SIMULTANEOUSLY DISPLAYING A GRAPHIC IMAGE AND A VIDEO IMAGE

Abstract: A method and arrangement for simultaneously displaying a graphic image (23) (e.g. a control menu of a television receiver or video recorder) and a video image (22) is disclosed. In order to achieve optimal legibility of the menu without fully obscuring the underlying video signal, the graphic image is accommodated within a window (21) and the window is given an adequate colour (24) which is additively mixed with the video signal. The mixing factor is preferably controlled in response to the amount of spatial and/or temporal activity of the video signal within the window.

Description: Simultaneously displaying a graphic image and a video image. FIELD OF THE INVENTION The invention relates to a method and arrangement for simultaneously displaying a graphic image and a video image, the graphic image being accommodated within a window. The invention also relates to a television receiver, a video recorder and a multimedia station comprising such an arrangement. BACKGROUND OF THE INVENTION A prior-art method of displaying a graphic image within a window of a video image is disclosed in UK Patent Application GB-A-2 242 594. To improve the visibility of the graphic image, the contrast of the video image is reduced within the window. OBJECT AND SUMMARY OF THE INVENTION It is an object of the invention to further improve the prior-art arrangement. In accordance with the invention, the method is characterized by generating a background colour signal for said window and mixing said background colour signal and the video signal representing said video image. It is thereby achieved that the window can be given a colour contrasting with the graphic image colour while maintaining the display of the video image within the window. The visibility of the graphic image is further improved by giving the window a background colour. More particularly, the legibility of text constituting a menu-on-screen is considerably improved, moreover, the appearance of menus-on-screen is considerably enhanced. The window resembles a coloured, partly transparent, piece of glass between the graphic image and the video image. As described in the patent application mentioned above, the step of generating a background colour signal is known per se. However, the video image is then not displayed within the window. The graphic image may have a perfect visibility, but a part of the video image is lost and cannot be observed. It is further known from international patent application WO 94/21080 to additively mix a graphic image with a video signal. 2 However, it is not desirable that the video signal remains visible through the graphic image itself, e.g. through the characters of a menu. To prevent the mixed signal within the window from assuming a too large amplitude, the video signal and the background colour signal may be given weighting factors, for example α and 1-α, respectively The larger the value of a, the better the underlying video signal remains visible. The weighting factor a will therefore also be referred to as "transparency" of the window. Experiments have shown that the legibility of on-screen-menus is substantially affected by the amount of activity in the video image. More particularly, video images having fine picture details with a high contrast (e g text) substantially interfere with graphic menus Said interference occurs in particular when the video image is static In a preferred embodiment of the invention, the weighting factors are therefore controlled in response to the amount of activity in the video image within the window The amount of activity in the graphic image may be taken into account as well so as to determine when interference between video and graphic image details is most annoying. It has also turned out that menus in television receivers have a different legibility for different categories (drama, documentaries, sports) of television programs. In a further embodiment of the invention, the weighting factors are therefore controlled in response to said categories. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter BRIEF DESCRIPTION OF THE DRAWINGS Fig. l shows a television receiver comprising an arrangement for simultaneously displaying a graphic image and a video image m accordance with the invention Fig.2 shows an example of a screen display. Fig.3 shows a further embodiment of the receiver in accordance with the invention. Fig.4 shows an embodiment of an activity measuring circuit which is shown in Fig.4 Fig.5 shows an example ot a relation between the amount of mixing and video image activity Fig 6 shows a further embodiment of the receiver in accordance with the invention DESCRIPTION OF EMBODIMENTS Fig 1 shows a television receiver comprising an arrangement for simultaneously displaying a graphic image and a video image in accordance with the invention The receiver comprises in further known manner an input 1 for receiving a composite video signal CVBS, a video processing circuit 2 receiving the composite video signal for generating a displayable output signal V, a display screen 3, a microprocessor 4 for controllmg the operation of the receiver, a remote control receiver 5, and a remote control transmitter 6 The receiver further compπses a graphic signal generator 7 which is controlled by the microprocessor and adapted to generate a graphic image representing, for example, an on-screen control menu The graphic signal generator 7 generates a graphic signal G, a fast blanking signal FB, and a window signal WIN An example of such a graphic signal generator is Philips integrated circuit SAA5240 When the fast blanking signal FB is active, the graphic signal defines a graphic image to be displayed on a screen (not shown) in terms of one or more foreground colours When the fast blanking signal is inactive, the graphic signal has been programmed to have a background colour The window signal WIN defines a window within which the graphic image is to be displayed Obviously, the window may have any shape The receiver further comprises mixing means 8 for mixing the graphic signal G and the video signal V In the embodiment shown, a first multiplier 81, a second multiplier 82, and an adder 83 form an additively mixed signal M =αV + ( l-α)G Generally, the graphic signal G and video signal V consist of three (red, green, blue) components The mixing means 8 are then provided for each component Switches 9 and 10 are provided for selecting either the graphics signal G, the video signal V or the additively mixed signal M More particularly, switch 9 is in the position shown when the fast blanking signal FB is active, and switch 10 is in the position shown when the window signal WIN is active Thus, the video signal V is selected for display when the window signal is inactive, the graphic signal G is selected when the fast blanking signal and the window signal are both active, and the mixed signal M is selected when the window signal is active and the fast blanking signal is inactive Fig.2 shows an example of a screen display. Outside the window 21, a video image 22 is displayed. Within the window, a graphic image 23 is displayed, using one or more foreground colours. As explained above, the graphic signal has a background colour when the fast blanking signal is inactive. Thus, the part 24 of the window which is not occupied by the graphic image 23 displays a mixture of said background colour and the video image. If the background colour is green, for example, it is as if a green glass is held between the graphic image and the video image. By programming a background colour having a sufficient contrast with the foreground colours, the menu has an excellent legibility, while the video signal remains visible through the window. The transparency of the glass is determined by the weighting factor α. The value thereof can be determined experimentally. It should be noted that the television receiver shown in Fig. l need not necessarily include the display screen 3. The display screen may be external to the receiver, which is the case if the receiver constitutes, for example, a video recorder or a personal computer. In the latter case, the video signal V may even be a locally generated virtual video signal. Fig.3 shows a further embodiment of the receiver. This receiver further comprises a circuit 11 for measuring an amount of image activity in the video image. A signal A k indicative of local video image activity is applied to the microprocessor 4 which, in response thereto, controls the value of weighting factors a and 1-α. The amount of spatial activity in a video image can be determined by subjecting the image, or at least the luminance component thereof, to a picture transform. Fig.4 shows an embodiment of activity measuring circuit 11. A block forming circuit 111 divides the image into blocks of, for example, 8*8 pixels. The blocks are subjected to a picture transform 112 such as the Discrete Cosine Transform or the less complicated Hadamard transform. Picture transform is well-known in the field of image coding. Each block of pixels is transformed into transform coefficients y υ in which index i (here i = O..7) denotes a horizontal spatial frequency and index j (j =0..7) denotes a vertical spatial frequency. Each transform coefficient indicates to what extent the corresponding spatial frequencies are present in the block of pixels. The coefficients y are applied to an evaluation circuit 113 which calculates the sum, possibly a weighted sum, of coefficients for those spatial frequencies that interfere most annoyingly with the graphic image. For example, if the spatial frequencies corresponding to i > 5 and j > 5 are interfering frequencies, the calculation circuit calculates a local activity A k of the current block k in accordance with the expression: y ±ό In an averaging circuit 114 the average activity A of all blocks within the window (21 in Fig.2) is calculated in accordance with the expression: N -v k wherein Ν is the number of blocks within the window. In order to determine this number, the circuit 114 receives the window signal WIN from graphic signal generator 6. The activity value A is applied to microprocessor 4 which, in response thereto, generates the weighting factors a and 1-α applied to the mixing means 8. An example of the relation between the weighting factors and activity is shown in Fig.5. Weighting factor a decreases (and 1-α increases accordingly) as activity A increases. Thus, if the video image within the window comprises a significant amount of detail, the "glass transparency" is reduced so as to render the graphic image within the window better visible. Further embodiments of the activity measuring circuit can easily be designed by those skilled in the art. For example, the transform coefficients y (J (circuit 113 in Fig.4) may also be evaluated by microprocessor 4. As the microprocessor has generated the graphic image and thus knows the spatial frequencies of said graphic image, the processor can be arranged to evaluate only the coefficients that actually interfere with the graphic image. The processor may also control the glass transparency on a block-by-block basis. Fig.6 shows a further embodiment of the receiver. The receiver now comprises a circuit 12 for receiving and decoding a program category code C. The transmission of program category codes which classify a television program in one of a plurality of categories such as drama, sports, documentaries, etc. is known in the art. For example, the category code may be part of the PDC (Program Delivery Control) data which is embedded in a teletext signal in the vertical blanking interval of the composite video signal CVBS. The code C is applied to the microprocessor 4 which comprises a table defining the weighting factor α for each category. Quiet television programs such as text-oriented educational courses are given a smaller value of α (i.e. less transparency) than highly dynamic programs such as sports events and action movies which affect the legibility of a menu to a lesser degree. It should be noted that the processor may also be adapted to cause the graphic signal generator to modify the background colour in accordance with image activity or program category. In summary, a method and arrangement for simultaneously displaying a graphic image (e.g. a control menu of a television receiver or video recorder) and a video image is disclosed. In order to achieve optimal legibility of the menu without fully obscuring the underlying video signal, the graphic image is accommodated within a window and the window is given an adequate colour which is additively mixed with the video signal. The mixing factor is preferably controlled in response to the amount of spatial and/or temporal activity of the video signal within the window.

Claims: Claims 1. A method of simultaneously displaying a graphic image (23) and a video image (22), the graphic image being accommodated within a window (21), characterized by generating a background colour signal for said window and mixing said background colour signal and the video signal representing said video image. 2. A method as claimed in Claim 1 , wherein the background colour signal and the video signal are given respective weighting factors (α, 1-α). 3. A method as claimed in Claim 2, wherein the weighting factors are adaptively controlled in response to the amount of activity (A) in the video image within said window. 4. A method as claimed in Claim 3, wherein the weighting factors are further adaptively controlled in response to the amount of activity in the graphic image. 5. A method as claimed in Claim 2, wherein the video image is a television program of a given category (C), the weighting factors being controlled in response to said category. 6. An arrangement for displaying a graphic image and a video image, comprising a graphic generator (7) for generating said graphic image accommodated within a window, characterized by means (4,7) for generating a background colour signal for said window and means (8) for mixing said background colour signal and the video signal representing said video image. 7. An arrangement as claimed in Claim 6, wherein the background colour signal and the video signal are given respective weighting factors (α, 1-α). 8 An arrangement as claimed in Claim 7, further comprising means (11) for adaptively controlling the weighting factors in response to the amount of activity in the video image within said window. 9 An arrangement as claimed m Claim 8, wherein the weighting factors are further adaptively controlled in response to the amount of activity in the graphic image. 1 0. An arrangement as claimed in Claim 7, wherein the video image is a television program of a given category, the arrangement further comprising means ( 12) for controlling the weighting factors in response to said category. H A television receiver including an arrangement as claimed in Claim 6. 12. A video recorder including an arrangement as claimed in Claim 6. 13. A multimedia station including an arrangement as claimed in Claim 6.

Inventors: BROUWER-JANSE MAGDALENA DINGEN, TANG HOK KONG, VAN GELDEREN TEDDE, WESTERINK JOANNE HENRIETTE DES, WESTRIK REMKO, BROUWER-JANSE, MAGDALENA, DINGENA, TANG, HOK, KONG, VAN GELDEREN, TEDDE, WESTERINK, JOANNE, HENRIETTE, DESIREE, MONIQUE, WESTRIK, REMKO
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1997
Main classification: H04N5
IPC codes: H04N5, H04N9