Method and apparatus for performing dynamic gamma contrast control

• US 5,394,195 A
• Filed: 06/14/1993
• Issued: 02/28/1995
• Est. Priority Date: 06/14/1993
• Status: Expired due to Fees

• Current Assignee: Philips Electronics North America Corporation (Koninklijke Philips N.V.)
• Original Assignee: Philips Electronics North America Corporation (Koninklijke Philips N.V.)
• Inventors: Herman, Stephen
• Primary Examiner(s): Groody, James J.
• Assistant Examiner(s): Harvey, David E.
• Application Number:
  US08/076,565
• Time in Patent Office: 624 Days
• Field of Search: 358/164, 358/32, 348/674, 348/675, 348/676, 348/677
• US Class Current: 348/675
• CPC Class Codes: H04N 5/202 Gamma control circuits for controlling camera response irrespective of the scene brightness H04N23/82
• CPC Class Codes: H04N 9/69 for modifying the colour signals by gamma correction controlling camera response for colour signals H04N23/83

Abstract

Ordinarily, television signals are subjected to gamma correction at the source to compensate for non-linearities in CRT-based displays in accordance with set standards. However, it has been found in practice that all CRT-based displays do not exhibit the same amount of non-linearity. Hence, some television receivers include gamma correction circuitry which compensate for the difference between the particular CRT display and that assumed by the transmission system standard. In addition the above, circuitry is added for adapting this correction to local areas on the display thereby achieving dynamic range equalization. The input video signal is first low-pass filtered and then subjected to a normalization between 0.0 and a first predefined maximum value A. A second predefined value B is then added to this normalized signal generating a gamma exponential. At the same time, the input video signal is normalized for the range 0.0 and 1.0. This normalized input signal is then raised to an exponential power equivalent to the gamma exponential. Finally, this resultant signal is re-scaled back to the dynamic range of the input video signal. The resultant video signal on display then has the dark portions thereof more visible without the bright parts looking washed out.

Claims

1. A method of providing dynamic gamma contrast control of an input 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, comprising the steps:

low-pass filtering the input video signal so that processing only occurs on the low frequency components therein;

normalizing, pixel-by-pixel, the low-pass filtered video signal so that the value thereof extends from 0 to a predetermined maximum value A, thereby forming a normalized signal;

adding a predetermined parameter B to the normalized signal thereby forming a gamma exponent;

normalizing the input video signal so that the values thereof lie between 0 and 1.0;

raising the normalized video signal, pixel-by-pixel, to an exponent equal to the gamma exponent forming a corrected normalized video signal; and

rescaling the corrected normalized video signal 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.

2. A method of providing dynamic gamma contrast control of a video signal as claimed in claim 1, wherein A is chosen in the range 1.2 and 2.5.

3. A method of providing dynamic gamma contrast control of a video signal as claimed in claim 2, wherein A is 1.34.

4. A method of providing dynamic gamma contrast control of a video signal as claimed in claim 1, wherein B is chosen from the range of values 0.1 and 0.9.

5. A method of providing dynamic gamma contrast control of a video signal as claimed in claim 4, wherein B is 0.45.

6. A method of providing dynamic gamma contrast control of a video signal as claimed in claim 1, wherein said low-pass filtering of the video signal comprises convolving the video signal with a square window of uniform unit weight and size 31-by-31.

7. A method of providing dynamic gamma contrast control of a video signal as claimed in claim 1, wherein said video signal is a color video signal and said method is performed on each of the R, G and B components in the color video signal individually.

8. 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.

9. 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_in, G_in, B_in signals;

means for determining, pixel-by-pixel, a maximum of said R_in, G_in, B_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_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_norm (x,y) thereby forming the signal h₁ (x,y) in accordance with the equation

space="preserve" listing-type="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 h₁ (x,y);

a first multiplier for multiplying an output of the subtracting means by g_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_norm (x,y) and for implementing a function r in accordance with the equation

space="preserve" listing-type="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_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

space="preserve" listing-type="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_out, B_out, G_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.

10. 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.