are in the middle
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