Image Processing Reference
In-Depth Information
regions of the image will be exposed. It is in this area that modern digital cameras are least
successful when compared to film. So if the footage you are digitizing has been shot on a
digital device as opposed to a film camera, gamma correction may help to reveal detail
that was hitherto invisible.
Some codecs do this gamma correction between platforms automatically. The
Sorenson codec does not automatically gamma-correct. The MPEG-4 Part 2 codec does.
Gamma changes have no effect whatsoever on the extreme black and extreme white
value. It is designed to affect the linearity and hence the mid-tones. This correction must
be applied after the brightness and contrast setting have corrected the luma.
These are some common gamma values:
Macintosh has a standard gamma setting of 1.8.
Video has a gamma setting of 2.2.
Windows has a gamma setting of between 2.2 and 2.5 but it is not rigorously
Increasing the gamma value makes images appear to be brighter, which is what people
want when they say they want a brighter image. It is only necessary to adjust the gamma;
leave the brightness and contrast settings untouched.
Figure 35-8 shows some example gradients with changes to the bit depth and the
gamma values.
In the top strip of pixels, the intensity is shown as a linear gradient from black to peak
white. Because the color depth represents a 10-bit value, there are no contours or steps.
In the second strip of pixels, the effect of reducing the bit depth from 10 bits to 8 bits
has been exaggerated. The reduction in bit depth does not actually cause such a gross con-
touring but it is there. It is exaggerated so the effect is visible in print. The contouring can
be removed by adding a little noise to the image before the thresholding takes place. Subtle
application of a static dithering pattern will not affect the compression process very much,
but if you apply a moving noise pattern it might decrease the compression efficiency.
The third strip of pixels applies an exaggerated gamma correction in order to show
it on the printed page. In reality, the effect is very subtle and when you see it on the screen,
the image becomes more pleasing as the gamma is increased.
Figure 35-8 Grey-scales and gamma.
Charles Poynton's gamma FAQ:
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