Abstract:
A chromakey signal producing apparatus is disclosed. A color to be keyed is set as a first reference color and a color not to be keyed is set as a second reference color. A clear chromakey signal is automatically produced by discriminating the difference between the first and second reference colors. According to the invention, even when a part of the TV picture has a color similar to that of the part to be keyed, a clearly discriminated chromakey signal is produced without the need for any complicated sensitivity factor judgment, simply by designating the color of that particular part as the second reference color corresponding to the maximum sensitivity.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a chromakey signal producing apparatus used for making montage pictures in TV program production, and more particulary to a line chromakey producing apparatus for producing a chromakey signal directly from an NTSC composite video signal. 
     Recently, an apparatus for producing a chromakey signal directly from a composite video signal, rather than from red (R), green (G) and blue (B) components, has been developed. In the prior art apparatus, a chromakey signal is produced by extracting Y, I and Q components from an incoming digital composite video signal, computing the differences between these components and corresponding Y, I and Q components of a set color (reference color) which is to be keyed, and multiplying the respective differences thus computed by sensitivity factors. In other words, only one reference color, i.e., one set of Y, I and Q components is designated as a key-color in the prior art. This prior art apparatus can generate a clear chromakey signal only when the incoming digital video signal has chrominance or luminance sufficiently distinguishable from the reference color in an area except the chromakey area. However, it is difficult to generate a clear chromakey signal when the incoming digital video signal has a color similar to the reference color in an area other than the chromakey area. Consequently, in the prior art, compensation must be made by manually adjusting the sensitivity factors in accordance with the color change of the incoming video signal or the reference color. Such manual adjustment is extremely complicated and requires a skilled technician. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a chromakey signal producing apparatus which can automatically produce a clear chromakey signal even when the color of an incoming video signal or a set reference color changes or the color of a part of an incoming video signal is similar to the set reference color outside the chromakey area. 
     A chromakey signal producing apparatus of the present invention includes designating two reference colors as an important feature. The color to be keyed (minimum sensitivity) is set as a first reference color and the color not to be keyed (maximum sensitivity) is set as a second reference color. The clear chromakey signal is automatically produced by discriminating the difference between the first and second reference colors. According to the present invention, even when a part of a TV picture has a color similar to that of the part to be keyed, a clearly discriminated chromakey signal is produced without the need for any complicated sensitivity factor adjustment, simply by designating the color of that particular part as the second reference color corresponding to the maximum sensitivity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the construction of a chromakey signal producing apparatus according to the prior art; 
     FIG. 2 is a block diagram of an embodiment of the present invention; and 
     FIG. 3 is a graph illustrating the operation of the chromakey signal producing apparatus shown in FIG. 2. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In order to facilitate an understanding of the present invention, a prior-art chromakey signal producing apparatus will be described first with reference to FIG. 1. As shown, a composite digital video signal v is separated into Y, I, and Q components by a luminance-chrominance separation circuit 1. In this case, if sample phases of the digital video signal v coincide with the ±I and ±Q axes of a color vector plane, the I, Q and Y components can be easily extracted from the composite digital video signal. A timing generator 2 generates a timing signal t having a certain width corresponding to an arbitrary position within the TV picture and Y, I and Q components are held in registers 3, 4, and 5, respectively, with that timing. A marker signal m is also outputted from the timing generator 2 so that the timing can be confirmed on a monitor. Therefore, any color on the TV picture can be selected as the reference color while the marker is monitored on the monitor picture. 
     The Y, I, and Q components thus held during the time period defined by the timing signal &#34;t&#34; are averaged by an arithmetic circuit 6 so as to improve the S/N (signal to noise) ratio, and are then sent as reference color components Y 0 , I 0 , Q 0  to subtractors 7, 8, and 9, respectively. The subtractors 7, 8, and 9 extract respective differences between Y, I, and Q delivered from the luminance-chrominance separation circuit 1 and Y 0 , I 0 , and Q 0 . The extracted differences (Y-Y 0 ), (I-I 0 ) and (Q-Q 0 ) of the corresponding components are multiplied by sensitivity factors α, β, and γ in multipliers 10, 11, and 12, respectively, to adjust the sensitivities of each component, and are then added by an adder 13. The result (pre-chromakey signal k) is clipped by a predetermined level in a clipper 14 to produce a chromakey signal. The pre-chromakey signal k delivered from the adder 13 can be expressed by Equation (1) as: 
     
         k=α|Y-Y.sub.0 |+β|I-I.sub.0 |+γ|Q-Q.sub.0 |          (1) 
    
     Since only one reference color, i.e., one set of Y 0 , I 0 , and Q 0  is designated, as is understood from Equation (1), the pre-chromakey signal k delivered from the adder 13 has a level corresponding to the distance from the reference points (Y 0 , I 0 , Q 0 ) on the color space defined by Y, I, and Q axes. As a result, any part on the TV picture having a color similar to the reference color which part should not be keyed cannot be clearly distinguished, and this part is undesirably replaced by another picture by a chromakey effect. In order to avoid this problem it is necessary to adjust the sensitivity factors α, β, and γ such that sufficient discrimination can be ensured. These adjustments are very troublesome because they have to be performed manually while observing a monitor picture. 
     Referring now to FIG. 2, the present invention will be described in connection with one possible embodiment thereof. Similar parts of FIG. 2 are indicated by the same numerals as those of FIG. 1. The chromakey signal producing apparatus of the present invention shown in FIG. 2 is different from the prior art apparatus shown in FIG. 1 in that any two positions on the TV picture can be designated as a first reference color to be keyed and a second reference color not to be keyed. The two timing signals which define the positions of the first reference color and the second reference color are generated by a timing generator 2&#39; and indicated by t and t&#39;. Here, the two positions of the first reference color and the second reference color correspond to the minimum sensitivity position and the maximum sensitivity position, respectively. 
     Similarly to FIG. 1, the Y, I, and Q components during a certain period are held in the registers 3, 4 and 5 at the instant defined by signal t and are sent to an arithmetic circuit 6&#39; for averaging treatment to produce the first reference color (a set of Y 0 , I 0  and Q 0 ). On the other hand, other Y, I, and Q components are held in registers 15, 16, and 17 at the instant defined by signal t&#39; and are sent to the arithmetic circuit 6&#39;  to produce the second reference color (a set of Y 1 , I 1  and Q 1 ). The timing generator 2&#39; also generates a first marker signal m indicating a position on a TV picture that is to be keyed and a second marker signal m&#39; indicating positions not to be keyed on the TV picture in response to an externally received control signal. These two marker signals m and m&#39; are sent to a monitor M. Similarly to FIG. 1, differences (Y-Y 0 ), (I-I 0 ), and (Q-Q 0 ) are obtained in the subtractors 7, 8, and 9 and are sent to multipliers 18, 19, and 20, respectively. Also, differences (Y 1  -Y 0 ), (I 1  -I 0 ), and (Q 1  -Q 0 ) are computed in the arithmetic circuit 6&#39;, and α/|Y 1  -Y 0  |, β/|I 1  -I 0  |, and γ/|Q 1  -Q 0  | are sent to the multipliers 18, 19, and 20. Hence, a output (pre-chromakey signal) delivered from the adder 13 can be expressed by Equation (2) as: ##EQU1## 
     In the present invention, as expressed by the Equation (2), the level of the pre-chromakey signal k is automatically compensated by the differences |Y 1  -Y 0  |, |I 1  -I 0  | and |Q 1  -Q 0  | to provide a clear chromakey signal. In other words, in accordance with the color difference between the color (Y 0 , I 0  and Q 0 ) to be keyed and the color (Y 1 , I 1 , Q 1 ) not to be keyed, the effective sensitivities are changed automatically even if the sensitivity factors α, β, and γ are fixed. Therefore, the clear chromakey signal can be obtained even if the color difference between the first and second reference colors is small. 
     Next, the chromakey signal produced by the present invention will be described with reference to FIG. 3. Within a three-dimensional vector space (Y, I, and Q axes), the chromakey signal defined by the Equation (2), is illustrated by an octahedron 100 in FIG. 3 after clipping treatment. This octahedron has its center at coordinates (Y 1 , I 1 , Q 1 ) and in an outermost face includes a point with coordinates (Y 0 , I 0 , Q 0 ). The key signal provides maximum gain at the coordinates (Y 1 , I 1 , Q 1 ) and the minimum gain outside the area defined by the coordinates (Y 0 , I 0 , Q 0 ). 
     As described above, the chromakey signal producing apparatus of the present invention enables simple and easy setting of sensitivities for the components indicating luminance (Y) and color hue which may be referred to alternatively as either the chrominance C or its components I and Q, by providing a plurality of reference colors. It is self-explanatory that, by initially setting the sensitivity factors α, β, and γ of the Equation (2) at arbitrary levels, it is possible to control a degree of softness or hardness of the chromakey effect, as well as to produce a special key effect such as a luminance key. It is apparent that the present invention should not be basically limited to a NTSC video signal. In the embodiment, a composite video signal has been used as the input signal, but it is apparent that the present invention can be applied with a similar principle to an apparatus using R, G, and B signals from a TV camera.