United States Patent
`
`Petro Vlahos
`(72) Inventor
`Tarzana, Calif.
`(21) Appl. No. 80,083
`Feb. 20, 1969
`22 Filed
`45 Patented July 27, 1971
`(73) Assignee The Association of Motion Picture &
`Television Producers Inc.
`v
`Hollywood, Calif.
`
`(54 ELECTRONIC COMPOSTE PHOTOGRAPHY
`16 Claims, 2 Drawing Figs.
`52 U.S.C........................................................ 178/5.2 R,
`17815.4 CR, 178/DIG. 6,352/131
`5ll at Cl......................................................... H04n 5122
`(50) Field of Search............................................ 178/6 ST, 6
`DIG. 6, 5.2, 5.4, 6.7 R, 5.4 CR, 5.2 D;352/131;
`355/4, 5, 7
`
`(56)
`
`References Cited
`UNITED STATES PATENTS
`1/1943 Batchelor...................
`2,307,661
`2,615,088 10/1952 Gordon......
`3,296,367
`l/1967 Cassagne......................
`
`178/DIG. 6
`178/DIG. 6
`178/DIG. 6
`
`
`
`(11) 3,595,987
`FOREIGN PATENTS
`720, 182 2/1954 Great Britain. ... ...
`1, 72,540, 6/1964 Germany ... . . . . :
`Primary Examiner-Robert L. Griffin
`Assistant Examiner-Joseph A. Orsino, Jr.
`Attorney-Charlton M. Lewis
`
`78/DIG. 6
`78/DIG. 6
`
`ABSTRACT: Separate foreground and background scenes are
`combined to form a composite color television picture or
`color motion picture film by electronic manipulation of
`respective sets of color component video signals, one set
`representing the foreground scene with an illuminated
`backing, typically blue, and the other set representing the
`background scene, Blue from the foreground backing is
`eliminated from the composite picture by electronically limit
`ing the foreground blue signal to a selected function of the
`green signal. Portions of the background that are covered by
`foreground objects are eliminated in the composite picture by
`gating the background video signals under control of color dis
`criminating circuitry which compares the foreground blue and
`green (or red) color component signals. Both the discriminat
`ing circuits and the gating circuits act proportionally, so that
`partially transparent objects of the foreground are correctly
`distinguished, making the background scene partially visible
`through such objects in the composite picture.
`
`

`

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`1 :
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`3,595,987
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`foreground and background video signals are mixed to
`ELECTRONIC coMPosITE PHOTOGRAPHY
`produce the composite picture,
`An important feature of the invention is that the resulting
`This invention has to do with electronic methods and ap
`switching orgating action is preferably not a simple on-off ac
`paratus by which a foreground scene and a background scene
`tion, but is proportional in its nature, and is thus capable of
`may be separately recorded and then combined to form a
`reproducing correctly partially transparent areas of the FG
`composite picture in which objects of the foreground appear
`scene through which the BG scene is partially visible. Such
`superposed over objects of the background.
`proportional gating of the picture components is made possi
`For convenience of description the abbreviations BG and
`ble by utilizing discriminating circuits responsive to a plurality
`FIG. will be used in referring to "background' and "-
`O
`of color component video signals, rather than attempting to
`foreground.'
`distinguish between FG and BG areas of the picture on the
`A particular object of the invention is to permit objects of
`basis of the single chrominance signal, as has been previously
`both FG and BGscenes to be portrayed in full color, with spe
`proposed.
`cial attention to accuracy of reproduction of such delicate
`The video signals representing the BG scene are propor
`colors as normally occur in flesh tones and eyes.
`15
`tionally gated under control of a signal that typically
`A further object of the invention is to permit BG objects to
`represents the excess of the blue light over a specified function
`be seen to a realistic extent through objects of the FG that are
`of the green light received from the FG scene. That gating
`partially or wholly transparent, and to achieve a normal
`reduces the BG signals to zero when the FG blue light does not
`degree of seethrough for such special situations as FG objects
`exceed that function and transmits the full BG signals when
`20
`that are out of focus or blurred by rapid motion.
`the FG blue has its maximum value, corresponding to an area
`Whereas the invention is particularly useful in connection
`of the illuminated blue backing. The gating of the FG color
`with television or motion picture scenes involving movement,
`signals typically acts only on the blue signal, and is essentially
`and in connection with color reproduction, many aspects of .
`a clipping action, limiting the blue FG signal to a value no
`the invention are useful for still pictures and for producing
`larger than a specified function of the green FG signal.
`25
`composite pictures in black and white or in partial color.
`A composite picture produced by the present invention may
`In purely photographic processes of composite photog
`have the form of a video signal suitable for television broad
`raphy, areas of the BG scene that are occupied by FG objects
`casting or for video tape recording, or may be recorded on
`are blocked out by printing the BG through a specially
`photographic film such as a motion picture film suitable for
`prepared matte, which is referred to as a traveling matte when
`conventional optical projection. Such production of com
`30
`motion pictures are involved. In the electronic system no such
`posite motion picture films by electronic procedures is prac
`physical matte is employed, but a suitable alternative capabili
`ticable only if the process is capable of correct reproduction
`ty must be provided by which the system can recognize for
`of partially transparent areas of the FG scene. Such areas
`every spot of the picture whether the video signal should cor
`occur in motion pictures not only from presence of inherently
`respond to the FG or BG component.
`35
`transparent objects, such as glassware, smoke and wisps of
`For that purpose, the present invention utilizes the conven
`hair, but also from edges of opaque objects that are blurred by
`tional procedure of arranging the objects of the FG scene be
`movement. Since the present process can handle such areas
`fore a backing of a distinctive color. FG objects are then
`properly, it can take the place of known photographic
`distinguished from the colored backing by suitable com
`processes for producing composite motion pictures from
`parison of the electronic color component video signals, such
`40
`separate FG and BGscenes.
`as are developed directly by a television color camera, for ex
`When so used for motion picture composite photography,
`ample. Those color component signals normally correspond to
`the present invention permits greater speed of operation and
`the colors blue, green and red, characterized typically by the
`far greater flexibility of control than the previously known
`respective wavelength regions of 400 to 500, 500 to 600 and
`photographic processes. Whereas purely photographic travel
`600 to 700 millimicrons, and the color component signals then
`45
`ing matte processes require more than one day to produce a
`represent directly the relative blue, green and red light values
`composite film, the present electronic process can produce a
`of the scene. Such signals for the FG and BGscenes may be
`completed film for viewing the next day.
`developed directly from the natural scenes, as by use of televi
`The present invention further permits a motion picture
`sion cameras or equivalent apparatus. Alternatively, the video
`director to observe on a television monitor a composite pic
`color component signals for one or both of the picture com
`50
`ture of the FG and BG scenes during photography of the FG
`ponents may be derived from a previously prepared record of
`scene. For example, the FG scene that is before the motion
`the FG or BG scene. Such a record may comprise a photo
`picture camera can be picked up also by a television camera
`graphic record such as a conventional motion picture film, or
`and combined electronically with a BG scene that is in
`may comprise a video tape in which the color information has
`troduced from an existing film. The composite picture on the
`55
`the form of a chrominance signal.
`monitor then permits the director to locate the FG action and
`The color of the illuminated backing for the FG scene is typ
`lighting to match elements in the BG scene.
`.
`ically restricted to one of the wavelength regions represented
`A full understanding of the invention, and of its further ob
`by the color component signals. In theory, and under special
`jects and advantages, will be had from the following descrip
`circumstances in practice, any of those component colors may 60 tion of certain illustrative manners in which it may be carried
`be used as backing for the FG scene. However, blue is or
`out. The particulars of that description, and of the accom
`dinarily the most practical backing color. That is because the
`panying drawings which form a part of it, are intended only as
`selected backing color should not ordinarily be used in pure
`illustration and not as a limitation upon its scope, which is
`form in the FG scene itself, and since a saturated blue is rarely
`defined in the appended claims.
`found in nature, its avoidance in the FG scene does not impose
`In the drawings:
`a serious limitation. Accordingly, for the sake of clarity the
`FIG. 1 is a schematic block diagram representing an illustra
`present description will be based on the use of blue as backing,
`tive system for carrying out the invention; and
`with the understanding that other colors may be preferred
`FIG. 2 is a schematic block diagram corresponding to a por
`under special circumstances.
`tion of FIG. 1 and representing a modification.
`The present invention provides discrimination circuitry that
`70
`As illustratively represented in FIG. 1, the FG scene 10 is
`is typically responsive to the blue and green components of
`arranged before the illuminated backing 12 and is recorded by
`the light received from the foreground scene and that
`the television camera represented at 20. The FG scene is typi
`develops a control signal representing the extent to which that
`cally illuminated in conventional manner, as by the lamp 14.
`light was derived from the blue backing. That control signal is
`That lamp requires no special filtering, and may be of any type
`then employed to control the relative proportions in which the 75 called for by the color reproduction process that is employed.
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`As illustratively shown in FIG. 1, the red and green com
`Backing 12 may be illuminated in many different ways, de
`ponent signals for the FG scene are transmitted directly via
`pending upon its nature. If the backing material is a painted
`the respective lines 24 and 25 to mixer 20. The blue com
`canvas having a reflectivity limited to the blue region of the
`ponent signal is modified by circuitry indicated schematically
`spectrum, it may be illuminated by the same lamps as the FG
`objects, though additional light is usually desirable. If the
`at 60, which receives the blue signal from line 26 and delivers
`the modified blue signal via the line 62 to mixer 50. Circuitry
`backing is a white opaque surface it may be placed out of the
`60 acts under control of an input control signal, received via
`range of FG lamps 14 and be lighted by special lamps, such as
`the line 63, which may be derived via the amplifier 64 from
`those shown at 16, which are provided with the blue filters 17
`either the green or the red FG signal, according to the position
`or otherwise constructed to emit only blue light. Alternatively,
`of the switch 66. For normal FG scenes switch 66 is ordinarily
`the backing may be of translucent material and be illuminated
`maintained in the position shown, supplying the green com
`from the rear, with the color limited to blue by use of either
`ponent signal from line 25 for control of circuit 60, and that
`blue material or blue lamps or both.
`position will be assumed for clarity of description. The func
`Television camera 20 may be of any conventional type
`tion of circuit 60 is then essentially to apply the green com
`which scans the FG scene and its backing under control of a
`ponent signal as a floating peak limiter or clipper upon the
`synchronizing signal received over the line 22 from the control
`blue signal. If the blue signal is equal to or less than the limit.
`unit 40, and which produces on the lines 24, 25 and 26 respec
`ing threshold, it is transmitted without modification to output
`tive video signals corresponding to the red, green and blue
`line 62 and mixer 50.
`color components of the scene, designated R, G and B.
`The limiting threshold thus imposed by circuit 60 upon the
`As represented in FIG. 1, the BG scene is illustratively pro
`FG blue signal may directly equal the green signal. However, it
`vided in the form of a motion picture film 32. That film is ad
`is ordinarily preferred to introduce biasing circuitry such that
`vanced intermittently by known mechanism indicated at 31 in
`the permitted maximum value of the blue signal increases
`response to suitably timed signals received over the line 33
`somewhat faster than the green control signal, typically cor
`from control unit 40. Each frame of film 32 is scanned in
`responding approximately to the product of the green signal
`synchronism with the FG scanning action of camera 20, as by
`and a factor that exceeds unity by a selected fraction, typically
`the flying spot scanner represented in simplified and sche
`of the order of 20 percent. Such a bias may be introduced in
`matic form at 30, Flying spot scanner 30 typically comprises
`any suitable manner, as by the amplifier 64 which has again M
`the cathode ray tube (CRT) 34 with deflection means, not ex
`that is preferably variable, as indicated by the control 65.
`plicitly shown, for causing a spot of light to scan an area on the
`Variation of M from unity to about 1.5 is sufficient for most
`30
`face of the tube under control of a synchronizing signal
`scenes. Limiter 60 then limits the blue FG signal reaching
`received over the line 36. That signal is developed by control
`mixer 50 to a maximum value equal to the green signal mul
`unit 40 in suitable time relation to the similar scanning control
`tiplied by M.
`signal delivered to camera 20. Those two control signals are
`A primary result of that limitation of the FG blue signal is to
`represented in FIG. 1 as being supplied by a common line to
`prevent any contribution to mixer 50 from the FG scene when
`emphasize their common time relation, but in practice distinct
`the scanning action of camera 20 is confined to the blue
`signals may be developed and employed for control of dif
`backing 12. When camera 20 is receiving only blue light, the
`ferent scanning devices. The "flying spot" on the face of CRT
`green and red component signals are necessarily zero. Though
`34 is focused onto a frame of film 32 by the lens 37, so that the
`the blue signal on line 26 is large, it is reduced to zero by the
`transmitted light 48 is modified in accordance with the color
`described limiting action of circuit 60.
`40
`. and density of the BG scene at the rapidly shifting illuminated
`On the other hand, when camera 20 is scanning a FG object,
`spot. The transmitted light 48 is separated in known manner
`the described limitation of the blue component ordinarily has
`by the dichroic mirrors 38 and 39 into red, green and blue
`no effect upon the reproduction of normally occurring FG
`color components, which are directed to the respective light
`colors. The exceptional effects that do occur, especially at
`sensor 41, 42 and 43, represented as photocells. The respec
`semitransparent areas of the FG scene, are discussed more
`tive photocell outputs on the lines 44, 45 and 46 are video
`fully below.
`signals representing the red, green and blue color components
`The gating of the BG scene is carried out in FIG. 1 by cir
`of the BG scene and designated R, G and B. Those signals cor
`cuitry indicated schematically at 70, acting under control of a
`respond directly to the FG component video signals on lines
`control signal E supplied via the line 72. That control signal is
`50
`developed by color discriminating circuitry represented at 74,
`24, 25 and 26, already described. That is, at any instant the FG
`component video signals and the BG component video signals
`which receives the blue FG signal from line 26 via the limiter
`are derived from directly corresponding points of the FG
`76 and the line 77, and receives a reference signal from the
`scene and of the BG scene, respectively.
`line 79. That reference signal is typically the same as the con
`The color component signals for the FG and BGscenes are
`trol signal for limiter 60, already described, being derived via
`55
`amplifier 64 from either the green or the red FG signal, de
`mixed in the mixer 50, to produce on the output lines 54, 55
`and 56 color component signals for the desired composite pic
`pending upon the position of switch 66.
`ture. The resulting composite picture can then be displayed,
`Circuit 74 is typically a different amplifier, and its output
`signal E on line 72 represents essentially the excess of the
`for example, by means of a three color cathode ray tube 58 in
`blue FG signal the output is zero. The input blue signal, how
`which the beam scanning is synchronized with that in camera
`60
`20 and CRT 34 by means of suitable synchronizing signals
`ever, is preferably first limited by variable limiter 76 to a value
`that will be denoted by B, and that is adjustable at 78. B is
`supplied via the line 59 from control unit 40. In accordance
`made no larger than the value corresponding the least brightly
`with the present invention, the separate FG and BG color
`component signals are suitably modified in intensity, before
`illuminated portion of backing 12. It is then immaterial
`whether the backing is lighted with strict uniformity, so long as
`being supplied to mixer 50, in such a way as to make each
`point of the resulting composite picture correspond properly
`all areas received at least the selected threshold intensity. As a
`matter of fact, B is ordinarily set at the level corresponding to
`to either the FG or the BG scenc, or to a properly weighted
`the maximum illumination of the FG objects, for reasons that
`combination of both. That signal modification thus performs
`will appear.
`fundamentally a selection function, and will be referred to for
`Whenever camera 20 or its equivalent is scanning the
`convenience as a "gating action.' However, the present gating
`70
`backing, the reference signal on line 79 is essentially zero.
`action is preferably quite different from the crude switching
`Control signal E then represents the full value of the input
`that is sometimes associated with that terms. The gating action
`blue signal, corresponding to the threshold or minimum illu
`of the present invention is carried out under control of color
`discriminating circuits which operate in response to color
`mination of backing 12. If camera 20 scans a FG object, con
`trol signal E is ordinarily sharply reduced for two reasons.
`component signals for the FG scene only.
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`First, the blue content of the FG object is normally far less
`mitted to the extent of 50 percent by the object. That blue
`than the described threshold illumination of the blue backing.
`signal is reduced by limiter 60 to the same level as the green
`Secondly, most FG objects have an appreciable green content,
`signal, so that the total contribution from the FG scene to
`so that the reference signal on line 79 is appreciable. Subtrac
`mixer 50 represents white light at the intensity actually
`tion of that reference signal from the input blue signal further
`reflected by the object. The BG color signals are also reduced
`reduces the value of E. In fact, for all ordinary opaque FG ob
`by 50 percent, since control signal E=B-G has half its max
`jects the green content (especially after amplification at 64)
`imum value. That result follows from setting of limiter 76 to
`equals or exceeds the blue content, so that the output control
`make B equal to the full normal blue reflection from an
`signal is zero. Special cases, including transparent or partially
`opaque white FG object, which makes the green reflection G
`transparent FG objects, are discussed more fully below.
`from the present partially transparent object equal to half of
`Gating circuit 70 for the BG scene comprises essentially
`B. The output of mixer 50 therefore correctly represents
`three variable gain amplifiers 7,73 and 75 for the respective
`equal contributions from the FG and BGscenes.
`color components. Each amplifier receives one of the BG
`A corresponding analysis shows that the system gives cor
`color component signals on the line 44, 45 or 46 and delivers
`rect reproduction also for other degrees of transparency than
`15
`the modified signal to mixer 50 via the corresponding line 44a,
`50 percent, and for all FG colors having an equal blue and
`45a or 46a. Each amplifier also receives the control signal E
`green content. Such colors include not only the grey scale but
`from line 72 and responds by amplifying its BG color com
`also red, flesh tones, pinks and cyan. However, if the FG in
`ponent signal with a gain substantially proportional to E.
`cludes a color having a blue content much less than the green
`Thus, when E is zero the amplifiers of circuit 70 act as open
`content, such as a highly saturated green or yellow, such
`20
`switches, and mixer 50 receives no input corresponding to the
`colors will be somewhat distorted if they occur on transparent
`BG scene. On the other hand, when the control signal has its
`objects or at edges that are blurred by motion. For example, a
`maximum value, corresponding to the described threshold il
`green FG object with 50 percent transparency due to move
`lumination of backing iO, the BG signals are transmitted with
`ment will reproduce as a rather dark cyan. At the blurred area
`full normal amplitude to mixer 50. For intermediate values of
`the blue signal will correspond to half the brightness of the
`25
`the control signal, corresponding primarily to partially trans
`backing, seen through the moving object, and the green signal
`parent objects of the FG scene, the BG color component
`will also have half its normal value, producing cyan. The BG
`signals are uniformly attenuated and contribute to mixer 50
`scene will not appear through that blurred edge, since the
`only an appropriate fraction of the BG brightness sensed by
`equal blue and green FG signals produce a BG control signal
`BG scanner 30.
`E=0. Fortunately, highly saturated colors, such as bright
`30
`In describing more fully the operation of the system of FIG.
`green and yellow, rarely occur in foreground objects and are
`1, it will first be assumed that the colors blue and magenta do
`ordinarily avoided as much as possible because of a tendency
`not occur in the objects of the FG scene. Magenta is defined as
`to appear fluorescent and unrealistic. Moreover, the described
`blue plus red, with little or no green content. With that as
`color distortion applies only to partially transparent objects or
`sumption all FG colors have a blue content that is equal to or
`to edges that are blurred by motion. Since motion is usually
`35
`less than the green content. Thus, for all grey scale objects
`transient the effect is not easily noticed. No corresponding
`from black to white the blue and green contents are equal.
`distortion results, of course, if bright green or yellow occurs in
`The color cyan includes equal amounts of blue and green with
`the BG scene behind a blurred edge of a FG object of normal
`color.
`little or no red. In the case of red, yellow, green, gold, copper
`The assumption made above that the FG objects contain no
`and flesh tones the blue content is less than the green content.
`40
`blue colors is not always feasible. Of particular significance
`For all such colors, biasing amplifier 64 can be set to a gain
`are pastel blues, as in blue eyes. Such shades of blue are highly
`of unity. Limiter 60 then transmits to mixer 50 only so much
`of the input blue signal from line 26 as equals the green signal
`unsaturated, including a large content of white, and hence in
`cluding green and red in appreciable and approximately equal
`from line 25. That does not affect the color of opaque FG ob
`amounts. The blue content of such pastel blues typically ex
`jects, since their blue content has been assumed not to exceed
`45
`the green content.
`ceeds the green content by a factor of the order of 5 percent to
`25 percent. All colors having that property are accom
`Also, control signal E then directly equals Bo-G, where Bo
`represents the output from limiter 76, and G represents the
`modated, in accordance with the present invention, by suita
`ble biasing of the control circuits.
`green component signal on line 25. That control signal distin
`That biasing is typically represented in FIG. 1 by the biasing
`guishes effectively between points of the blue backing and
`50
`amplifier 64, which boosts the green signal relatively to the
`points of the FG scene itself. At any point of the backing the
`blue signal at the input both to limiter 60 of the FG control cir
`control signal has the full value B, while for any opaque ob
`cuit and to difference circuit 74 of the BG control circuit.
`ject of the FG scene the control signal is zero, since the blue
`Considering first the FG control, the biasing action increases
`content has been assumed not to exceed the green content.
`55
`the level at which the blue signal is clipped by limiter 60 by the
`Hence for such objects, the BG gating action of circuit 70 es
`factor M, from the level of the green signal to M times that
`sentially switches the BG color signals between full transmis
`level, where M represents the gain of amplifier 64. If M=1.25,
`sion to mixer 50 when backing 10 is being scanned, and full
`for example, a FG color containing 25 percent more blue than
`suppression when a FG object is being scanned. Thus there is
`green will still be correctly reproduced, since limiter 60 will
`zero superposition of the BG scene on any opaque object of
`60
`transmit the blue signal without reduction to mixer 50. Yet
`the FG scene, zero veiling of the BG scene by blue derived
`during scanning of blue backing 12 the resulting large blue
`from backing 10, and fully correct color reproduction of both
`signal is still reduced to zero by limiter 60, since the absence
`the FG and BG objects.
`of any green light from the backing makes the clipping level
`With the same assumptions as to FG colors, the present
`zero. Since the described biasing action is independent of the
`system reproduces correctly most objects of the FG that are
`red content, it provides correct reproduction of such colors as
`partially transparent, or are blurred by motion, which causes
`low saturated magenta which combine red with the described
`essentially the same effect as partial transparency of a sta
`mutual proportions of blue and green.
`tionary object. For example, a fully illuminated white FG ob
`Turning now to the gating of the BG scene, when biasing
`ject that is 50 percent transparent will reflect equal amounts
`amplifier 64 is set for a gain of 1.25, as just described, dif
`of blue, green and red, but only at half the intensity that would
`70
`ference circuit 74 produces a BG control signal E equal to the
`result from an opaque white object. The green and red com
`excess of the blue signal over 1.25 times the green signal.
`ponent signals from such an object are therefore half the nor
`Hence E is zero for a light blue FG object, as well as for all
`mal maximum. The blue component signal has the full max
`other colors having a blue content no greater than 1.25 times
`imum value, half resulting from blue light reflected by the ob
`the green content. Therefore such FG colors are reproduced
`ject, and half resulting from blue light from the backing, trans
`75
`without any superposition of light from the BGscene.
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`7
`Application of the baising technique just described has the
`potential disadvantage of extending the range of FG colors
`that are subject to the color distortions, described above,
`which occur at blurred edges or otherwise partially trans
`parent areas. The color range in which those distortions may
`be encountered is characterized mainly by a blue content
`much less than the green content. With bias amplifier set to a
`gain of 1.25, say, instead of unity, the ratio of blue to green at
`which such distortions may occur is increased cor
`respondingly. It is emphasized, however, that distortions of
`this type are limited to blurred or otherwise semitransparent
`FG objects, and are also limited to types of colors that tend to
`be rarely used.
`if it is desired to use colors containing much more green
`than blue for FG objects that will be subject to blurring, the
`tendency to color distortion can often be reduced by adjusting
`the gain of bias amplifier 64 to a value less than unity, That af
`fects reproduction of unblurred FG areas only if their blue
`content nearly equals the green content, and it is often possi
`ble to avoid such colors in a particular scene. Thus a judicious
`selection of color combinations and appropriate adjustment of
`the bias adjustment can usually reduce the described potential
`color distortion to negligible proportions.
`in the above description it was initially assumed for clarity
`of discussion that the color magenta was excluded from the
`FG scene. As already mentioned, magentas of low saturation
`are correctly reproduced together with low saturation blues by
`suitable adjustment of bias amplifier 64. If it is desired to in
`clude brilliant or highly saturated magenta in a particular
`scene, switch 66 is shifted from the position shown in FIG. ,
`making the control circuits for both FG and BG scenes subject
`to the FG red color component signal on line 24 in place of the
`green signal on line 25. The system then operates in a manner
`similar to that already described, except for substitution of red
`for green throughout, which includes the interchange of
`magenta and cyan. Many colors, including in particular pastel
`shades having a high white content, are reproduced equally
`well with switch 66 in either position. The fact that switch 66
`is ordinarily preferred in the position illustrated is not due to
`40
`any peculiarity of the system, but results from the greater rela
`tive frequency of occurrence and use of colors related to cyan
`(blue plus green) as compared to colors related to magenta
`(blue plus red).
`Many circuit details which would be included in a practical
`system have been omitted from FIG. for clarity of illustra
`tion. Such features include, for example, optical and elec
`tronic filters, biasing and clipping circuits, phase control
`devices for maintaining proper phase relations among the vari
`ous signals, and variable gain amplifiers for such purposes as
`adjusting the effective contrast or gamma of the various color
`components, equalizing circuit gain, compensating filter
`losses, and compensating the relative spectral sensitivity of
`photographic emulsions or of the output cathode ray tube.
`Such amplifiers may be designed in known manner to produce
`a nonlinear response, as to compensate photographic effects
`at the toe portions of the characteristic curve of a photo
`graphic emulsion. Signal controls of such types are well
`known, in and of themselves, and can be provided as needed
`to meet the requirements of any particular system.
`It will be understood without detailed discussion that the
`color component video signals for the FG scene and for the
`BG scene can be developed in any desired manner. In particu
`lar, the FGscene 10 can be photographed with an illuminated
`backing 2, and the resulting motion picture film can then be
`scanned by mechanism such as that represented in FIG. 1 at
`30 in synchronism with whatever mechanism is used for
`recording the BG scene. Also, the BG scene may be recorded
`directly by a television camera that is suitab