FR2464606A1 - IMAGE CUTTING SYSTEM FOR COLOR TELEVISION - Google Patents

Abstract

CHROMA KEY SYSTEM PERFECTED TO OBTAIN SPECIAL EFFECTS IN TELEVISION, SUCH AS REPLACING SELECTED PARTS OF A CLOSE-UP IMAGE WITH A BACKGROUND IMAGE, AND TO PREVENT GENERATION OF THE CHROMA KEY SIGNAL WHEN BASIC COLOR SIGNALS ARE SENSITIVELY EQUAL IN AMPLITUDE. THIS SYSTEM INCLUDES A VIDEO 2 SIGNAL SOURCE, A MATRIX 20 CONNECTED TO THIS SOURCE AND WHICH GENERATES COLOR DIFFERENCE SIGNALS, A COLOR SELECTOR 30 SENSITIVE TO COLOR DIFFERENCE SIGNALS IN ORDER TO PRODUCE A CHROMA KEY DECOUPAGE SIGNAL WHICH A TINT OR RANGE OF TINTS THAT CAN BE SELECTED IN THE VIDEO SIGNALS, A COMPARATOR 25 SENSITIVE TO THE FUNDAMENTAL COLOR SIGNALS TO PRODUCE AN INHIBITION SIGNAL APPLIED TO THE SELECTOR 30 WHEN THE FUNDAMENTAL COLOR SIGNALS ARE SENSITIVELY EQUAL, A PROCESS MIRE 60 AND A VIDEO MIXER 10. APPLICATION TO TELEVISION CAMERAS.

Description

The present invention relates to an electronic circuit system

  designed to produce special effects in color television,

  and more particularly to systems for replacing

  selected from a close-up image by a background or background image when desired. Systems are called "chroma key". There are already various systems for cutting selected portions of a close-up image, such as may be formed by a close-up television camera, and for replacing the thus extracted portions with a rear image. background or background, formed for example by a television camera for backgrounds. Known examples of such systems are described in U.S. Patent No. 3,778,452, entitled "Blue Screen Traveling Matte System" granted to LC Hanseman on December 11, 1973, as well as in US Pat. 3,560,638 entitled "Chroma Keying System Utilizing Remote Controlled Chroma Keyer" (Chroma Keying System Using a Chroma Key Device Controlled Remotely), granted to

  Ole Skrydstrup and John D. Ross on February 2, 1971.

  The Hanseman patent is a typical example of a caching system

  in which the close-up and background signals are

  algebraically, for example by using networks

  suppressors, to produce the complex signal of colors. In the

  particular position described in this Hanseman patent, one obtains a

  a color difference graph by subtracting the sum of the red and green components of the close-up signal from twice the value of the blue component. The close-up signal is selectively deleted by a network

  which has the effect of subtracting the component from the blue of the total signal.

  The components of the background signal are introduced into multi-

  pliers where they are selectively deleted by the signal of

color.

  The Skrydstrup patent also discloses a cutting system

  image in which the output is effectively switched between the

  plane and the background in order to generate the complete color signal.

  In one embodiment of this invention, the uncoded outputs

  of the camera are stamped to form (luminance) signals

  respectively red (R), blue (B) and yellow (J). These signals are passed through multipliers whose control terminals include a DC input proportional to the sine and cosine of a hue applied to these multipliers. The outputs of the multipliers are added to obtain the switching signal Vk = (RJ) sinG +

  (B-J) cose. The clipping signal is then delayed to the extent

  to link it to the correct coded camera signals for the background and the close-up. The clipping signal is then amplified and clipped before using it to switch between close-up and background cameras. A clipping command is provided to set a threshold below which it

  no clipping signal occurs.

  The optimum setting of the clipping control depends on the

  ambient lighting of the television production studio. By

  example, in a studio with a blue background where lighting is

  gal, it may be necessary to adjust the clipping control close to

  mite of the zero, so that all the blue parts of the scene engender

  a clipping signal. Thus, it would be possible to obtain that the system

  chroma key performs cutting from monochrome parts of -

  close-up signal. These monochrome parts contain extra areas

  dark and other extremely bright areas due to

specular flexures.

  A chroma key handling system according to the present invention

  There are some improvements which prevent the generation of a clipping signal in response to the presence of the monochrome portions of the camera signal: foreground. More particularly, the red-green-blue (RGB) signals are compared with each other and when they are equal, the prohibition or inhibition of the clipping signal occurs. This

  The system is based on the well-known fact that during the mono-

  chromed color television signals the RGB signals are equal

between them.

  Thus, the present invention aims to provide a chroma key system made so that it can not generate a clipping signal under the influence of monochrome parts of the camera signal

close-up

  However, the invention, both from the point of view of its organization and its method of operation, as well as other advantages and characteristics that are specific to it, will be better understood if one

  refers to the following description and the accompanying drawings o the same

  numbers and reference signs or symbols refer to similar elements. In the drawings FIG. 1 shows in block diagram form the application of the present invention to a video switching system; FIGURE 2 is a vector diagram of the color circle showing the color difference vectors;

  FIGURE 3 is a detailed block diagram of the present

  vention; FIGURE 4 is a diagram relating to the comparator 25 of Figure 3; FIGURE 5 is a graphical representation of the voltage at point A in FIG. 4, and FIGURE 6 is a diagram of the function generator 200 of FIG.

Figure 3

  If we first refer to Figure 1, where we have

  in the form of a synoptic diagram a chroma key system to which the present invention has been applied, it is seen that video or image signals

  corresponding to a background or background image in the course of televi-

  are generated from an appropriate source represented only

  in simplified form and referred to as video source 1;

  in addition, video or image signals corresponding to a gross image

  current television plan are generated from another suitable source shown only in simplified form and designated as the video source 2. The image signals generated in the source 1 are full encoded image signals that the it is applied to a device 10 constituted for example by a mixer stage which serves to combine

  limeal way of the distinct input signals, in the proportion

  read to produce an output signal. The image signals coming out of the

  2 are a plurality of signals, i.e. complete coded picture signals, which are applied to the same mixing device 10, as well as signals of the fundamental colors red (R), green (V), and blue (B) close-up image. The video source 2 may include a decoder for transforming the coded picture signals into signals of the fundamental colors mentioned above. A source of

  backup or spare image signals (not shown) may be

  view to provide additional coded or color-coded signals

  doms. Such a source may be a video recorder or other similar device.

246 4606

  Each of the fundamental color signals is applied to a

  device 20 constituted, for example, by a resistive matrix which transmits

  forms the signals of the three fundamental colors into three fundamental color signals and a luminance signal (J). These signals are then applied to another device 30 which in fact is a chroma key chopper, designed to generate a chroma clipping signal and which may include remote controlled means for selecting a color represented by the chroma key. chroma key cutting. Means provided within the signal generator 30

  chroma key decoders use the aforementioned signals of color

  the luminance signal to generate two of the three

  their well-known fundamentals: (B-J), (R-J) and (V-J). Of course,

  the signals generated depend on the input signals. In the pre-mode

  As can be seen from Figure 1, the matrix 20 produces the R, B and J signals. Therefore, the color difference signals produced by the chroma key switch generator 30 are (BJ) and (RJ) . Experts in this field know that one can

  also use the difference signals (R-J) and (V-J). The general

  chroma key 30 provides the subject difference signals with mixers whose control terminals have a DC input signal proportional to the sine and cosine of a hue voltage applied to these devices. The

  outputs of these multipliers are added together to produce the

  chroma key: Vk = (R-J) sin8 + (B-J) cos9.

  This chroma key signal generator is well known to specialists in

  lists in the art to provide a signal to alternate input selection of the switching system in accordance with a color

  selected, and is generally operated by means 40 for selecting

  code that receives the chroma key clipping signal through c. the switching device 50. The chroma key is then applied to a device 60, for example a target processor, to process the chroma key and other effects.

  conventional devices applied to this device via the circuit 70.

  Outputs "Command A" and "Control B" from Processor 60, obtained according to current practice, are applied to the mixer

  to control the combination of video signals or background image

  plan and close-up generated by the video sources 1 and 2, so as to form an output signal which is displayed on the output device 15.

  The system described above differs from existing systems,

  if it is set out below, in that it constitutes an improved system

  born chroma key cutting. A device 25, connected to receive the RGB signals, is a three-level comparator. This comparator produces an inhibitory signal when the RGB signals have substantially identical amplitudes. The inhibitory signal is subtracted from the chopper signal and thereby inhibits the chroma key system to prevent it from generating a chopper signal for

  monochrome scene in close-up.

  The present invention takes advantage of the well-known principle of

  which, when coding monochrome television signals, is

  according to the color standards of N. T. S. C., the amplitudes

  respective individual signals R, V and B are substantially equal.

  The principles of color coding according to N. T. S. C. are set forth in the publication entitled "Color Television", Vol. 1, "Principles and Practice", Carnt, P.S. and Townsend, G.B., Iliffe Books Ltd. London:

  1961, pp.78-116, an article to which reference should be made for a better

  their understanding of the invention.

  For a maximum white signal, there are

their followers:

R = V = B = 1

J = 0.30 (1) + 0.59

B-J = 1-1 = 0

R-J = 1-1 = 0

(1) + 0.11 (1) = 1

  In a similar way, for a gray signal, there are the following color signals:

R = V = B = 0.2

  o: J = 0.30 (0.2) + 0.59 (0.2) = 0.11 (0.2) = 0.2 therefore: BJ = 0.2-0.2 = 0 and : RJ = 0.2 - 0.2 = 0 The above derivation can be carried out for all shades of gray, up to black, where R = V = B = 0, with the same results. Therefore, all monochrome colors

  would be located at the origin of the vector diagram of Figure 2.

  This diagram illustrates the well-known fact that any color can be of o: therefore: and: s defined on a circle of colors in terms of the difference vectors

between two colors (R-J) and (B-j).

  Reference should now be made to Figure 3 which shows in more detail the new aspects of the present invention. As seen above, the non-coded RGB signals from the video source 2 are applied to the matrix 20 in which they are transformed into R, B and J signals. These signals are combined in the multipliers 100 and 110. to produce the well-known signals of RJ and BJ color differences. These multipliers may be of a commercially available model, such as MC1496. These multipliers are controlled by a function generator 200. The products of the multipliers 100 and 110 are the (R-J) sino and (B-J) sino signals. The (RJ) sine and (BJ) cose signals are linearly added in the adder 120 to thereby obtain the chroma key chopping signal which is in the following form: V = (RJ) cose + (BJ) sinG k which constitutes the equation of a hue carried on the circle of colors of Figure 2. This principle is known from the aforementioned US Pat. No. 3,560,638 issued to Ole Skrydstrup et al. February 2, 1971, to which reference should be made. The chopping signal is applied to a device 130 which may be an amplifier having both non-inverting and inverting inputs. In this case, the clipping signal

  is applied to the non-inverting input, while an inhibit signal

  from the comparator 25 is applied to the inverting input.

  The inhibition signal is produced as a result of the comparison of

  RBVs. When R = B = V, an inhibit signal is generated and subtracted from the chopper signal by the amplifier 130 to produce a zero and thereby override the switching function of the chopper.

chroma key.

  If a chopping signal is generated, it is applied to the

  positive retarder 140 which is necessary to compensate for delays in

  encoder that normally occur in television cameras.

  The clipping signal thus delayed is then applied to a multiplication

  controller 160 controlled by the gain. The amplifier 170 amplifies and limits

  the clipping signal at the value of about one volt.

  An embodiment of the comparator 25 is shown schematically

  Figure 4. The RGB signals received from the video source 2 are the input signals of this comparator 25. The red signal (R) is applied to the base of the NPN transistor 320, the green signal (V) is applied to the signal. base of the NPN transistor 310, and the blue signal (B) is applied to the base of the NPN transistor 300. These transistors 300, 310 and 320 are selected so that they have very close operating characteristics. The collector of the transistor 300 is connected through a resistor 301 to a suitable positive supply source + V. Collectors of

  transistors 310 and 320 are connected in a similar manner via

  respective resistors 311 and 321, at the same positive current source + V. Resistors 301, 311 and 321 have the same value. The emitter of each transistor 300, 310 and 320 is connected to the current source 305

  connected in turn to an appropriate negative voltage supply -V.

  The collector of the transistor 300 is connected to the base of the NPN transistor

  350. The collectors of the transistors 310 and 320 are connected respectively

  base of the NPN transistors 340 and 330. These transistors

  330, 340 and 350 are chosen so that they have characteristics of

  very close operation. The collector of each transistor

  330, 340 and 350 is connected directly to the voltage source + V.

  The emitter of each transistor 330, 340 and 350 is connected via

  diode at a point designated at A on the circuit of Figure 4, and from there to the source of negative voltage -V. This point A is connected to the base of the PNP transistor 360 whose collector is connected to the source

  positive voltage + V across a diode and a resistor. The collection

  transistor 360 is connected to the negative voltage source -V through a resistor. The output or inhibition signal is

  taken from the collector of the transistor 360.

  The circuit described above works as a comparator to

  three levels. Whereas such circuits are well known to specialists in

  lists in this area, it is not considered useful to give a descrip-

  detailed description. When the RGB signals are equal, the voltage at the point

  A approaches its maximum negative value. Figure 5 is a diagram

  setting the voltage at point A while two of the input signals are held constant; for example, R = V, while the signal B varies between 0.3 volts above and 0.3 volts below the levels of both

other signals.

  When the RGB signals are equal, the transistors 300, 310 and 320 draw the same amount of current from the source 305. Therefore, the base excitation of the transistors 330, 340 and 350 is equal and the voltage at point A found at its maximum negative value. However, if the level of one of the signals were to increase, one of the transistors 300, 310 or 320 would draw more current than the other two. This state of imbalance would have the effect of increasing the voltage at point A according to the curve of Figure 5. The transistor 360 reverses the

  signal at point A before it is applied to the amplifier 130.

  The case described above is simplified for the sake of clarity.

  More complex situations may exist and are effectively

  in the actual operation of the system. However, the overall result is the same: when R = V = B, the voltage at point A (Figure 4) reaches its maximum negative value -and when these RGB signals differ by more than a few tenths of a volt, the voltage at point A

  rises to a minimum negative value.

  Figure 6 shows the diagram of an embodiment of the general

  200. Such circuits are well known in the art and,

  therefore, only a brief description will be given here. The

  Sine and cosine voltages developed by function generator 200

  command the multipliers 100 and 110. The cosine voltage is de-

  developed by the transistors 400 and 405, and by the amplifier

  410. This amplifier 410 may be of the type MC1458 or else

  be appropriate operational amplifier. When the potentiometer 190

  the color control is grounded, the transistor 400 is conductive

  and transistor 405 is turned off. The amplifier 410 behaves

  then as a non-inverting amplifier with unity gain. Lors-

  when the hue control potentiometer 190 is set to its average position, the transistor 405 begins to turn on and lowers the voltage at the non-inverting input of the amplifier 410. This makes the amplifier 410 operate as an amplifier. that amplifies to non-inversion with gain of unit. The sinus voltage is generated by the transistors

  415, 425, 430 and 435, as well as the amplifier 420. When the

  tintometer 190 of the hue is to ground, the base of the tran-

  sistor 425 is also and the base of the transistor 430 is about

  -3.3 volts. Similarly, the transistor 415 is saturated and the voltage at the

  non-inverting tree of amplifier 420 is close to ground.

  As the voltage from the control potentiometer 190

  the hue increases, the voltage at the non-inverting input of the ampli-

  controller 420 tracks the voltage at the base of transistor 425 until the

  transistor 415 ceases to be saturated. The amplifier 420 operates manually

  holding as an inverting amplifier and unity gain, until the transistor 435 is saturated. When this saturation occurs, the voltage at the non-inverting input of the amplifier 420 again follows the voltage of the base of the transistor 425. The outputs of the function generator change their direction rather abruptly, which gives an output triangular. However, the tips are softened by an overexcitation

  control terminals of the multipliers 100 and 110.

  In summary, a chroma key cutting system has been described above which prevents the production of a chroma key signal when the RGB signals are equal. This has the effect of preventing chroma key cutouts

  on monochrome parts of camera signals of coarse color

  plan. These monochrome parts are mainly due to lighting

unequal and reflections.

  Although a preferred mode of reproduction has been described and

  It is obvious to any skilled technician in this field that many variations and modifications can be made without departing from the basic principles of the invention taken in their broadest aspects. Therefore, these variants and possible modifications will have to be considered as remaining in the

framework of the invention.

Claims (23)

  1. An apparatus for switching a plurality of video signals so as to generate a video signal which produces a complex image according to a selected color in one of the video signals, this apparatus   being characterized in that it comprises -   a) a source (2) of a plurality of color image signals, including signals of fundamental colors; b) a resistive matrix (20) connected to said video source (2) and adapted to develop color difference signals; c) a color selector (30) responsive to said color difference signals for developing an electrical control signal   which represents a hue or range of hues that can be selected   in one of said video signals, and d) a comparator (25) responsive to said fundamental color signals for producing an inhibition signal when said signals   of fundamental colors are substantially equal, this signal of inhibition   being applied to said color selector to prevent the production of   said electrical control signal.   2. Apparatus according to Claim 1, characterized in that the   comparator (25) is of the three-level video type.   Apparatus according to Claim 1, characterized in that said   color picker includes means for adding the-   said electrical control signal and said inhibition signal.   An apparatus for switching a plurality of video signals to provide a video signal capable of generating a complete image according to a selected color in one of the video signals, which apparatus is characterized by comprising: a) a source ( 2) multiple color video signals, including fundamental color signals; b) a resistive matrix (20) connected to said video source and adapted to develop color difference signals; c) a color selector (30) responsive to said color difference signals for producing a chroma key clipping signal   which represents a hue or range of hues that can be selected   in one of said video signals;   d) a comparator (25) responsive to said color signals   damentales for producing an inhibition signal when said fundamental color signals are substantially equal, said inhibition signal being applied to said color selector (30) for pocketing the production of said chroma key; e) a pattern processor (60) responsive to said chroma key signal for developing control signals, and   f) a video mixer (10) responsive to at least two of said   video signals and said control signals to provide a signal complex video.   Apparatus according to Claim 4, characterized in that said   comparator (25) is of the three-level type.   Apparatus according to Claim 4, characterized in that   comprises an effect amplifier (70) for producing special effects   in the complex image obtained.   Apparatus according to Claim 4, characterized in that the   said color selector (30) includes a chroma key generator which   comprises a control device (35) which can be controlled from the   to select said color in one of said deo of colors.   Apparatus according to Claim 4, characterized in that said color selector (30) comprises a chroma key signal generator having a control device (35) operable from the outside to select said color from among the one of said signals color video.   Apparatus according to Claim 8, characterized in that the   said control device (35) which can be operated from the outside   comprises means for generating at least two control signals de-   90 phases between them, and that said signals have the form (R-J) sine and (B-J) cose, o @ is a function of said control device that one   can operate from the outside while (B-J) and (R-J) designate the-   said color difference signals.   Apparatus according to Claim 8, characterized in that the   said chroma key generator (30) includes means (120) for adding   said chroma key signal and said inhibition signal.   Apparatus for switching a plurality of video signals so as to generate a video signal which forms a complex image according to a color selected by one of the video signals, said apparatus being characterized by comprising a) a source of a plurality of video signals of colors, including color coded signals; b) a decoder responsive to said color coded signals for producing fundamental color signals therefrom;   c) a resistive matrix connected to said decoder and suitable for   to derive color difference signals;   d) a color picker responsive to said difference signals;   this color to generate an electrical control signal which re-   has a hue or range of hues that can be selected in one of said video signals, and   e) a comparator responsive to said fundamental color signals   to produce an inhibition signal when said signals   fundamental colors are substantially equal, said signal of inhi-   bition being applied to said color selector to prevent   duction of said electrical control signal.   Apparatus according to Claim 11, characterized in that   said comparator is of the three-level type.   Apparatus according to Claim 11, characterized in that said color picker comprises means for adding said   electrical control signal and said inhibition signal.   An apparatus for switching a plurality of video signals so as to generate a video signal which produces a complex image according to a selected color in one of the aforementioned video signals, said apparatus being characterized by comprising: a) a source of a plurality of color video signals comprising color coded signals; b) a decoder responsive to said color coded signals so   to produce fundamental color signals from these encoded signals;   c) a resistive matrix connected to said decoder and suitable for   generate color difference signals; d) a color picker responsive to said color difference signals for generating a chroma key signal which represents a hue or range of hues that can be selected in one of the aforementioned video signals;   e) a comparator responsive to said fundamental color signals   in order to generate an inhibition signal when said signals   fundamental colors are approximately equal, this signal of   hibition being applied to said color selector to prevent production of said chroma key signal; f) a pattern processor responsive to said chroma key signal for generating control signals, and g) a video mixer responsive to at least two of said video signals and said control signals, said video mixer producing a complex video signal.   Apparatus according to Claim 14, characterized in that   said comparator comprises a three-level video comparator.   Apparatus according to Claim 14, characterized in that   includes an effects amplifier for producing special effects in the complex image.   Apparatus according to Claim 14, characterized in that said effect selector comprises means for adding   said chroma key signal to said inhibition signal.   Apparatus according to claim 14, characterized in that said color picker comprises a chroma key signal generator having a control means operable from outside   to select said selectable color in one of said video color channels.   Apparatus according to Claim 18, characterized in that said externally operable control means comprises means for generating at least two out-of-phase control signals 900 between them, and said signals are in the form of (RJ) sinQ and (BJ) cos & oi is a function of said externally operable control means, while (BJ) and (RJ) define said color difference signals. Apparatus according to Claim 18, characterized in that   said chroma key signal generator comprises means for adding   the chroma key signal and said inhibition signal.   An apparatus for switching a plurality of video signals to provide a video signal that can generate a complex image according to a selected color in one of the video signals, which apparatus is characterized by comprising: a) a video source for generating a plurality of signals video which defines a complex image according to a selected color in one of the video signals, including the fundamental color signals;   b) a matrix connected to said video signal source for   generate color difference signals;   c) a color picker responsive to said difference signals;   color coder for generating an electrical control signal which represents a hue or range of hues that can be selected in one of said video signals, and   d) a comparator responsive to said fundamental color signals   to generate an inhibition signal when these fundamental color signals are substantially equal, said muting signal   being applied to said color selector means to prevent the generation of   ration of said electrical control signal.   22. Apparatus for switching a plurality of video signals to   create a video signal which in turn generates a complex   to a selected color in one of the video signals, this   being characterized in that it comprises   a) a video source for generating a plurality of video signals,   ci defining a plurality of video signals having color coded signals; b) a decoder responsive to said color coded signals for generating from them fundamental color signals; c) a matrix connected to said decoder for generating color difference signals;   d) a color picker responsive to said difference signals;   this color to generate an electrical control signal which re-   has a hue or range of hues that can be selected in one of said video signals, and   e) a comparator responsive to said fundamental color signals;   to cause an inhibition signal when said fundamental color signals are substantially equal, this inhibition signal being applied to said color selector for pocketing the generation   of said electrical control signal.   23. An improved generator according to the chroma key system for producing a chroma key signal representative of a selected color contained in one of several video signals, characterized in that it comprises a comparator sensitive to the fundamental color signals   contained in one of said video signals, so that the general   the chroma key signal is prohibited when the said   their fundamentals are equal in amplitude.