Ross Vision - Gamma

Ops 19–10 • Appendix B: Color Correction Parameters Vision Operator’s Manual (v12.0 MD)
the output contains 0% (i.e. some amount of color component is present in all output
values).
• Lower Offset — Affects the A section of the video signal and how much, if any, of the
input video color component (R,G, or B) is mapped to 0% (i.e., no color component) in
the output signal. Increasing the lower offset decreases the amount of input signal values
that will be mapped to 0%. It is possible to increase the lower offset so all parts of the
video signal contain some of the color component (i.e., there are no output values with a
0% color component). Decreasing the lower offset increases the number of input video
signal values that will be mapped to 0%.
Gamma
The previous corrections are all linear. No matter which correction was applied, the output versus
input mapping is still a straight line. Gamma corrections introduce non-linear corrections to a
video signal. A gamma correction can best be described as taking a point on the output versus
input video signal line and then pulling it perpendicularly away from the line. The resulting line is
a Bezier curve that flows smoothly from the start, through the new point and then continues to the
end point. The following graph (Figure 19.12) illustrates the effect of adding positive and
negative gamma corrections to an RGB component of a video signal.
Figure 19.12 Effects of Increased and Decreased Gamma (Red Component)
The point C, called the Gamma Offset point can be any point along the AB line. The Gamma
Value indicates how far perpendicularly off the line the Gamma Offset point is moved. Once the
Gamma Offset point and Gamma Value are determined, a Bezier curve is drawn from point A,
through point C and ends at point B.
Generally, increasing the gamma value adds more of the component to the video signal in the
location of the gamma offset point. Decreasing the gamma value reduces the amount of the
component in the video signal in the location of the gamma offset point. Moving the gamma
offset point allows you to select which part of the input video signal receives the gamma
correction. For example, if you increase the red gamma correction to the part of the video signal
that has no red component you will add red to those areas while having little effect on areas that
already contain a significant amount of red. This allows you to add a red tint to the image while
minimizing the amount of red-clipping that occurs.
The Vision switcher can perform gamma corrections on each RGB component.
Red Input Value
Red Output Value
A
B
C
100%
100%
0
Standard
Increased Gamma
Decreased Gamma