Method and apparatus for performing dynamic gamma contrast control

A circuit for providing dynamic gamma contrast control of a video signal in a television receiver, said input video signal forming, on display, image frames, each of which exhibits varying brightness in differing areas of the respective image frame, said circuit comprising:

• an input for receiving an input video signal;
• means for low-pass filtering said input video signal;
• means for normalizing said low-pass filtered video signal between the range of 0.0 and a first predetermined quantity A, providing a normalized output signal;
• means for adding a second predetermined quantity B to said normalized output signal, forming a gamma exponential signal;
• means coupled to said input for normalizing said input video signal between the range of 0.0 and 1.0;
• means for raising said normalized video signal by a quantity indicated by said gamma exponential signal, forming a corrected normalized video signal;
• and
• means for re-scaling said corrected normalized video signal back to the full dynamic range of the input video signal, whereby dark portions of the image frames formed when said corrected normalized video signal is displayed are more visible than in the image frames of said input video signal.

A digital circuit for providing dynamic gamma contrast control of a video signal in a television receiver, said input video signal forming, on display, image frames, each of which exhibits varying brightness in differing areas of the respective image frame, said circuit comprising:

• input means for receiving input R, G, B signals;
• means for digitizing said input R, G, B signals forming R.sub.in, G.sub.in, B.sub.in signals;
• means for determining, pixel-by-pixel, a maximum of said R.sub.in, G.sub.in, B.sub.in signals thereby forming a signal g(t);
• storing means for storing the signal g(t);
• means coupled to an output of said storing means for normalizing g(t) forming an output signal g.sub.norm (x,y) which ranges in value from 0.0 to 1.0;
• convolving means for performing a two-dimensional averaging and rescaling on g.sub.norm (x,y) thereby forming the signal h.sub.1 (x,y) in accordance with the equation
• h1(x,y)=g.sub.low (x,y)*A/maximum(g.sub.low (x,y)),
• where A is a constant selected from the range 1.2 to 2.5;
• subtracting means for subtracting 1.0 from h.sub.1 (x,y);
• a first multiplier for multiplying an output of the subtracting means by g.sub.norm (x,y);
• adding means for adding 1.0 to an output of said first multiplier;
• a look-up table for receiving the signal g.sub.norm (x,y) and for implementing a function r in accordance with the equation
• g.sub.2 (x,y)=r(g.sub.1 (x,y),c)=ln{cg.sub.1 (x,y)+sqrt(cg.sub.1 (x,y))2+1}
• where c is a constant selected from the range 2.0 to 15.0;
• a second multiplier coupled to the output of the adding means and the look-up table forming the signal g.sub.out (x,y);
• a divider coupled to an output of the second multiplier and an output of the normalizing means for forming the factor z(x,y) in accordance with the equation
• z(x,y)=g.sub.out (x,y)/g.sub.norm (x,y);
• and
• multipliers coupled respectively to outputs of said digitizing means and each receiving the function z(x,y) thereby forming the output signals R.sub.out, B.sub.out, G.sub.out, whereby dark portions of the image frames formed when said output signals are displayed are more visible than dark portions in the image frames of said input video signal.

An analog circuit for providing dynamic gamma contrast control of a video signal in a television receiver, said input video signal forming, on display, image frames, each of which exhibits varying brightness in differing areas of the respective image frame, said circuit comprising:

• an input for receiving input R, G, B signals;
• means for determining a maximum, pixel-by-pixel of the input:
• R, G, B signals forming the signal g(t);
• means for low-pass filtering the signal g(t);
• means for sampling an output of the low-pass filtering means;
• delaying means for delaying an output of said sampling means, said delaying means having a plurality of stages each of which having an output;
• means for summing the outputs from said plurality of stages in said delaying means;
• a first multiplier having a first input coupled to the output of said summing means and a second input coupled to the output of said maximum determining means;
• a nonlinear circuit having an input coupled to the output of said maximum determining means;
• a second multiplier having a first input coupled to an output of said first multiplier and a second input coupled to an output of said nonlinear circuit;
• a divider coupled to an output of said second multiplier and the output of said maximum determining means for forming the function z(x,y);
• and
• means for receiving the input R, G, B signals and for multiplying these signals by the function z(x,y), whereby dark portions of the image frames formed when said input R, G, B signals multiplied by the function z(x,y) are displayed are more visible than dark portions in the image frames of said input video signal.