GB2258782A

GB2258782A - Television signal gamma correction

Info

Publication number: GB2258782A
Application number: GB9214305A
Authority: GB
Inventors: Byung Ee An
Original assignee: Samsung Electronics Co Ltd
Current assignee: Samsung Electronics Co Ltd
Priority date: 1991-07-30
Filing date: 1992-07-06
Publication date: 1993-02-17
Anticipated expiration: 2012-07-06
Also published as: GB9214305D0; GB2258782B; JPH05211659A

Abstract

An input video signal is analysed S1 to determine in which of three magnitude ranges its magnitude falls. Different arithmetic operations S2, S3, S4 are carried out on the video signal in dependence on the magnitude range into which the input signal falls. This results in a reduction of processing time in comparison with conventional look-up table techniques. <IMAGE>

Description

METHOD AND APPARATUS FOR GAMMA CORRECTON DESCRIPTION The present invention relates to a method and apparatus for gamma correction.

The display tube (CRT) of a television does not have a linear relationship between the input signal and the light intensity produced. Light output is related to signal input by a power law, the Greek letter #(gamma) being used to denote the power, that is: Light output a V.

in This non-linearity in television display tubes is conventionally overcome by processing the television signal according to an inverse function before transmission.

Figure 2 illustrates a conventional gamma correction circuit. R (red), G (green) and B (blue) signals, which have undergone 8-bit analogue-to-digital conversion, are received through switches SW1 to SW3.

Gamma corrected data is stored in three RAM arrays, each having a capacity of 256 x 8 bits.

These RAM arrays are called "Look-up Tables". Referring to Figure 1, each RAM array has 256 addresses because the R, G and B signals consists of 8-bit signals.

Thus, in order to represent 256 patterns corresponding to the characteristics of the input signals, the mentioned memory structure is provided.

Gamma corrected signal data is read from a RAM array by designating a particular memory address using the R, G or B data as the address. The RAM array is filled with gamma corrected signal data and the gamma corrected signal data is output by reading the address corresponding to the input signal data, under the control of a central processing unit CPU. The above described conventional gamma correction circuit requires the use of a digital picture processing ASIC (Application Specific Integrated Circuit), including a RAM array, during the blanking period after a control signal is generated by a controller.

Furthermore, a microcomputer or a central processing unit is required, and in addition, the central processing unit and the digital picture processing chip are separate from each other. Consequently, the hardware for interfacing two ICs is complex. It is also difficult to know the relationship between an input signal and the gamma corrected signal which means that the correct data must be established by trial and error during the development of an ASIC chip. This trial and error process involves repeatedly storing gamma corrected signal data. Consequently, much time is consumed in storing the gamma corrected signal data in the RAM array.

An aim of the present invention is to overcome the above-described disadvantages of conventional techniques.

According to a first aspect of the present invention, there is provided, a gamma correction apparatus, comprising: determining means for determining into which of a plurality of magnitude ranges a video signal falls; and processing means responsive to an output from the determining means for performing an arithmetic operation on said video signal in dependence on the magnitude range into which said video signal falls.

According to a second aspect of the present invention, there is provided a method of real time gamma correction, comprising the step of: (a) processing a video signal according to an arithmetic function, selected from a plurality of functions in dependence on the magnitude of said video signal.

Preferably, the method includes the step of: (b) determining into which of a plurality of magnitude ranges the magnitude of said video signal falls, wherein step (b) is performed before step (a) and the function selected in step (a) is selected in dependence on the result of step (b).

Preferably, the video signal is an R, G or B signal.

Preferably, there are three magnitude ranges, said magnitude ranges being a low range, a middle range and a high range.

Preferably, the video signal is processed according to the arithmetic functions Y = ax, Y = bx + c and Y = dx + e when the magnitude of said video signal is in the low range, the middle range and the high range respectively, where Y is the magnitude of the resultant gamma corrected signal, x is the magnitude of said video signal and a, b, c, d and e are constants.

Preferably, 4 5 a < 6, 1 S b S 1.5, 20 5 c S 25, 0.55 S d 5 0.50 and 41 5 e 5 45.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic view of a look-up table used in the conventional gamma correction circuit; Figure 2 illustrates a conventional gamma correction circuit; Figures 3A and 3B are respectively first and second level block diagrams of a gamma correction circuit using a real time digital picture processor according to the present invention; Figures 4A and 4B illustrate waveforms showing the input/output transmission characteristics of a circuit according to the present invention; and Figure 5 is a flow chart showing a gamma correction method using a real time digital picture processor according to the present invention.

Referring to Figures 3A and 3B, a comparator 100 and a switching part 200 are connected to input terminals IN for R, G and B signals (to be called "X" hereafter).

The comparator 100 compares the magnitudes of the values X with the magnitudes of threshold values A and B under the assumption that the value X is 100% when the final video signal is LOIRE. The switching part 200 switches over a switch SW4 so that gamma corrections are carried out according to the output from the comparator 100. In the above, it is assumed that 5 s A s 7 and 28 S B s 32.

A gamma correction circuit 300 for carrying out gamma correction is connected to the switching part 200.

The gamma correction circuit 300 includes a first gamma correcting part 310 for carrying out gamma correction based on an arithmetic formula Y = ax when the input value X is smaller than the above mentioned threshold value A, a second gamma correcting part 320 for carrying out gamma corrections based on an arithmetic formula Y = bx + c when the input value X is between the threshold values A and B, and a third gamma correcting part 330 for carrying out gamma corrections based on an arithmetic formula Y = dx + e when the input value X is larger than the threshold value B.

Referring to Figure 5, the gamma correction method includes a step L1 for comparing the magnitudes of the input R, G and B signals with the magnitudes of the threshold values A and B under the assumption that the R, G and B signals are 100% when the resultant video signals are 100 IRE and a step L2 for carrying out gamma corrections according to the comparison result of step L1.

The comparator 100 compares the input values X with the threshold values (referred to as "A" and "B" hereinafter) under the assumption that the value X is 100% when the final video signal is lOOIRE, and then the switch SW4 of the switching part 200 switches in response to the results of the comparisons.

That is, after the above described comparison, if the value X is smaller than the value A, the switch SW4 is switched over to a terminal P1, so that a gamma correction is carried out based on the formula Y = ax.

If the value X is an intermediate value between the values A and B, the switch SW4 is switched over to a terminal P2, so that gamma correction is carried out based on the formula Y = bx + c. If the value X is larger than the value B, the switch SW4 is switched over to a terminal P3, so that gamma correction is carried out based on the formula Y = dx + e.

In the above, relations 4 S a S 6, 1 5 b S 1.5, 20 5 c < 25, 0.5 5 d < 0.59, and 41 5 e s 45 are experimental results which give effective gamma correction.

To describe the procedure of gamma correction in more detail referring to Figure 5, different subroutines are carried out for the different values of X using respective formulas.

That is, after comparing the value X with the values A and B (S1), in the case of X < A, gamma correction is carried out by a subroutine based on the arithmetic formula Y = ax (S2), while in the case of A S X < B, gamma correction is carried out by a subroutine based on the arithmetic formula Y = bx + c (S3). Finally, in case of B < X, gamma correction is carried out by a subroutine based on the arithmetic formula Y = dx + e (S4).

The gamma corrections which are carried out in the above described manner have waveforms having the input/output transmission characteristics as shown in Figure 4A.

Now the process of gamma correction will be described exemplarily based on the above arithmetic formulas. If gamma corrections are carried out under the assumption of A = 6, B = 30, a = 5, b = 5/4, c = 22.5, d = 4/7 and e = 300/7, the input/output transmission characteristic is as shown in Figure 4B when gamma corrections are carried out on such a basis, a first range is processed according to the relationship Y = 5x, a second range is processed according to the relationship Y = (5/4)x + 22.5, and a third range is processed according to the relationship Y = (4/7)x + (300/7).

The value E is a threshold value of 100, under the assumption that the input value X is 100% when the final video signal is lO0IRE, as in the case of A and B.

As a result, the magnitudes of the input R, G and B signals are compared with the magnitudes of the threshold values A and B under the assumption that the values of the R, G and B signals are 100% when the final video signals are lOOIRE. The switch SW4 of the switching part 200 is switched according to the results of such comparisons and as a result the gamma correction circuit 300 carries out gamma corrections based on the respective arithmetic formulas according to the magnitude of the input R, G or B signals.

The embodiment of the present invention, described above, uses a high speed real time digital picture processor. Using such a processor, the input is divided into a plurality of ranges, and subroutines are set up for the respective ranges, thereby carrying out gamma correction. Consequently, no separate hardware is required, so that its circuit is simplified, and the gamma corrections can be carried out by means of software or firmware, resulting in a simplified procedure and a reduction in processing time compared with the case of using a conventional RAM array look-up table.

Claims

1. A gamma correction circuit using a real time digital picture processor, comprising: comparing means connected to input terminals of R, G and B signals, for comparing magnitudes of said R, G and B signals input through said input terminals with mangitudes of counterpart values A and B under the assumption that the values of said R, G and B signals are 100% when the final video signals are 100 IRE; switching means connected to said input terminals for carrying out switchings in such a manner as to perform gamma corrections according to outputs from said comparing means; and a gamma correction circuit means connected to said switching means, for carrying out gamma corrections according to connection state of said switching means.

2. The gamma correction circuit as claimed in claim 1, wherein said gamma correction circuit means comprises: a first gamma correcting means for carrying out gamma corrections based on an arithmethic formula Y "Y = ax (value of R, G or B signals)", in case that the value of the input R, G or B signals is smaller than said counterpart value A; a second gamma correcting means for carrying out gamma corrections based on an arithmetic formula Y "Y = bx (value of R, G or B signals) + c" in case that said value of said R, G or B signals is located between said counterpart signals A and B; and a third gamma correcting means for carrying out gamma corrections based on an arithmetic formula Y "Y = dx (value of said R, G or B signals) + e" in case that said value of said R, G or B signals is larger than said counterpart value B.

3. The gamma correction circuit as claimed in claim 1, wherein ranges of the magnitudes of said counterpart values are 5 5 A < 7 and 28 5 B S 32.

4. The gamma correction circuit as claimed in claim 2, wherein range of the magnitude of "a" in said arithmetic formula is 4 5 a 5 6.

5. The gamma correction circuit as claimed in claim 2, wherein ranges of the magnitudes of "b" and "c" in said arithmetic formulas are 1 < b < 1.5 and 20 c c 5 25, respectively.

6. The gamma correction circuit as claimed in claim 2, wherein ranges of the magnitudes of "d" and "e" in said arithmetic formulas are 0.55 5 d 5 1.5 and 20 5 c S 25, respectively.

7. A gamma correction method using a real time digital picture processor, comprising the steps of: comparing magnitudes of R, G and B signals with the magnitudes of counterpart values A and B under the assumption that the values of said R, G and B signals are 100% when the final video signals are 100IRE; and carrying out gamma corrections according to the compared results of said comparing step.

8. The gamma correction method as claimed in claim 7, wherein said gamma correcting step comprises the steps of: carrying out gamma corrections based on an arithmetic formula Y "Y = ax (the value of said R, G or B signals)" in case that the values of said input R, G or B signals are found to be smaller than the value of said counterpart value A as a result of the comparison as said comparing step; carrying out gamma corrections based on an arithmetic formula Y "y = bx (the value of said R, G or B signals) + c" in case that the values of said input R, G or B signals are found to be located between said counterpart values A and B as a result of the comparison at said comparing step; and carrying out gamma corrections based on an arithmetic formula Y "Y = dx (the value of said R, G and B signals) + e" in case that the values of said input R, G or B signals are found to be larger than said counterpart value B as a result of the comparison at said comparing step.

9. The gamma correction method as claimed in claim 8, wherein ranges of the magnitudes of said counterpart values A and B are 5 5 A 5 7 and 28 5 B 5 32, respectively.

10. The gamma correction method as claimed in claim 8, wherein range of the value of "a" in said formula is 4 5 a 5 6, respectively.

11. The gamma correction method as claimed in claim 8, wherein ranges of the values "b" and "c" in said formula are 1 5 b S 1.5 and 20 d c S 25, respectively.

12. The gamma correction method as claimed in claim 8, wherein ranges of the values "d" and "e" are 0.55 < d 5 0.59 and 41 5 e 5 45, respectively.

13. A gamma correction apparatus, comprising: determining means for determining to which of a plurality of magnitude ranges a video signal falls; and processing means responsive to an output from the determining means for performing an arithmetic operation on said video signal in dependence on the magnitude range into which said video signal falls.

14. An apparatus according to claim 13, wherein the video signal is an R, G or B signal.

15. An apparatus according to claim 13 or 14, wherein there are three magnitude ranges, said magnitude ranges being a low range, a middle range and a high range.

16. An apparatus according to claim 15, wherein, if the magnitude of said video signal is within the low range, the processing means performs the arithmetic operation Y = ax, where a is a constant, x is the magnitude of said video signal and Y is the magnitude of the resultant gamma corrected signal.

17. An apparatus according to claim 15 or 16, wherein, if the magnitude of said video signal is within the middle range, the processing means performs the arithmetic operation Y = bx + c, where b and c are constants, x is the magnitude of said video signal and Y is the magnitude of the resultant gamma corrected signal.

18. An apparatus according to claim 15, 16 or 17, wherein, if the magnitude of said video signal is within the high range the processing means performs the arithmetic operation Y = dx + e, wherein x is the magnitude of said video signal, Y is the magnitude of the resultant gamma corrected signal and d and e are constants.

19. An apparatus according to any one of claims 14 to 18, wherein the middle range is defined by first and second threshold values.

20. An apparatus according to claim 19, wherein the first threshold value is greater than or equal to 5% of peak x and less than or equal to 7% of peak x.

21. An apparatus according to claim 19 or 20, wherein the second threshold value is greater than or equal to 28% of peak x and less than or equal to 32% of peak x.

22. An apparatus according to claim 16, wherein 4 S a > 6.

23. An apparatus according to claim 17, wherein 1 S b S 1.5 and 20 5 c s 25.

24. An apparatus according to claim 18, wherein 0.55 S d 2 0.59 and 41 5 e 5 45.

25. A method of real time gamma correction, comprising the step of: (a) processing a video signal according to an arithmetic function selected from a plurality of functions in dependence on the magnitude of said video signal.

26. A method according to claim 25, including the step of: (b) determining into which of a plurality of magnitude ranges the magnitude of said video signal falls, wherein step (b) is performed before step (a) and the function selected in step (a) is selected in dependence on the result of step (b).

27. A method according to claim 26, wherein the video signal is an R, G or B signal.

28. A method according to claim 25 or 26, wherein there are three magnitude ranges, said magnitude ranges being a low range, a middle range and a high range.

29. A method according to claim 28, wherein if the magnitude of the video signal is within the low range, processing the video signal according to the arithmetic function Y = ax, where a is a constant, x is the magnitude of said video signal and Y is the magnitude of the resultant gamma corrected signal.

30. A method according to claim 28 or 29, wherein, if the magnitude of the R, G or B signal is within the middle range, the R, G or B signal is processed according to the arithmetic function Y = bx + c, where b and c are constants, x is the magnitude of said R, G or B signal and Y is the magnitude of the resultant gamma corrected signal.

31. A method according to claim 28, 29 or 30, wherein, if the magnitude of the R, G or B signal is within the high range, the R, G or B signal is processed according to the arithmetic function Y = dx + e, where x is the magnitude of said R, G or B signal, Y is the magnitude of the resultant gamma corrected signal and d and e are constants.

32. A method according to any one of claims 28 to 31, wherein the middle range is defined by first and second threshold values.

33. A method according to claim 32, wherein the first threshold value is greater than or equal to 5% of peak x and less than or equal to 7% of peak x.

34. A method according to claim 32 or 33, wherein the second threshold value is greater than or equal to 28% of peak x and less than or equal 32% of peak x.

35. A method according to claim 29, wherein 4 5 a < 6.

36. A method according to claim 30, wherein 1 < b # 1.5 and 20 5 c S 25.

37. A method according to claim 31, wherein 0.55 5 d 5 0.59 and 41 5 e 5 45.

38. A gamma correction apparatus substantially as hereinbefore defined with reference to Figures 3A, 3B, 4A, 4B and 5 of the accompanying drawings.

39. A method of real time gamma correction substantially as hereinbefore described with reference to Figures 3A, 3B, 4A, 4B and 5 of the accompanying drawings.