DIGITAL CORRECTION OF LINEAR APPROXIMATION OF GAMMA

Abstract: A circuit for gamma correcting a video signal incorporates both linear approximation (16) as well as a look-up table (12'), in which the disadvantages of linear approximation, i.e., inaccurate representation of the gamma function, and of a gamma look-up table, i.e., low resolution at low grey levels, are minimized. The transfer function of the look-up table (12') is chosen such that the inaccuracies of the linear approximation are compensated.

Description: Digital correction of linear approximation of gamma. BACKGROUND OF THE INVENTION Field of The Invention The subject invention relates to the display of color video and data graphic signals, and more particularly, to displaying color video and data graphic signals on a linear display. All video and data graphic sources assume that the information will be displayed on a CRT-like device, i.e., a display device with a non-linear luminance output/electrical input function. The assumed function is the so-called gamma function, where V OUT = IM 7 , where γ = 2.2-2.8. However, various flat displays, including plasma display panels (PDP's), have a linear response, i.e., γ = 1.0. If this discrepancy is not corrected, the displayed picture has a pale appearance with little color saturation. Description of The Related Art Typically, digital video signal processing is carried out using 8 bits. As shown in Fig. 1, after analog-to-digital conversion in A/D converter 10, a gamma look-up table 12 can be used to interface between the 8-bit processing and an 8-bit PDP 14. Here, the look-up table 12 uses a list of 256 8-bit values for each color, each of these values being corrected with respect to gamma. The most common way of realizing the look-up table 12 is by using a (P)ROM or a RAM. In the case of a 10-bit PDP, U.S. Patent Application Serial No. 09/191,842, filed November 13, 1998 (Attorney Docket No. PHA 23,515), assigned to the Assignee of this application, discloses a circuit for converting an 8-bit input video signal into a 10-bit gamma corrected output video signal for application to a PDP. Fig. 2 shows a graph of a gamma curve (solid line), where gamma (γ) = 2.2. As can be seen, the curve is very flat at the beginning, at low grey levels. This means that a large number of input codes are mapped onto a few output codes resulting in a loss in resolution. Alternatively, it is known to use linear approximation to correct for gamma in a video signal. As shown in Fig. 3, the output from the A/D converter 10 is applied to a linear approximation circuit 16 which then applies the gamma corrected video signal to the linear - WO Oα/38412 2 PCT/EP99/09598 display 14. Samsung Electronics markets a color decoder containing a programmable gamma correction unit KS0127 which allows a user to apply many different types of corrections. Depending on the type of correction needed, only a limited number of values are stored in the unit, and the unit uses linear interpolation to generate the in between values. Fig. 2 also shows a graph of a linear approximation curve (dotted line) in which 4 linear segments are used. However, the results have not proven to be acceptable, in that errors occur where the linear approximation curve deviates from the (ideal) gamma curve. SUMMARY OF THE INVENTION It is an object of the present invention to provide a circuit for gamma correcting an input video signal in which greater resolution at low grey level is achieved. It is a further object of the present invention to provide a circuit for gamma correcting an input video signal in which inaccuracies due to linear approximation of an ideal gamma function are minimized. To these ends, the invention provides a method and a circuit as defined in the independent claims. An advantageous embodiment is defined in the dependent claim. Applicant realized that by combining linear approximation with a gamma lookup table, the disadvantages of both approaches could be minimized. In particular, the transfer function of the look-up table is chosen such that it corrects for any differences between the linear approximation and the gamma function. As such, this transfer function exhibits an initial rapid rise thereby improving the resolution of dark gray levels, i.e., less codes are lost when compared to a gamma look-up table. BRIEF DESCRIPTION OF THE DRAWINGS With the above and additional objects and advantages in mind as will hereinafter appear, the invention will be described with reference to the accompanying drawings, in which: Fig. 1 shows a block schematic diagram of prior art gamma correction using a look-up table; Fig. 2 shows a graph of a gamma function in a solid line, and a linear approximation thereof in dotted lines; Fig. 3 shows a block schematic diagram of a prior art gamma correction using linear approximation; Fig. 4 shows a block schematic diagram of a gamma correction circuit of the subject invention using a combination of a look-up table and linear approximation; Fig. 5 shows a graph of the transfer function of the look-up table of Fig. 4; and Fig. 6 shows a graph of the initial portion of the transfer functions of the look- up tables of Fig. 1 (solid line) and of Fig. 4 (dashed line). DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 4 shows a block diagram of an embodiment of the subject invention. In particular, an input video signal is applied to the A/D converter 10. The output of the A/D converter 10 is applied to a look-up table 12'. The output from the look-up table 12' is then applied to the linear approximation circuit 16 which applies the gamma corrected video signal to the linear display 14. The transfer function of the look-up table 12' is chosen such that it compensates for the inaccuracies of the linear approximation circuit 16. Fig. 5 shows the transfer function of the look-up table 12' when the linear approximation circuit 16 uses 4 linear segments. The first 64 bytes of the transfer function of the look-up table 12' are shown in Fig. 6 as a dotted line. This should be contrasted with the transfer function of the look-up table 12 of Fig. 1, shown as a solid line. It should be apparent that the resolution of low (dark) grey levels is better, i.e., less codes are lost, as compared to a standard gamma look-up table. It should be noted that the order of the look-up table 12' and the linear approximation circuit 16 may be reversed. In addition, if the linear display 14 requires a 10- bit signal, a look-up table in accordance with U.S. Patent Application Serial No. 09/191,842, filed November 13, 1998 (Attorney Docket No. PHA 23,515) may be used in which the transfer function is adjusted accordingly. While the present embodiment only shows a single look-up table 12' and a single linear approximation circuit 16 implying a monochrome video signal, it should be apparent that the subject invention is equally applicable to color video signals, wherein separate look-up tables and separate linear approximation circuits are used for the three color •signals, or, alternatively, for the luminance signal and two chrominance signals. Numerous alterations and modifications of the structure herein disclosed will present themselves to those skilled in the art. However, it is to be understood that the above described embodiment is for purposes of illustration only and not to be construed as a limitation of the invention. All such modifications which do not depart from the invention are intended to be included within the scope of the appended claims. In the claims, any reference signs placed oetween parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of other elements or steps than those listed in a claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention can be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means can be embodied by one and the same item of hardware.

Claims: CLAIMS: 1. A circuit for gamma correcting an input video signal for application to a linear display, said circuit comprising: input means for receiving the input video signal; means (10) for digitizing the input video signal; linear approximation means (16) for linearly approximating a gamma function using a predetermined number of linear segments; and look-up table means (12') containing gamma table values, wherein a transfer function of the look-up table corrects differences between said gamma function and said linear approximation. A circuit as claimed in claim 1, wherein said predetermined number of linear segments is 4. 3. A method of gamma correcting an input video signal for application to a linear display, the method comprising the steps of: receiving the input video signal; digitizing (10) the input video signal; linearly approximating (16) a gamma function using a predetermined number of linear segments; and wherein a transfer function of a gamma look-up table corrects differences between said gamma function and said linear approximation.

Inventors: VAN MOURIK JOHANNES G R, VAN MOURIK, JOHANNES, G., R.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1999
Main classification: H04N5
IPC codes: G09G3, H04N5, H04N9