SYSTEM AND METHOD FOR ENHANCING THE SHARPNESS OF A COLOUR IMAGE

Abstract: A colour image is locally discoloured in the vicinity of edges. The edges extend, for example between a superposed foreground and a background in a computer-generated image, but also between different regions in natural images which are processed, for example in a television receiver. Consequently, the edges are perceived as being sharper upon display.

Description: System and method for enhancing the sharpness of a colour image. The invention relates to an image display system, comprising: an image display device which is arranged to receive an image signal representing an input colour image and to display an output colour image which is the same as the input colour image in a general sense, - edge location signalling means for signalling a location in the input colour image where an edge is situated between two regions having a different image characteristic, which means are coupled to the image display device which is also arranged to make a colour saturation of the output colour image locally, in a vicinity of the location, different from that of the input colour image. The invention also relates to a method of forming an image for display, which method comprises the following steps: receiving an input image signal representing an input colour image, signalling an edge location in the input colour image where an edge is situated between two regions having a different image characteristic, - outputting an output image signal representing an output colour image which is the same as the input colour image in a general sense but in which a colour saturation is changed locally, in a vicinity of the location, relative to the input image. A system of this kind is known from European Patent Application No. 327 107. The cited Patent Application concerns the display of colour images with enhanced image sharpness. Generally speaking, the output colour images then remain the same as the input colour images, be it that changes are made only around selected locations. In prior art this is achieved by forming an enhancement factor by way of spatial high-pass filtering from a grey component of the input colour image. This enhancement factor is added to the input colour image. Near an edge between two regions having different grey components, the region at the "light" side of the edge thus becomes locally lighter with a comparatively less saturated colour, and the region at the "dark" side becomes locally darker with a comparatively more saturated colour. The image remains unchanged in parts of the input colour image in which no distinct edges are present. The strength of the grey component of the edge is thus exaggerated, thus creating the impression of more sharpness. It is, inter alia an object of the invention to display colour images in such a manner that they are perceived as being sharper by the human eye. This applies notably to computer images in which windows are reproduced against a background or television images on which an "on-screen display" image is superposed for control purposes, or a "Picture-in-Picture" image of two input images, but also to natural images. The image display system in accordance with the invention is characterized in that the image display device is arranged to make the colour saturation of the output image locally smaller, at both sides of the edge, than that of the input colour image. It has been found that this improves the perception of sharpness, notably in the vicinity of edges. Because the human eye perceives spatial colour variations less sharply than spatial grey variations, the presence of a colour component reduces the sharpness perceived, by reduction of the colour saturation, so that the colour component is attenuated relative to the grey component and the perception of sharpness is enhanced. An embodiment of the image display system in accordance with the invention is characterized in that the image display device is arranged to limit the output colour image to shades of grey at both sides of the edge. By removing all colour from the visible image in the vicinity of the location of the edge, the perception of sharpness is maximized. The reduction of the colour saturation preferably diminishes gradually as the distance from the location is greater; the fact that the ouφut image is a processed version of the input image thus becomes less evident. Beyond a given distance, the input image and the ouφut image are the same. The invention is notably suitable for systems in which the colour component of the image is displayed with a resolution which is lower than that of the intensity component; this is the case in most television systems. Colour decoding artefacts, occurring due to edges, are then also rendered less visible. An embodiment of the invention is characterized in that the location signalling means comprise an edge detector for detecting the edge in the image signal and for measuring the location. The use of an edge detector prevents the occurrence of artefacts in the case of gradual variations in the image characteristic. The system can thus be used for standard image signals such as television signals. An embodiment of the invention is characterized in that the edge detector is arranged to check a length of the edge and to signal the location only if the edge has at least a predetermined length and/or to check at least one of the two regions in a zone adjacent the edge for the presence of edges and to signal the location only in the absence of an edge. The restriction to edges of minimum length and edges which adjoin an edge-free region prevents the discolouring of "texture" regions with many small edges (for example, a lawn with individually visible blades of grass). The restriction to edges of minimum length, moreover, prevents discolouring in the case of short edges caused by noise. An embodiment of the invention is characterized in that the image signal represents an underlying image and a superposition image, the input colour image corresponding to the underlying image except for a part or parts in which the underlying image has been replaced by the superposition image, and that the edge location signalling means are arranged to signal the location wherever in the input colour image the underlying image changes over into the superposition image. Examples of underlying images with superposition images are computer background signals with windows superposed thereon, or television images on which an "On Screen Display" image is superposed for control purposes, or a "Picture-In-Picture" image from a different image source. Upon composition of the image it is then also known where the edge between the underlying image and the superposed image is situated. In accordance with the invention, the vicinity of this edge is reproduced with less colour saturation. The reduction of the colour saturation may be restricted to a small region along the edge; for example, for the display of windows the contents of the window can be displayed mostly with the normal colour saturation. An embodiment of the invention is characterized in that the location signalling means are arranged to signal the location wherever a colour of the image changes over from a predetermined background colour to another colour. The background colour is, for example that of a specially coloured rear wall in the input image, or that of the grass of a soccer pitch. The sharpness of the outline of persons and objects visible against this background colour is thus enhanced in a simple manner. An embodiment of the invention is characterized in that the image display device is arranged to render an intensity of the ouφut image locally lower, in a part of the vicinity of the location, than that of the input colour image. An embodiment of the invention is characterized in that the image display device is arranged to make an intensity transition, in a part of the vicinity of the location, upon crossing of the edge in the ouφut image locally larger than in the input colour image. The edge is displayed with emphasized intensity value in these two manners, whereas the colour saturation is present with less emphasis or is even absent. The perception of sharpness is thus also enhanced. The method of the invention is characterized in that at both sides of the edge the colour saturation of the ouφut image is locally rendered lower than that of the input colour image. The ouφut image thus formed can be transmitted, for example via the air or a cable system for reception and display in television receivers. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the drawings: Fig. 1 shows a first embodiment of an image display system in accordance with the invention, Fig. 2 shows image contents in a section of an image, Fig. 3 shows a second embodiment of an image display system in accordance with the invention, Fig. 4 shows an image display device for use in a system in conformity with a further aspect of the invention, and Figs. 5a and 5b show further image contents in a section of an image. Fig. 1 shows an image display system in accordance with the invention. The system comprises an input 10 which is coupled to an image display device 12. The input 10 of the image display device 12 is coupled to a colour separation unit 121 via a delay member 120. The colour separation unit 121 comprises a first and a second output. The first ouφut is coupled directly to an image control unit 122 and the second ouφut is coupled to the image control unit 122 via a controllable colour attenuator 126. The image control unit 122 is coupled to a display screen 124. The system also comprises an edge detector 14, an input of which is coupled to the input 10 of the system and an ouφut of which is coupled to a control input of the controllable colour attenuator 126. During operation the input 10 receives an image signal which represents the contents of an image line-by-line in the form of a time-dependent signal. This signal is applied, after delay by the delay member 120, to the colour separation unit 121 which splits it into an intensity component and a colour component (generally comprising two signals). The intensity component is applied directly to the image control unit 122 whereas the colour component is applied to the image control unit 122 via the controllable colour attenuator 126. The image control unit 122 controls the display screen 124 in such a manner that the represented image is displayed as a function of the location on the display screen 124. The edge detector 14 receives the image signal from the input 10, detects edges in this image signal and signals such detection at the instant at which the edge occurs in the image signal; this instant represents the location in the image signal in which the edge occurs. The edge detector can detect the occurrence of an edge in the image signal only with some delay. The delay member 120 serves to compensate for this delay and it also introduces some additional delay, so that the signalling of the edge on the control input of the attenuator 126 reaches the attenuator 126 sometime before the edge in the image signal. For the edge detector 14 a generally known edge detector can be used, for example one of the examples discussed in section 2 of an article by J. Bernsen in "Philips Journal of Research", 46 (1991), pp. 57-94: "An objective and subjective evaluation of edge detection methods in images" and the references included therein. The amplitude of the colour component represents the colour saturation of the image. The attenuator 126 attenuates this amplitude in a vicinity of the location (locations) signalled by the detector 14. The image arrives line-by-line on the input of the attenuator 126 and one line thereof contains the signalled location. In order to attenuate the amplitude in a vicinity, the colour component is attenuated during an interval from the instant at which the edge detector 14 signals the edge until sometime after the arrival of the edge in the image signal at the attenuator. The attenuation gradually increases as a function of time until it reaches a maximum at the instant at which the edge in the image signal itself arrives at the attenuator, after which the attenuation gradually diminishes again. The colour component is also attenuated in one or more lines preceding and succeeding the line in which the location is situated, be it to an extent which is less as the relevant line is situated further from the line in which the location is present. Preferably, the edge locations are signalled to reduce of the colour saturation only if they form part of an edge having at least a predetermined length and/or if no edge detections occur in a region of predetermined size on at least one side adjacent the edge. An increase of the noise is prevented by the restriction to an edge of at least predetermined length. The restriction to edges adjacent a region without edges prevents discolouring of regions of non-uniform texture in the image. Both restrictions can be realised, for example by way of software by including an elementary edge detector, for example as described in the cited publication by J. Bernsen, in the edge detector 14 and by topographically storing the detections produced thereby in an image memory. In order to check whether the edge has at least a minimum length, for example it is checked whether an edge has also been detected in locations neighbouring a location in which the elementary edge detector has detected an edge; for such a neighbouring location checking is recursively continued until an edge of minimum length has been found, the location then being signalled. Signalling is omitted if no neighbouring location with a detection is recursively found before the minimum length is reached. Alternatively, such a check can also be performed by means of known template matching techniques. In order to check whether a detection-free region occurs adjacent the edge, the direction of the edge is determined and, for example a rectangular region is chosen which extends parallel to this direction and is situated at a few pixels from the edge in a direction transversely of the edge. This region is checked for the absence of detections; if such detections are absent, the edge is signalled and otherwise it is not. Alternatively, this can also be performed by means of template matching techniques. Instead of an edge detection function as described in the cited article by J. Bernsen, the edge detector 14 can also perform a background/foreground detection. This is realised, for example by comparing the Euclidean distance between the colour and/or intensity value of the input image and a predetermined value with a threshold value. An edge location is then signalled in a location having both neighbouring locations in which this distance exceeds the threshold value and neighbouring locations in which this distance is below the threshold value (neighbouring locations may include the location itself). Fig. 2 shows different aspects of the image signal as a function of time. The Figure concerns the line of the image signal in which the location is present and shows the following signals: the intensity component 30 of the image signal, two colour components 32, 36 of the image signal, and the signal 39 signalling the location of the edge. The Figure also shows an attenuation profile 301. The edge corresponds to an intensity transition 31 in the intensity component 30. The arrival of this transition is preceded by a signalling pulse in the edge signalling signal 39. In response to this pulse the two colour components are temporarily attenuated. The attenuated colour components 34, 38 are denoted by dashed lines. At the area of the transition no colour component remains for the time being. On the lines situated adjacent the line in which the location is present the colour components are attenuated in a similar manner, be it that the attenuation is less as the lines are situated further from the line in which the location is present. In order to realise the attenuation, the colour components in the line in which the location is present are, for example multiplied by a time-dependent attenuation profile 301. The attenuation profile 301 is shaped, for example as an inverted bell or as a trapezium with first a linear increase of the attenuation, subsequently a constant attenuation and, after that a linear decrease of the attenuation. When a standard television monitor is used (resolution 625 lines per screen), the overall length of the trapezium corresponds, for example to 5 pixels with a constant level of 1 pixel before and after the location. The image control unit 122 and the display screen 124 convert these signals into a visible image which has been discoloured in a vicinity of the edge. Discolouring is, for example complete but the sharpness enhancing effect already commences in the case of partial discolouring. Furthermore, the Figs. 1 and 2 are detailed, only by way of example, in terms of an intensity component and a colour component. Evidently, the invention can be used equally well for other representations of colour information, for example RGB components. Fig. 3 shows a second image display system in accordance with the invention. It deviates from the image display system shown in Fig. 1 notably as regards the signalling of the edge. Components which correspond to Fig. 1 are denoted by the same reference numerals as used in Fig. 1. The system comprises an input 10 which is coupled to an image display device 12. The input 10 of the image display device 12 is coupled to a colour separation unit 121 via a multiplexer 128. The colour separation unit 121 comprises a first and a second ouφut. The first ouφut is coupled directly to an image control unit 122 whereas the second ouφut is connected to the image control unit 122 via a controllable colour attenuator 126. The image control unit 122 is coupled to a display screen 124. The input 10 is also coupled to an address generator 20 which comprises an address ouφut which is coupled to an address input of two image memories 22, 24. A data ouφut of one of the memories is coupled to the colour separation unit 121 via the multiplexer 128. A data ouφut of the second memory is coupled to a control input of the multiplexer 128 and to the control input of the colour attenuator 126. The address generator 20 forms successive address values from the image signal during operation, each address corresponding to the location represented by the image signal at the instant of formation of the address. In response to the address, information is read from the memories 22, 24. The information from one memory (22) represents a superposition image intended to replace locally the underlying image represented by the image signal on the input. The information from the second memory (24) indicates where the superposition image is to replace the underlying image. The information from the one memory 22 may describe, for example a window with information which is to be superposed on the underlying image, or On Screen Display information. o The information from the second memory 24 controls the multiplexer 128 in order to pass on alternately the image signal received on the input 10 and the information from the one memory 22. The information from the second memory 24 also serves to control the colour attenuator 126, so that the colour saturation is locally reduced, as described with reference to Fig. 2, before and after the alternation between the image signal received on the input 10 and the information from the one memory 22. Fig. 4 shows an image display device for use in a system in conformity with a further aspect of the invention. This device corresponds mainly to the devices shown in the Figs. 1 and 3 and corresponding components are denoted by corresponding reference numerals. The difference with respect to the Figs. 1 and 2 consists in that the first ouφut of the colour separation unit 21 is not coupled directly to an image control unit 122, but via a controllable intensity adapter 60. The control input of the intensity adapter 60 is coupled to the control input of the colour attenuator 126. During operation the intensity adapter 60 also adapts the intensity component near the signalled edges. Figs. 5a and b show two feasible adaptations of the intensity component 30. In Fig. 5a the adapted intensity component has been locally reduced to zero in a vicinity of the edge which is smaller than the vicinity of the edge in which the colour saturation is reduced. In the ouφut image this creates the impression of a black line emphasizing the edge. At the side of the edge having the highest intensity in Fig. 5b, the adapted intensity component 50 is gradually made stronger than in the original image signal as the edge is approached; at the side of the edge having the lowest intensity, the adapted intensity component 50 is gradually made less strong than in the original image signal as the edge is approached. In the ouφut image this creates the impression of an exaggerated edge which makes the edge seem sharper. Even though the invention has been described on the basis of an independent colour image, such as displayed on a television/computer monitor and on a printer, it will be apparent that the invention can also be used for moving images represented by a series of separate colour images. This is advantageous notably because in moving images an impression of artificiality, sometimes created by accentuation of the edges, is less evident. The invention can be used wherever the sharpness of images is to be automatically enhanced, for example in a computer system, in a television receiver, in a camera, in an image printer or in a distribution centre for image signals, such as a video-on- demand centre.

Claims: CLAIMS: 1. An image display system, comprising an image display device which is arranged to receive an image signal representing an input colour image and to display an ouφut colour image which is the same as the input colour image in a general sense, - edge location signalling means for signalling a location in the input colour image where an edge is situated between two regions having a different image characteristic, which means are coupled to the image display device which is also arranged to make a colour saturation of the ouφut colour image locally, in a vicinity of the location, different from that of the input colour image, characterized in that the image display device is arranged to make the colour saturation of the ouφut image locally smaller, at both sides of the edge, than that of the input colour image. 2. An image display system as claimed in Claim 1 , characterized in that the image display device is arranged to limit the ouφut colour image to shades of grey at both sides of the edge. 3. An image display system as claimed in Claim 1 or 2, characterized in that the location signalling means comprise an edge detector for detecting the edge in the image signal and for measuring the location. 4. An image display system as claimed in Claim 3, characterized in that the edge detector is arranged to check a length of the edge and to signal the location only if the edge has at least a predetermined length and/or to check at least one of the two regions in a zone adjacent the edge for the presence of edges and to signal the location only in the absence of an edge. 5. An image display system as claimed in Claim 1 or 2, characterized in that the image signal represents an underlying image and a superposition image, the input colour image corresponding to the underlying image except for a part or parts in which the underlying image has been replaced by the superposition image, and that the edge location signalling means are arranged to signal the location wherever in the input colour image the underlying image changes over into the superposition image. 6. An image display system as claimed in Claim 5, characterized in that the part or the parts in which the underlying image has been replaced by the superposition image are larger than a vicinity of the location in which the colour saturation of the ouφut image is made less than in the input colour image. 7. An image display system as claimed in Claim 1 or 2, characterized in that the location signalling means are arranged to signal the location wherever a colour of the image changes over from a predetermined background colour to another colour. 8. An image display system as claimed in any one of the Claims 1 to 7, characterized in that the image display device is arranged to render an intensity of the ouφut image locally lower, in a part of the vicinity of the location than that of the input colour image. 9. An image display system as claimed in any one of the Claims 1 to 7, characterized in that the image display device is arranged to make an intensity transition, in a part of the vicinity near the location, upon crossing of the edge in the ouφut image locally larger than in the input colour image. 10. A method of forming an image for display, which method comprises the following steps: receiving an input image signal representing an input colour image, signalling an edge location in the input colour image where an edge is situated between two regions having a different image characteristic, - ouφutting an ouφut image signal representing an ouφut colour image which is the same as the input colour image in a general sense but in which a colour saturation is changed locally, in a vicinity of the location, relative to the input image, characterized in that at both sides of the edge the colour saturation of the output image is locally rendered lower than that of the input colour image.

Inventors: DE LANGE ALPHONSIUS ANTHONIUS, DE LANGE, ALPHONSIUS, ANTHONIUS, JOZEF
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1995
Main classification: H04N1
IPC codes: H04N9