Abstract:
A chroma-key tracking apparatus capable of automatically eliminating an unwanted partial spot noise from a chroma-key signal, to enable the generation of a correct frame around a desired portion of a television picture. The apparatus includes a noise eliminating circuit which by use of counters and comparators is capable of eliminating partial spot noises whose size in the horizontal and vertical directions is smaller than a predetermined value.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a chroma-key tracking apparatus. 
     A chroma-key tracking apparatus is used to produce a special effect by compressing a television picture to a small size corresponding to a chroma-key frame, and inserting the compressed television picture into another television picture. An apparatus of this kind is disclosed in U.S. Pat. No. 4,200,890. In the apparatus disclosed in this patent, a frame position signal representing the frame size to which the television picture is compressed is generated based on a chroma-key signal, and a picture compressing circuit compresses a television picture in response to this frame position signal, or to a frame position signal which is a little larger. The compressed television picture is supplied to a mix-keyer circuit, and combined with another television picture in response to the chroma-key signal. 
     In the disclosed apparatus, minimum and maximum positions in the horizontal and vertical directions with respect to the chroma-key signal are detected for generating the frame position signal in such a manner as shown in FIG. 4. This method can be practiced by a comparatively simple circuit construction. However, when an unwanted partial spot noise is included in the chroma-key key signal, a chroma-key frame which includes this partial noise spot is inevitably generated. When such an erroneous detection occurs, the television picture is not compressed to the size that corresponds to the desired size represented by the chroma-key signal in the picture compressing circuit, so that the desired special effect is not obtained. When the key signal, especially the chroma-key signal, is generated by discriminating a hue difference, the partial spot noise is likely to occur in certain television pictures. Furthermore, the partial spot noise also occurs intermittently, so that the stable chroma-key tracking operation can not be performed. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the present invention to provide a chroma-key tracking apparatus capable of automatically eliminating an unwanted partial spot noise from a chroma-key signal, thereby enabling the correct detection of the frame position. 
     According to the present invention, a chroma-key tracking apparatus including a noise eliminating circuit cab be obtained, the noise eliminating circuit being adapted to eliminate a partial spot noise by counting the width of the partial spot noise contained in an chroma-key signal in the horizontal and vertical directions, and masking the partial spot noise whose width is less than a predetermined width. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIGS. 1(a) and 1(b) show a shape of a chroma-key signal on a television screen and a frame position corresponding to the chroma-key signal, respectively; 
     FIGS. 2(a) and 2(b) show another shape of a chroma-key signal including a partial spot noise and a corresponding frame position, respectively, for explaining the operation of a prior art frame position detection; 
     FIG. 3 is a block diagram of a noise eliminating circuit according to the present invention; 
     FIG. 4 shows the operation of the noise eliminating circuit shown in FIG. 3 with respect to the horizontal direction; 
     FIG. 5 shows the operation of the noise eliminating circuit shown in FIG. 3 with respect to the vertical direction; and 
     FIG. 6 shows a block diagram of a chroma-key tracking apparatus including the noise eliminating circuit according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     First, the detection of a frame position in a chroma-key signal will be described. Assuming the chroma-key signal K shown in FIG. 1(a) within a television picture, the frame shown in FIG. 1(b), which surrounds the chroma-key signal K, is detected. In a prior art frame position detection step, a minimum position (start position) A SH  and a maximum position (end position) A EH  in the horizontal direction are detected. Further, a minimum position A SV  and a maximum position A EV  in the vertical direction are also detected. However, when unwanted noise, i.e., a partial spot noise Z is included in the chroma-key signal K, as shown in FIG. 2(a), a frame which includes the partial spot noise Z is unavoidably generated in the prior art at shown in FIG. 2(b). Consequently, a compressed television picture having only desired portion can not be obtained and the desired special effect is not generated. 
     Next, a noise eliminating circuit according to the present invention will now be described with reference to FIG. 3. The elimination of noise in the horizontal direction will be described. The chroma-key signal K is applied to an AND-gate 1 in which the chroma-key signal is processed with a pulse H representating a horizontal scanning period. The output delivered from the AND-gate 1 is applied to a counter 2 and while this output is supplied, for example when this output is at high level, a clock pulse CL having a frequency sufficiently higher than the horizontal scanning frequency, for example, 14.3 MHz is counted. When the output delivered from the AND-gate 1 to the counter 2 turns to low level, the count is reset. The counted value A delivered from the counter 2 is sent to a comparator 3 in which the value A is compared with a predetermined mask width B corresponding to the width of a partial spot noise to be eliminated, for example, &#34;3&#34;. When the result of the comparison is A&gt;B, a horizontal key signal K h  is sent to an AND-gate 4. Because of the provision of the comparator 3, any partial spot noise having the width less than the mask width B is eliminated. 
     In FIG. 4 showing the operation for eliminating the partial noise spot in the horizontal direction, the partial spot noise having the width t A , i.e., &#34;2&#34; is eliminated. After the counter 2 is reset in a next period t B , it starts to count again in a period t CH . When the count value in the counter 2 exceeds &#34;3&#34;, the key signal K h  is delivered from the comparator 3. The width t DH  of the key signal K h  is less than that of the period t CH  by a period ΔH, i.e., &#34;3&#34;. However, the period ΔH is compensated for very easily when the frame detection operation is carried out by a chroma-key tracking apparatus. 
     The elimination of partial spot noise in the vertical direction will now be described with reference to FIGS. 3 and 5. The chroma-key signal K is first sent to a retriggerable multivibrator 5 where the chroma-key signal is converted into a continuous key signal S 5  of high level, including a horizontal blanking period. To this end, the retriggering period in the retrigger multivibrator is set to a period between one horizontal period and two horizontal periods, for example, 100 μs. The continuous key signal S 5  from the retrigger multivibrator 5 is sent to an AND-gate 6 with a vertical pulse V representing the vertical scanning period. The output S 6  delivered from the AND-gate 6 is sent to a counter 7. When the level of this output S 6  is high, the counter 7 counts up using a horizontal pulse H as a clock pulse. The counter 7 is reset when the level of the output S 6  from the AND-gate 6 falls to low level, in the same manner as the counter 2. The counter value C delivered from the counter 7 is compared, by a comparator 8, with a mask width D corresponding to a vertical width of the partial spot noise to be eliminated. When C&gt;D, a vertical key signal K v  is delivered from the comparator 8 and applied to the AND-gate 4. As may be clearly seen from FIG. 5, the width of the vertical key signal K v  is less than that of the key signal K by the period ΔV corresponding to the mask width. The width of this period ΔV is also compensated for easily by the chroma-key tracking apparatus. Since the retriggerable multivibrator 5 is used, a trailing end of the continuous key signal S 5  extends beyond that of the key signal K by ΔV&#39;, which is extremely short, and can be neglected. The horizontal key signal K h , the vertical key signal K v , and the input chroma-key signal K are supplied to the AND-gate 4, so that a noise-eliminated key signal K&#39; can be obtained from the AND-gate 4. 
     In FIG. 6 showing the chroma-key tracking apparatus including the noise eliminating circuit 9 according to the present invention, two video signals are supplied to terminals 10 and 11, and a chroma-key signal generator 12 generates a chroma-key signal by discriminating a predetermined hue within the video signal from the terminal 10. This chroma-key signal is sent to a position signal detector 15 through the noise eliminating circuit 9 and a gate circuit 14, and the frame position corresponding to the chroma-key signal is detected thereby. The frame position signal delivered from the detector 15 is sent to an arithmetic circuit 16 in which the shortages ΔH and ΔV generated in the noise eliminating circuit 9 are compensated, and in which the frame position signal is enlarged by a predetermined width and applied to a picture compressing circuit 17. In the picture compressing circuit 17, the video signal delivered from the terminal 11 is compressed to the size of the frame. The compressed video signal is sent to a mix/keyer circuit 18. In the mix/keyer circuit 18, the compressed television picture is inserted into the television picture from the terminal 10 in accordance with the chroma-key signal K to form the special effect. Since the detailed operations of these parts are described in the specification of U.S. Pat. No. 4,200,890, detailed descriptions thereof are omitted here. 
     According to the present invention described above, any partial spot noise contained in the input chroma-key signal is eliminated so that the accurate frame position of the chroma-key signal can be detected, and thereby the desired special effect can be performed reliably.