METHOD FOR DIFFERENTIATING BETWEEN BACKGROUND AND FOREGROUND OF SCENERY AND ALSO METHOD FOR REPLACING A BACKGROUND IN IMAGES OF A SCENERY

Abstract

The present invention relates to a method for differentiating between background and foreground in images or films of scenery recorded by an electronic camera. The invention relates in addition to a method for replacing the background in recorded images or films of scenery whilst maintaining the foreground.

Claims

1. Method for differentiating between background and foreground of a scenery in images recorded by an electronic camera, the background showing an arbitrary image coded with a coding such that the image is visible for a direct observer of the background, and the background being differentiated from the non-coded foreground in a signal produced by an image sensor, recording the images, of the camera by means of the coding.

2. Method according to the preceding claim, wherein the coding comprises the background showing the image as a periodic sequence of components composing the image, at least one imaging parameter which influences the display of the image being respectively changed in the course of each period, and an exposure of the image sensor of the camera being effected synchronously with the display of a specific component.

3. Method according to the preceding claim, wherein a temporal averaging of the coded image over at least one period corresponds to the image.

4. Method according to one of the two preceding claims, wherein a period duration of a recording cycle of the image sensor is less than or equal to a period duration of the periodic sequence of the display of the components.

5. Method according to one of the preceding claims, the coding comprising the background displaying periodically alternating different components of the image, the components of respectively each period composing the complete image, the display of the components being synchronised with an exposure of the camera such that the camera records only one of the components in at least one, preferably each period.

6. Method according to the preceding claim, wherein the component recorded by the camera is chosen such that it allows a differentiation of the foreground from the background by means of keying, preferably by means of chroma-keying, and in that the background is differentiated from the foreground in the images recorded by the image sensor by means of keying, preferably by means of chroma-keying.

7. Method according to the preceding claim, wherein the component of one period recorded by the image sensor displays increased a colour component essentially not occurring in the foreground and the other components display this colour component correspondingly reduced such that the components together compose the colours of the image.

8. Method according to one of the claims 2 to 7, wherein two components being shown alternating with a frequency which is twice as high as an exposure frequency of the camera.

9. Method according to one of the preceding claims, characterised in that the coding of the image comprises the background displaying periodically alternating patterns which complement each other within one period to form the complete image and in that the background is differentiated from the foreground in the signal of the image sensor by means of the periodically alternating pattern.

10. Method according to the preceding claim, wherein an exposure frequency of the image sensor and the display of the pattern is synchronised with each other and/or a Fourier transform is implemented in the signal of the image sensor and the background is differentiated from the foreground by means of the frequency of the alternating display of the patterns.

11. Method according to one of the two preceding claims, wherein the pattern exists in a mathematically distinguishable structure, preferably being periodic in a scanning direction of the image sensor, the pattern preferably being a chessboard pattern.

12. Method according to one of the claims 9 to 11, each of the patterns of one period is a binary pattern which is completely light-permeable or self-illuminating in regions and being less light permeable or less permeable for light of one colour in regions or impermeable for the corresponding light or illuminating more weakly or not at all in the corresponding colour.

13. Method according to claim 1, wherein the coding comprises the background displaying the image only with colours selected from one, preferably at least two, ranges of the visible spectrum at a spacing from each other, and light emanating from the background running through a colour filter, preferably an interference filter, before impinging on the image sensor, which colour filter filters out or tones down these ranges of the visible spectrum, and letting other ranges of the visible spectrum pass through without being toned down.

14. Method according to claim 1 or 13, characterised in that the coding comprises the image being represented in a grid of coloured dots on a backdrop, the coloured dots respectively together with the backdrop surrounding them composing the colour of the image, and a colour filter being disposed in front of the image sensor, which colour filter tones down the colours of the coloured dots and preferably lets through the colours of the backdrop without being toned down, the background being differentiated from the foreground by means of keying, preferably chroma-keying, on the colour of the backdrop.

15. Method according to the preceding claim, wherein the colours of the coloured dots and the colours of the backdrop form disjoint parts of the visible spectrum, preferably the light of the coloured dots running through at least one spectral filter, preferably interference filter, which the light emitted from the backdrop does not run through, and in that the filter disposed in front of the image sensor being a spectral filter, preferably interference filter, which is complementary to the interference filter or filters in front of the coloured dots.

16. Method according to one of the two preceding claims, wherein the coloured dots are self-illuminating, preferably light diodes.

17. Method according to claim 1 or 13, wherein the coding comprises the background radiating only colours selected from at least one, preferably at least two, ranges of the visible spectrum at a spacing from each other in the spectrum, the light emanating from the scenery running through a colour filter before impinging on the image sensor, which colour filter tones down those ranges of the visible spectrum from which the colours of the image are selected, and letting through ranges of the visible spectrum situated between these ranges without being toned down, the background being differentiated from the foreground by keying onto the ranges of the spectrum toned down by the colour filter in front of the image sensor.

18. Method according to the preceding claim, wherein an interference filter is disposed in the scenery behind the foreground and in front of the image, which interference filter tones down those ranges of the visible spectrum which are not those ranges from which the colours of the background are selected and wherein the colour filter which the light runs through before impinging on the image sensor is an interference filter which is complementary to the mentioned interference filter.

19. Method according to claim 1, wherein the coding comprises the background radiating, letting through or reflecting electromagnetic radiation of at least one non-visible spectral range and a conversion device being disposed in front of the image sensor, which converts the non-visible-electromagnetic radiation into visible light which is detected by the image sensor.

20. Method according to claim 19, wherein the conversion device is planar and converts, at least on a part of its surface, the non-visible radiation which is radiated, let through or reflected by the background into visible light which is detected by the image sensor and the background being differentiated from the foreground by keying on the converted light.

21. Method according to the preceding claim, wherein the filter has dots of fluorescent or phosphorescent material distributed over its surface, preferably disposed in a grid, which dots illuminate in specific wavelengths by being excited by the non-visible radiation, and also a device, preferably a lens, assigned to each of the dots in order to image the corresponding fluorescent or phosphorescent dots onto the image sensor.

22. Method according to claim 1, wherein the coding comprises the background radiating or reflecting polarised light only of one specific polarisation direction or polarisation rotation, the light emanating from the scenery running through a colour filter and a polarisation filter before impinging on the image sensor, which filter is orientated such that it filters out light of that polarisation which is radiated and reflected from the background and differentiation of the background from the foreground being effected by means of keying on darker regions of the image recorded by the image sensor which have no colour tone of the colour filter, the light emanating from the scenery preferably firstly running through the colour filter and subsequently the polarisation filter.

23. Method according to claim 19, wherein the electromagnetic radiation emanating from the scenery runs through the conversion device before impinging on the image sensor, preferably after running through an imaging lens system of the camera, which conversion device converts the non-visible electromagnetic radiation into visible light and blends it into the scenery from the perspective of the camera at the location of the background.

24. Method according to the preceding claim, wherein the conversion device has a beam splitter which deflects a part of the radiation and/or the non-visible electromagnetic radiation towards a converter which blends an image corresponding to the background into the beam path in front of the image sensor, preferably via a partially permeable mirror, by means of the radiated electromagnetic radiation.

25. Method according to one of the preceding claims, wherein the coding of the image comprises an oscillation over the image sequence and the oscillation being taken into account in the differentiation of the foreground from the background.

26. Method according to one of the preceding claims, wherein the foreground is reconstructed in the image signal after differentiation of the background and foreground.

27. Method according to one of the preceding claims, wherein the image is located in the image of the image sensor with the help of camera tracking and/or the camera tracking assists the differentiation of the background and foreground.

28. Method for replacing a background in an image of a scenery, the background is differentiated from a foreground by means of a method according to one of the preceding claims and the background being replaced by a different background.

29. Electronic display system having at least one display and also at least one camera, a method according to one of the preceding claims being able to be implemented with the electronic display system.

Description

[0055] There are shown:

[0056] FIG. 1 a construction for implementing the method according to the invention according to a first embodiment of the present invention;

[0057] FIG. 2 a construction for implementing the method according to the invention according to a second embodiment of the present invention;

[0058] FIG. 3 a construction for implementing the method according to the invention according to a third embodiment of the present invention;

[0059] FIG. 4 a construction for implementing the method according to the invention according to a fourth embodiment of the present invention;

[0060] FIG. 5 a construction for implementing the method according to the invention according to a fifth embodiment of the present invention;

[0061] FIG. 6 a construction for implementing the method according to the invention according to a sixth embodiment of the present invention;

[0062] FIG. 7 a construction for implementing the method according to the invention according to a seventh embodiment of the present invention; and

[0063] FIG. 8 two spectra of two mutually complementary interference filters as schematic representation.

[0064] FIG. 1 shows a device for implementing the method according to the invention according to a first embodiment of the invention. Scenery with a foreground 2 and a background 3 is filmed by means of a camera 1. The camera 1 has an image sensor, not shown, which produces an image signal 4 which is supplied to a device for image analysis and/or image processing 5. The camera 1 can have for example an imaging lens system 1a and a camera housing 1b in which the image sensor is disposed.

[0065] The foreground 2 is a simple cube in the illustrated example. In all embodiments, the method according to the invention can however be implemented with any foregrounds 2, in particular for example with sportspersons of a sporting event. The background 3 can likewise be of any kind as long as it allows a coding as is required for implementing the invention. For example, the background can be a studio background or pitch-perimeter advertising in a stadium.

[0066] In the example shown in FIG. 1, the background 3 shows an image. Periodically alternating different components of respectively one period of the image are thereby represented, the components composing the complete image. The display of the components of the image at times t.sub.1, t.sub.2, t3 t.sub.8, . . . is now synchronised with the exposure of the camera such that the camera records, in at least one period, preferably in each period, only a specific one of the components ti, t3, t5, . . . which is chosen such that it allows a differentiation between foreground and background in the image signal by means of keying. The keying can be for example colour-based keying, preferably chrome-keying. The analysis device 5 then differentiates the background from the foreground in the images 4 recorded by the image sensor of the camera 1.

[0067] In the illustrated example, the image of the background can be divided for example into two images, the first image comprising an optionally reduced colour component from the original image of the background and the second image the complementary colours so that the original image is produced in the combination of both images. Both images can then be shown alternating in such a high frequency that they are no longer individually perceptible to an observer (e.g. 100 Hz). The recording camera 1 can then operate synchronously with half the frequency (e.g. 50 Hz) and with a reduced exposure time (e.g. 1/100 sec.) so that only the first of the two images with the chosen colour component is recorded by it. The method can be produced here also passively with a reflecting background. For this purpose, an actuatable LCD colour filter can be disposed in front of the background, which filter lets through respectively the corresponding component of the image. In the active case in which the background is self-illuminating (for example as LCD display), the background can be actuated specifically in order to show correspondingly the components of the image.

[0068] FIG. 2 shows a further advantageous setup for implementing the method according to the invention. The background for the coding hereby shows periodically alternating patterns which complement each other within one period to form the complete image. If all the patterns of one period are therefore regarded together, then the complete image is produced.

[0069] The scenery comprising background 3 and foreground 2 is recorded by the image sensor of the camera 1 and the image signal 4 is analysed by means of an analysing device 5. In the image recorded by the image sensor, the background 3 can now be differentiated from the foreground 2 by means of the periodically alternating pattern. For this purpose, a Fourier transform can be implemented for example in the signal 4 of the image sensor and the background 3 can be differentiated from the foreground 2 by means of the frequency of the alternating display of the patterns. The image sensor can hereby be scanned for example in lines and the analysis can be implemented directly in the scan signal. The periodically alternating pattern produces a frequency component in the Fourier transform with the alternating frequency wherever the background is imaged.

[0070] The analysis can be effected both over time by means of analysis of successive images and within one image. Hence, the corresponding pattern in the image analysis of the camera 1 can be sought. The analysis can be assisted by information from a camera tracking. For example, information from the camera tracking can help in the determination of the pattern size.

[0071] Different patterns are possible. In the illustrated case, the background 3 shows a chessboard pattern with alternating black squares and squares which show the cut-out from the image at the location of the corresponding square. The chessboard pattern and the chessboard pattern complementary hereto, in the case of which the black squares and squares showing the image are exchanged relative to the actual chessboard pattern, is shown here alternating.

[0072] The frequency of the oscillation is chosen so high here that it cannot be perceived by the observer (e.g. 50 Hz). The frequency can also be coupled to the image frequency of the camera.

[0073] The pattern of the background can be produced passively, for example by an actuatable filter, such as an LCD filter. It can also be configured actively as a self-illuminating background, for example as LED display.

[0074] FIG. 3 shows a further possible arrangement for implementing the method according to the invention. For the coding, here the image is represented by means of coloured dots 6 on a backdrop 7. The coloured dots 6 on the backdrop 7 are preferably arranged in a grid. The coloured dots 6, respectively together with the backdrop surrounding them, represent the colour of the image at the corresponding location. The backdrop can also be black so that the complete contribution to the colour of the image by the coloured dots is provided. A colour filter 8 is now disposed in front of the image sensor of the camera 1, preferably in front of the lens 1a of the camera 1, which coloured filter tones down or filters out precisely the colours radiated from the coloured dots. Preferably, the colour of the backdrop 7 of the background 3 is chosen such that it does not occur or only to a small extent in the foreground 2. In the image signal 4 produced by the image sensor of the camera 1, the foreground 2 can then be differentiated from the background 3, in the analysing device 5, by means of keying onto the colour tone of the backdrop 7. The backdrop 7 forms here therefore the colour to be analysed (or a pattern) for production of a mask for the keying.

[0075] Preferably, the dots are disposed such that the grid does not appear to be grid-shaped for a direct observer at a specific minimum distance. This minimum distance can be provided for example by the normal location of observers in the scenery itself, in the case of pitch-perimeter advertising in a stadium for example by the distance of the nearest seats from the pitch-perimeter advertising. If then the coloured dots 6 are disposed in a grid on a specially coloured or black backdrop 7 at a small spacing relative to each other, the actual image is visible for the observer. Furthermore, the grid should also be chosen to be so narrow that the grid cannot be detected by the camera from the position thereof at a specific minimum distance. Both for the camera and for an observer in the scenery, the desired coloured impression is then produced from a corresponding distance relative to the background.

[0076] The coloured dots now radiate only specific wavelengths from the visible range of light which are filtered out precisely by the colour filter 8 in front of the image sensor so that only the colour of the backdrop essentially remains left on the image sensor.

[0077] The solution with interference filters, which is shown in FIG. 3 and dealt with later in more detail with reference to FIG. 8, can be achieved particularly favourably. An interference filter which lets through only a part of the frequencies of the visible spectrum is hereby disposed in front of each coloured dot 6. The part thereby preferably has a plurality of regions at a spacing from each other which are distributed over the visible spectrum such that the colours of the image can be composed therefrom. The colour filter 8 is then an interference filter which is complementary to the colour filters in front of the coloured dots 6, which interference filter filters out or tones down precisely those colours which are let through by the colour filters in front of the coloured dots 6 without being toned down. The coloured dots 6 are preferably self-illuminating, i.e. for example light bulbs or LEDs.

[0078] FIG. 4 shows a further possible embodiment for implementing the method according to the invention. There is disposed hereby, between the background 3 and the foreground 2, a colour filter 9 which radiates only colours selected from at least one, preferably two, ranges of visible light which are at a spacing from each other in the spectrum. In front of the image sensor or the lens system 1a of the camera 1, there is then disposed a further colour filter 8 through which light emanating from the scenery runs en route to the image sensor. This colour filter 8 filters out or tones down precisely those ranges of the spectrum which are let through by the colour filter 9. As a result, on the image sensor the background appears toned down or black relative to the foreground 2 which radiates in particular light of those frequency ranges which are let through by the colour filter 8. The dark regions therefore form a mask in the image signal 4 of the image sensor, with which mask keying onto the dark regions and hence to the background can be implemented. In this way, the image analysing device 5 can differentiate the background 3 from the foreground 2.

[0079] In this embodiment, the filters 8 and 9 are also preferably interference filters which are complementary to each other, i.e. are permeable for different, preferably non-overlapping ranges of the visible spectrum.

[0080] The colour filter 9 can also be dispensed with if the background 3 radiates per se only light of specific frequency ranges which are filtered out by the filter 8. Such a self-illuminating background can be produced for example with LEDs which radiate a defined spectrum. The colour filter 8 is then designed such that it tones down or filters out precisely the frequencies radiated by the LEDs.

[0081] The filter 9 can be chosen such that the colour regions of the visible spectrum which are let through by it suffice to display the colours occurring in the background. Correspondingly, the colour filter 8 can also be chosen such that the frequencies let through by it suffice to display the colours occurring in the foreground 2. In this way, the image produced by the image sensor can be used further without correction. However, a correction of the colours of the foreground is always possible in the image recorded by the image sensor which corrects possible colour deviations because of the filter 8.

[0082] FIG. 5 shows a further construction for implementing the method according to the invention. For the coding, the background radiates non-visible radiation 10 which can for example be ultraviolet radiation or infrared radiation. A planar conversion device 11 is now disposed in front of the image sensor or in front of the lens 1a of the camera 1, which conversion device lets through radiation impinging on it in regions and has elements 15 in regions which are for example fluorescent or phosphorescent, by means of which the non-visible radiation 10 is converted into visible light 13. The elements 15 can thereby be disposed uniformly and in particular in a grid over a surface of the device 11 so that visible light 12 can pass through in every region, on the one hand, and, on the other hand, the non-visible radiation 10 can be converted into visible light. All radiation impinging on the image sensor runs through the device 11. On the image sensor, the light passing unchanged through the device is therefore represented, on the one hand, and, on the other hand, the light 13 produced by the conversion of the non-visible radiation 10. Since the non-visible radiation 10 emanates from the background 3, the image sensor of the camera 1 records the light 13 precisely where the background 3 is imaged. Preferably, the colour of the light 13 produced by the device 11 is chosen such that it can be differentiated from the colours occurring in the foreground 2. The produced light 13 then forms a mask, by means of which the background 3 can be differentiated from the foreground 2 by a keying process.

[0083] Provided that the elements 15 for the conversion of non-visible radiation into visible light 13 radiate undirected light 13, a lens 14 can be assigned to each of the elements 15, which lens is disposed such that it represents the unconverted light 13 from the perspective of the image sensor in the background 3. Preferably, screening can also be provided for each element, which screening lets light pass through only in the direction of the optical beam path.

[0084] FIG. 6 shows a further possible, arrangement for implementing the method according to the invention. The background 3 hereby radiates polarised light 16 of a specific polarisation direction. A colour filter 17 and also a polarisation filter 18 is now disposed in front of the image sensor of the camera 1, the passage direction of which is perpendicular to the polarisation direction of the light 16 radiated from the background. When using circular polarisation, the passage direction of rotation would be complementary to the polarisation rotation. Preferably, light emanating from the scenery firstly runs though the colour filter 17 and then the polarisation filter 18 before it impinges on the image sensor. In the image recorded by the image sensor, the background appears dark, without colour tone, in this arrangement. For direct observers of the scenery, the background 3, in contrast, appears in its normal colours since these observers can perceive the polarised light 16. A colour tone, which the foreground 2 has in the image produced by the image sensor of the camera 1 because of the colour filter 17, can subsequently be compensated for in the image signal 4 by the computer.

[0085] In this embodiment of the method, the dark image components without colour tone, i.e. those image components resulting from the background 3, form a mask, by means of which keying can be implemented in order to differentiate the background 3 from the foreground 2.

[0086] FIG. 7 shows a further arrangement for implementing the method according to the invention. The background 3 hereby radiates non-visible radiation 10, for example ultraviolet radiation, or infrared radiation in addition to the actual image. In front of the image sensor, preferably in the direction of the beam path behind the imaging lens system 1a of the camera 1, a conversion device 19 is disposed in this embodiment, by means of which a visible image of the background 3 can be produced from the non-visible light. The device 19 thereby has a beam splitter 20 and a semipermeable mirror 21 which are disposed in succession in the beam path of the light impinging on the image sensor from the scenery. The beam splitter 20 deflects the non-visible light at least partially onto an image sensor 22 of the conversion device 19. A converter 24 records the image produced by the image sensor 22 from the non-visible radiation 10 and produces, on an image display 23, an image of visible light which corresponds to the image recorded by the image sensor and therefore shows precisely the pattern which is preset by the non-visible radiation 10. The image produced by the image display is then blended into the beam path of the light impinging on the image sensor of the camera 1 from the scenery by means of the semipermeable mirror 21 and serves in the image signal produced by the image sensor of the camera 1 as pattern for the production of a keying mask, by means of which the foreground 2 can be differentiated from the background 3 by keying.

[0087] FIG. 8 shows, in the upper and lower part, spectra of two mutually complementary interference filters in a schematic representation. The frequency of the light is plotted on the horizontal axis and is intended to span essentially the visible range in the illustrated example. The respective intensity which light which has passed through the corresponding interference filter has is plotted on the vertical axis if the interference filter is radiated with white light which comprises all frequencies of the illustrated range with the same intensity. It can be detected that the interference filters let a plurality of ranges of visible light at a spacing in the spectrum pass through. The regions of the passing light can thereby be chosen in both interference filters such that all required colours of the background 3 or of the foreground 2 can be composed from them. Both illustrated interference filters are mutually complementary which means that those regions of the spectrum which are let through by the interference filter shown in the upper part are filtered out precisely by the interference filter shown in the lower part, whilst those frequencies which are filtered out by the upper interference filter are let through precisely by the lower interference filter. Reference may be made to the fact that the filters need not completely filter out the corresponding colour components in order to make the method according to the invention possible. Toning down is also adequate. Furthermore, it is also not required that the frequency ranges are separated from each other so clearly, as is represented schematically in FIG. 8. A certain degree of overlapping of the ranges let through by the different filters is permissible.