Abstract:
Disclosed is a moving picture compression apparatus for performing intra-frame or inter-frame compression every block in frames in a moving picture, which comprises: means for detecting scene change in a moving picture; means for generating code containing information on components in a transformed domain&#39;s range gradually extending as the picture advances for a block in a new scene portion of a frame at the scene change detection during plural frames from the frame at the scene change detection, wherein the information is obtained by compressing the block in the new scene portion with the intra-frame compression process in the first frame from the scene change detection, and the information is obtained by compressing the block in the new scene portion with the inter-frame compression process in the second and following frames from the scene change detection.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a moving picture compressing apparatus and a moving picture compressing method and in particular, to a moving picture compressing apparatus and a moving picture compressing method for compression-encoding a picture signal on real time basis corresponding to a compression-encoding system mainly using a discrete cosine transform (DCT) process and a quantizing process. 
     2. Description of the Prior Art 
     When picture information is digitized and transmitted to a communication medium such as a telephone line, it is transmitted after it is compressed and encoded since the data amount of the picture information is vast. When a picture is compressed, a DCT encoding system has been widely used. In the DCT encoding system, picture data is compressed using the characteristic that magnitudes of spatial frequencies thereof tend to concentrate on lower spatial frequencies. This system has been adopted in international standards such as MPEG (Moving Picture Experts Group) video and ITU-T Recommendation H.263. 
     FIGS. 9A,  9 B,  9 C,  9 D,  9 E, and  9 F are schematic diagrams showing a hierarchy of code formats of MPEG video. FIG. 1A shows a video sequence as the top hierarchical level. The video sequence is composed of a plurality of GOPs (Group of Pictures). Each GOP is composed of a plurality of pictures as shown in FIG.  1 B. Each picture represents one frame. Pictures are categorized into three types of pictures that are I picture, P picture, and B picture. The I picture is composed of only intra-frame codes. The P picture is composed of inter-frame codes in forward direction as well as intra-frame codes. The B picture is composed of inter-frame codes in both of forward and backward directions as well as intra-frame code. 
     Each picture is composed of a plurality of slices arbitrarily divided into areas as shown in FIG.  1 C. Each slice is composed of a plurality of macro-blocks arranged rightwardly or downwardly. Macro-blocks are categorized as intra-blocks and inter-blocks. The intra-blocks are composed of intra-frame codes. The inter-blocks are composed of inter-frame codes in forward direction or two directions. The I picture is composed of only intra-blocks. On the other hand, the P picture and B picture are composed of inter-blocks as well as intra-blocks. 
     Each macro-block is composed of a total of six blocks that are luminance components (Y 1 , Y 2 , Y 3 , and Y 4 ) and two color difference components (Cb and Cr) as shown in FIG.  1 E. Each of the six blocks is composed of 8×8 pixels. A block of 8×8 pixels shown in FIG. 1F is the minimum encoding unit. 
     Next, with reference to FIG. 2, a conventional picture compressing process will be explained. FIG. 10 is a block diagram showing an example of the structure of a conventional picture compressing apparatus. In the apparatus shown in FIG. 10, central processing unit (CPU)  2  executes a program in apparatus controlling means  1  so as to control the whole of the apparatus. A user inputs a desired command to CPU  2  through keyboard  3 . Picture data is compression-encoded by picture compressing means  4 . The resultant compressed code is transmitted to a communication line. 
     Picture data is supplied to color converting means  5  in picture compressing means  4 . Color converting means  5  converts the picture data into three types of data that are Y, Cr, and Cb (hereinafter referred to as YCrCb data as a whole). A motion estimating means  6  searches a macro-block in the preceding/following frame so that the difference between the macro-block and a macro-block in the current frame becomes the minimum and calculates a motion vector corresponding to the motion between he two macro-blocks. When the difference is small, predicting means  7  calculates the difference between the frames so as to perform the inter-frame compressing process. 
     Output data of motion estimating means  6  is supplied to DCT means  8  along with the output of predicting means  7 . When the intra-frame compressing process is performed, DCT means  8  performs a DCT process for the YCrCb data. When the inter-frame compressing process is performed, DCT means  8  performs the DCT process for the difference data. Thereafter, quantizing means  9  quantizes the resultant data. Next, variable length code encoding means  10  encodes the resultant data into variable length code. 
     The quantized data that is outputted from quantizing means  9  is supplied to inversely quantizing means  14 . Inversely quantizing means  14  inversely quantizes the quantized data. Inverse DCT means  13  performs an inverse DCT process for the resultant data. When the intra-frame compressing process is performed, the resultant data is stored in reference frame portion  11 . When the inter-frame compressing process is performed, the difference in the inverse DCT process is added to the macro-block compensated in motion with the motion vector stored in reference frame portion  11 . The resultant data is stored in reference frame portion  11 . 
     The picture code that has been compression-encoded on the transmitting side is transmitted to the communication line. The picture to be transmitted need to be compressed in a compression ratio corresponding to the bit rate of the communication line. In particular, when picture data and audio data are transmitted at a low bit rate of 64 kbps used in a TV telephone system, they should be compressed at a high compression rate. Thus, the ratio of inter-frame compressed blocks to intra-frame compressed blocks is increased because code amount of an inter-frame compressed blocks is smaller than that of an intra-frame compressed block. 
     For example, in the first frame, all macro-blocks are compressed in the intra-frame compressing process. In the second or later frames, all the macro-blocks are compressed in the inter-frame compressing process unless the difference between different frames is large. Thus, the ratio of macro-blocks that are compressed in the intra-frame compressing process is decreased in second or later frames. However, when a scene change takes place, a code amount abruptly increases since the difference between different frames becomes large. Thus, it takes a long time to reproduce frames following a scene change on the reproducing side. Consequently, the picture looks like a still picture. 
     In order to solve such a problem, various picture compressing apparatuses that effectively compress pictures preceded by a scene change have been proposed in, for example, JPA-8-56361, JPA-2-174387, JPA-7-38895, and JPA-3-13792. In the picture compressing apparatus disclosed in JPA-8-56361, several consecutive frames preceded by a scene change including frames which are originally compressed in the intra-frame compressing process are forcedly compressed in the inter-frame compressing process, so that a code amount is decreased. In the picture compressing apparatus disclosed in JPA-2-174387, the intra-frame encoding process or the inter-frame prediction encoding process is used so that the prediction error becomes minimum and blocks are encoded using codebooks of which each is dedicated for intra-frame encoding process or the inter-frame prediction encoding. In the picture compressing apparatus disclosed in JPA-7-38895, after a scene change, higher-spatial-frequency components of blocks that are compressed in the intra-frame compressing process are deleted. In the picture compressing apparatus disclosed in JPA-3-13792, after a scene change due to a switching operation of a camera, picture data of several frames are sub-sample encoded so as to decrease a code amount generated. 
     Among the foregoing prior art references, in the picture compressing apparatus disclosed in JPA-8-56361, the ratio of macro-blocks that are compressed in the inter-frame encoding process is increased so as to transmit code at a low bit rate. But, the effect of decreasing a code amount is small. In addition, when data is compressed at a high compression rate in order to reduce a code amount, the picture quality deteriorates. The deteriorated picture does not recover its quality for a long time because it recovers its quality when all of the blocks has been compressed in the intra-frame compression process after deterioration. In the picture compressing apparatus disclosed in JPA-2-174387, the effect is low in case that the ratio of blocks that are compressed in the intra-frame compressing process is originally decreased. In the picture compressing apparatus disclosed in JPA-7-38895, since higher-spatial-frequency components are deleted, the picture quality deteriorates. And the deteriorated picture does not recover its quality until a frame of the picture is compressed in the intra-frame compression process. In the picture compressing apparatus disclosed in JPA-3-13792, a special decoding means is required on the reproducing side. 
     SUMMARY OF THE INVENTION 
     In order to overcome the aforementioned disadvantages, the present invention has been made and accordingly has an object to provide a picture compressing apparatus and a picture compressing method in which a code amount of a picture after a scene change is decreased and picture quality is quickly recovered after the scene change even at a low bit rate without need to use a special means on the reproducing side. 
     According to a first aspect of the present invention, there is provided a moving picture compression apparatus for performing intra-frame or inter-frame compression every block in frames in a moving picture, which comprises: means for detecting scene change in a moving picture; means for generating code containing information on components in a transformed domain&#39;s range gradually extending as the picture advances for a block in a new scene portion of a frame at the scene change detection during plural frames from the frame at the scene change detection, wherein the information is obtained by compressing the block in the new scene portion with the intra-frame compression process in the first frame from the scene change detection, and the information is obtained by compressing the block in the new scene portion with the inter-frame compression process in the second and following frames from the scene change detection. 
     According to a second aspect of the present invention, there is provided a moving picture compression apparatus for performing intra-frame or inter-frame compression every block in frames of a moving picture, which comprises: transforming means for transforming data in space domain to data in transformed domain; inversely-transforming means for transforming the data in the transformed domain to data in the space domain; scene change detecting means for detecting scene change; a first storage for storing the data in the transformed domain; a second storage for storing data used as reference data; a first writing means for writing the data in the transformed domain to the first storage when the scene change is detected; invalidating means for invalidating a part of the data in the transformed domain when the scene change is detected; a second writing means for writing the data obtained by inversely converting data which remains valid among the data in the transformed domain to the second storage as reference data when the scene change is detected; replacing means for replacing data in a current block by replacement data obtained by reading a part of the data in the transformed domain in the first storage and inversely transforming the part of data by using the inversely-transforming means; difference data generating means for generating difference data by subtracting reference data read from the second storage from the replacement data and supplying the difference data to the transforming means; and updating means for updating reference data in the second storage memory by reading reference data from the second storage, adding the output of the inverse transform means to the read reference data to get sum data, and writing the sum data to the second reference storage; wherein the part of the data in the transformed domain read from the first storage by replacing means is increased in range as a picture advances. 
     The moving picture compression apparatus may further comprises: quantizing means for quantizing the data in the space domain; inversely quantizing means for inversely quantizing the quantized data in the transformed domain to obtain inversely quantized data in the transformed domain which is supplied to the inverse-transforming means; encoding means for coding the quantized data in the transformed domain. 
     The encoding means may generate a variable length code. 
     The transformed domain may be spatial frequency domain. According to a third aspect of the present invention, there is provided a moving picture compression method for performing intra-frame or inter-frame compression every block in frames in a moving picture, which comprises steps of: detecting scene change in a moving picture; generating code containing information on components in a transformed domain&#39;s range gradually extending as the picture advances for a block in a new scene portion of a frame at the scene change detection during plural frames from the frame at the scene change detection, wherein the information is obtained by compressing the block in the new scene portion with the intra-frame compression process in the first frame from the scene change detection, and the information is obtained by compressing the block in the new scene portion with the inter-frame compression process in the second and following frames from the scene change detection. 
     These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of the best mode embodiment thereof, as illustrated in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a block diagram showing the structure of a picture compressing apparatus according to an embodiment of the present invention; 
     FIGS. 2A,  2 B, and  2 C are flow charts for explaining the operations of the picture compressing apparatus shown in FIG. 1 and a picture compressing method according to the present invention; 
     FIG. 3 is a flow chart showing an example of a special compressing process shown in FIG. 2B; 
     FIGS. 4A and 4B are flow charts showing examples of the inter-frame compressing process and the intra-frame compressing process shown in FIG. 2C; 
     FIGS. 5A and 5B are flow charts showing examples of a scene change determining process shown in FIG. 2B; 
     FIGS. 6A and 6B are flow charts showing examples of a special inter-frame compressing process and a special intra-frame compressing process shown in FIG. 3; 
     FIGS. 7A,  7 B,  7 C, and  7 D are schematic diagrams for explaining a case that a frame with a scene change is compressed; 
     FIGS. 8A,  8 B,  8 C,  8 D, and  8 E are further schematic diagrams for explaining another case that a frame with a scene change is compressed. 
     FIGS. 9A,  9 B,  9 C,  9 D,  9 E, and  9 F are schematic diagrams showing a hierarchy of code formats corresponding to MPEG video standard; and 
     FIG. 10 is a block diagram showing an example of the structure of a conventional picture compressing apparatus; 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 is a block diagram showing the structure of a picture compressing apparatus according to an embodiment of the present invention. In FIG. 1, the picture compressing apparatus comprises apparatus controlling means  21 , central processing unit (CPU)  22 , keyboard  23 , and picture compressing means  24 . Apparatus controlling means  21  controls the entire apparatus. Central processing unit  22  executes a program. Keyboard  23  inputs a user&#39;s command. Picture compressing means  24  compresses a picture. 
     Picture compressing means  24  comprises color converting means  25 , motion estimating means  26 , predicting means  27 , DCT means  28 , quantizing means  29 , a variable length code encoding means  30 , reference frame portion  31 , motion compensating means  32 , inverse DCT means  33 , inversely quantizing means  34 , encoding controlling means  35 . Color converting means  25  converts picture data into YCrCb data. Motion estimating means  26  detects a motion vector of each macro-block in a picture. Predicting means  27  calculates the difference between macro-blocks in different frames. DCT means  28  performs a discrete cosine transform (DCT) process as an example of an orthogonal transform process. Quantizing means  29  performs a quantizing process. Variable length code encoding means  30  performs a variable length code encoding process. Reference frame portion  31  stores preceding and following frames referenced to detect a motion of a picture between different frames. Motion compensating means  32  adds the difference between different frames to data in a reference frame. Inverse DCT means  33  performs an inverse DCT process. Inversely quantizing means  34  performs an inversely quantizing process. Encoding controlling means  35  controls motion estimating means  26 , predicting means  27 , DCT means  28 , and inverse DCT means  33  so as to gradually encode higher-spacial-frequency components in a scene change picture. 
     In the picture compressing apparatus shown in FIG. 1, CPU  22  executes a program in apparatus controlling means  21  so as to control the whole of the apparatus. A user&#39;s command is inputted from keyboard  23 . Picture compressing means  24  compresses picture data. The compressed code is transmitted to a communication line. 
     In common with the aforementioned conventional picture compressing apparatus, color converting means  25  in picture compressing means  24  converts input picture data into YCrCb data. Motion estimating means  26  searches a macro-block of a preceding/following frame so that the difference between the macro-block and a macro-block in the current frame becomes the minimum. In addition, motion estimating means  26  calculates a motion vector corresponding to the motion between the two macro-blocks. When the difference is small, predicting means  27  calculates the difference between the frames so as to perform the inter-frame compressing process. 
     When the intra-frame compressing process is performed, DCT means  28  performs the DCT process for the YCrCb data. When the inter-frame compressing process is performed, DCT means  28  performs the DCT process for the difference data. Quantizing means  29  quantizes frequency components received from DCT converting means  28 . Variable length code encoding means  30  encodes the quantized data received from quantizing means  29  into variable length code. 
     Inversely quantizing means  34  in picture compressing means  24  inversely quantizes the quantized data received from quantizing means  29 . Thereafter, inverse DCT means  33  performs an inverse DCT process for the data received from inversely quantizing means  34 . When the intra-frame compressing process is performed, the data received from inverse DCT means  33  is stored in reference frame portion  31 . When the inter-frame compressing process is performed, a difference value received from inversely DCT means  33  is added to a macro-block compensated in motion by motion compensating means  32  with a motion vector searched from the preceding/following frame and the sum is stored in reference frame portion  11 . The resultant data is stored in reference frame portion  31 . 
     When the number of blocks compressed in the intra-frame compressing process increases because of the large difference obtained by motion estimating means  26  or a code amount of a frame largely increases, encoding controlling means  35  in picture compressing means  24  controls motion estimating means  26 , predicting means  27 , DCT means  28 , and inverse DCT means  33  so as to perform the DCT process for only lower-spatial-frequency components in intra-frame encoding blocks or in all the blocks for the first frame, i.e. a scene change frame, and store all the spatial frequency components that have been DCT-processed in reference frame portion  31 . Encoding controlling means  35  execute inter-frame encoding in two ways. The first way includes a step of extracting all the spatial frequency components so as to obtain inter-frame encoded blocks or all the blocks for second and later frames after scene change. The second way includes a step of extracting spatial frequency components of a block in a scene change frame in a spatial frequency range extending gradually as a picture advances and performing inverse DCT process for the extracted spatial frequency components so as to obtain a inter-frame encoded blocks until the difference between each of inter-frame encoded blocks and each of locally decoded blocks decreases. 
     Next, with reference to a flow chart shown in FIG. 2A, a picture compressing process of the picture compressing apparatus according to the embodiment will be explained. 
     In the picture compressing process, color converting means converts picture data into YCrCb data (at step S 41 ). Motion estimating means  26  searches the motion between a macro-block of the preceding/following frame and a macro-block of the current frame (at step  42 ). Encoding controlling means  35  compresses the YCrCb data. Next, apparatus controlling means  21  determines whether or not encoding controlling means  35  has compressed data of all frames (at step  44 ). When the determined result at step S 44  is No, the flow returns to step  41 . When the determined result at step  44  is Yes, apparatus controlling means  21  completes the picture compressing process. 
     Next, with reference to a flow chart shown in FIG. 2B, the compressing process of encoding controlling means  35  will be explained. Encoding controlling means  35  determines whether or not a scene change has taken place (at step  51 ). Encoding controlling means  35  determines whether a scene change has taken place or a special compressing process is being performed after a scene change (at step  52 ). When the determined result at step  52  is No, encoding controlling means  35  performs a normal compressing process (at step  53 ). When the determined result at step  53  is Yes, encoding controlling means  35  performs a special compressing process (at step  54 ). The special compressing process will be explained later. 
     Next, with reference to a flow chart shown in FIG. 2C, the normal compressing process will be explained. Encoding controlling means  35  determines whether or not the total (the total of the absolute values or the square-sum) of the difference values between the macro-block searched by motion estimating means  26  and the current macro-block exceeds a threshold value  a  (at step  61 ). When the determined result at step  61  is Yes, picture compressing means  24  performs the intra-frame compressing process for the current macro-block (at step  62 ). When the determined result at step  61  is No, picture compressing means  24  performs the inter-frame compressing process for the current macro-block (at step  63 ). 
     Next, encoding controlling means  35  determines whether or not all the macro-blocks of the current frame have been compressed (at step  64 ). When the determined result at step  64  is No, the flow returns to step  61 . When the determined result at step  64  is Yes, the picture compressing means  24  completes the normal compressing process. The threshold value  a  is an optimum value that depends on the size of the picture, the frame rate of the picture, the bit rate of the communication line, and the performance of the compressing apparatus. 
     Next, with reference to a flow chart shown in FIG. 3, the special compressing process at step  54  shown in FIG. 2B will be explained. Encoding controlling means  35  determines whether or not the current frame is the first frame after a scene change (at step  71 ). When the determined result at step  71  is Yes, encoding controlling means  35  determines whether or not the total (the total of absolute values or the square-sum) of difference values between the macro-block searched by motion estimating means  26  and the current macro-block exceeds the threshold value  a  (at step  72 ). When the determined result at step  72  is Yes, encoding controlling means  35  performs the special intra-frame compressing process for the current macro-block (at step  73 ). 
     As will be explained later, in the special intra-frame compressing process for macro-blocks performed in the first frame after the scene change, higher-spacial-frequency components are deleted. Thus, the code amount of the scene change frame can be decreased. 
     When the determined result at step S 72  is No, encoding controlling means  35  performs the inter-frame compressing process for the current macro-block (at step  74 ). The inter-frame compressing process will be explained later. After encoding controlling means  35  has completed either the process at step  73  or the process at step  74 , encoding controlling means  35  determines whether or not all the macro-blocks of the current frame have been compressed (at step  75 ). When the determined result at step  75  is No, the flow returns to step  72 . When the determined result at step  75  is Yes, encoding controlling means  35  completes the special compressing process. 
     When the determined result at step  71  is No, encoding controlling means  35  determines whether or not the special intra-frame compressing process has been performed for the first frame after the scene change and in addition motion estimating means  26  has omitted the motion estimating process (at step  76 ). When the determined result at step  76  is Yes, encoding controlling means  35  performs the special inter-frame compressing process for the current macro-block (at step  77 ). When the determined result at step  76  is No, encoding controlling means  35  performs the inter-frame compressing process for the current macro-block (at step  78 ). As will be explained, in the special inter-frame compressing process at steps  73  and  77 , encoding controlling means  35  performs the inter-frame compressing process in such a manner that a scene change picture that has more higher-spatial-frequency components than the preceding frame is treated as the next frame. Thus, the picture quality can be quickly restored without requiring to use a special means on the reproducing side. 
     After encoding controlling means  35  has completed either the process at step  77  or the process at step  78 , encoding controlling means  35  determines whether or not all the macro-blocks of the current frame have been compressed (at step  79 ). When the determined result at step  79  is No, the flow returns to step  76 . When the determined result at step  79  is Yes, encoding controlling means  35  determines whether or not the number of macro-blocks that have been compressed in the special inter-frame compressing process is zero (at step  80 ). When the determined result at step  80  is Yes, apparatus controlling means  21  causes encoding controlling means  35  not to perform the special compressing process from the next frame (at step  81 ). When the determined result at step  80  is No, encoding controlling means  35  completes the special compressing process. 
     In the special compressing process shown in the flow chart of FIG. 3, macro-blocks in a scene change frame that are compressed in the inter-frame compressing process are compressed in the normal compressing process. However, the quantization step of quantizing means  29  can be increased in order to decrease a code amount. Although the normal compressing process or the special compressing process is selectively performed for each macro-block in the embodiment explained, there is another embodiment in which all the macro-blocks of the first frame after the scene change are compressed in the special intra-frame compressing process and all the macro-blocks in the second and later frames are compressed in the special inter-frame compressing process. 
     Next, with reference to flow charts shown in FIGS. 4A and 4B, the inter-frame compressing process at step  63  shown in FIG.  2 C and the intra-frame compressing process at step  62  shown in FIG. 2C will be explained. In the inter-frame compressing process for a macro-block, as shown in FIG. 4A, predicting means  27  calculates the difference between YCrCb data of a current macro-block and YCrCb data of a macro-block in a reference frame that has been motion-compensated and that has been stored in reference frame portion  31  (at step  91 ). Thereafter, DCT means  28  performs the DCT process for the difference and obtains frequency components (at step  92 ). Next, quantizing means  29  quantizes the frequency components (at step  93 ). Variable length code encoding means  30  encodes the quantized data into variable length code and transmits the resultant code to the communication line (at step  94 ). 
     Inversely quantizing means  34  inversely quantizes the quantized data received from quantizing means  29  (at step  95 ). Inverse DCT means  33  performs the inverse DCT process for the data received from inversely quantizing means  34  (at step  96 ). Motion compensating means  32  adds difference data received from inverse DCT means  33  to YCrCb data of a macro-block of the preceding/following frame that has been motion-compensated and stores the resultant data as a macro-block of the next reference frame in the reference frame portion  31  (at step  97 ). 
     Next, with reference to a flow chart shown in FIG. 4B, the intra-frame compressing process for a macro-block will be explained. DCT means  28  performs the DCT process for YCrCb data of the current macro-block (at step  101 ). Quantizing means  29  quantizes frequency components and obtains quantized data (at step  102 ). Thereafter, variable length code encoding means  30  encodes the quantized data into variable length code and transmits the resultant code to the communication line (at step  103 ). 
     Inversely quantizing means  34  inversely quantizes the quantized data (at step  104 ). Inverse DCT means  33  performs the inverse DCT process for the data received from inversely quantizing means  34  (at step  105 ). A macro-block received from inverse DCT means  33  is stored as a macro-block of the next reference frame in reference frame portion  31  (at step  106 ). 
     Next, scene change determining process at step  51  shown in FIG. 2B will be explained. There are two kind of scene change determining process as shown in FIGS. 5A and 5B. 
     In the first scene change determining process shown in FIG. 5A, encoding controlling means  35  calculates the total (the total of absolute values or the square-sum) of the difference values between a macro block in the current frame and a macro block in the reference frame which is compensated corresponding to the motion of macro-blocks searched by motion estimating means  26  (at step  111 ). Encoding controlling means  35  determines whether or not the total of difference values exceeds a predetermined threshold value α (at step  112 ). When the determined result at step  112  is No, the flow advances to step  116 . When the determined result at step  112  is Yes, encoding controlling means  35  counts up the number of macro-blocks (at step  113 ). Thereafter, encoding controlling means  35  determines whether or not the number of macro-blocks counted up at step  113  exceeds a predetermined threshold value β(at step  114 ). When the determined result at step  114  is Yes, encoding controlling means  35  determines that a scene change is taking place (at step  115 ). 
     When determined result at step  114  is No or when the determined result at step  112  is No, encoding controlling means  35  determines whether or not all the macro-blocks of the current frame have been completed (at step  116 ). When the determined result at step  116  is No, the flow returns to step  111 . When the determined result at step  116  is Yes, encoding controlling means  35  determines that a scene change is not taking place (at step  117 ). 
     Next, with reference to the flow chart shown in FIG. 5B, the second scene change determining process will be explained. Encoding controlling means  35  compresses one macro-block so as to obtain a code amount rather than performing a real compressing process (at step  121 ). Encoding controlling means  35  obtains a code amount (co) and the number of macro-blocks (mn) that have been compressed (at step  122 ) and calculates the average code amount (cm=br/(fr×mc)) per macro-block corresponding to the frame rate (fr) of the picture data, the bit rate (br) of the communication line, and the number (mc) of macro-blocks in one frame (at step  123 ). 
     Thereafter, encoding controlling means  35  determines whether or not the ratio (co/(mn×cm)) of the real code amount (co) to the calculated average code amount (mn×cm) exceeds a predetermined threshold value γ (at step  124 ). Normally, the threshold value γ is one or greater. When the ratio (co/(mn×cm)) exceeds the threshold value γ, encoding controlling means  35  determines that a scene change is taking place(at step  125 ). 
     When the determined result at step  124  is No, encoding controlling means  35  determines whether or not all the macro-blocks of the current frame have been completed (at step  126 ). When the determined result at step  126  is No, the flow returns to step  121 . When the determined result at step  126  is Yes, the encoding controlling means  35  determines that a scene change is not taking place (at step  127 ). 
     The threshold values α, β, and γ are optimum values that depend on the size of the picture, the frame rate of the picture, the bit rate of the communication line, and the performance of the compressing apparatus. 
     Next, with reference to flow charts shown in FIGS. 6A and 6B, the special inter-frame compressing process at step  75  shown in FIG.  3  and the special intra-frame compressing process at step  74  shown in FIG. 3 will be explained, respectively. 
     In the special inter-frame compressing process for macro-blocks, frequency components that were outputted from DCT means  28  when the special intra-frame compression was executed and have been stored in reference frame portion  31  since then are read out from reference frame portion  31  and spacial frequency components included in wider band than the preceding frame are validated and processed by inverse DCT means  33  to generate a macro block regarded as the current macro-block (hereinafter referred to as the pseudo current macro-block) (at step  131 ). Thereafter, predicting means  27  calculates the difference between YCrCb data of the pseudo current macro-block and YCrCb data of a macro-block at the same position in the reference frame stored in reference frame portion  31  (at step  132 ). 
     Thereafter, DCT means  28  performs the DCT process for the difference data calculated at step  132  (at step  133 ). Quantizing means  29  quantizes data received from DCT means  28  and obtains quantized data (at step  134 ). Variable length code encoding means  30  encodes the quantized data into variable length code and transmits the resultant code to the communication line (at step  135 ). Inversely quantizing means  34  inversely quantizes the quantized data (at step  136 ). 
     Inverse DCT means  33  performs the inverse DCT process for the inversely quantized data (at step  137 ). Motion compensating means  32  adds the difference data received from inverse DCT means  33  and YCrCb data of a reference macro-block of a reference frame stored in reference frame portion  31  and stores the resultant data as a macro-block of the next reference frame in reference frame portion  31  (at step  138 ). 
     Next, encoding controlling means  35  determines whether any of two conditions is satisfied (at step  129 ). The two conditions are that the total (the total of absolute values or the square-sum) of the difference data exceeds the threshold value  b  and that all the spatial frequencies are validated (at step  139 ). When the determined result at step  139  is Yes, encoding controlling means  35  causes motion estimating means  26  to search the motion of the macro-block at the current  25  position (at step  140 ). This operation triggers normal encoding process. When the determined result at step  139  is No, encoding controlling means  35  completes the special inter-frame compressing process. 
     The threshold value  b  is an optimum value that depends on the size of the picture, the frame rate of the picture, the bit rate of the communication line, and the performance of the compressing apparatus. 
     Next, with reference to the flow chart shown in FIG. 6B, the special intra-frame compressing process for a macro-block will be explained. DCT means  28  performs the DCT process for YCrCb data of the current macro-block. The resultant data is stored in reference frame portion  31 . Thereafter, values of higher-spatial-frequency components are set to zero. Thus, only the lower-spatial-frequency components are validated (at step  151 ). 
     Thereafter, quantizing means  29  quantizes data received from DCT means  28  and obtains quantized data (at step  152 ). Variable length code encoding means  30  encodes the quantized data into variable length code and transmits the resultant code to the communication line (at step  153 ). In addition, inversely quantizing means  34  inversely quantizes the quantized data (at step  154 ). Inverse DCT means  33  performs the inverse DCT process for the inversely quantized data (at step  155 ). A macro-block consisting of the resultant data is stored as a macro-block of the next reference frame in reference frame portion  31  (at step  156 ). Thereafter, encoding controlling means  35  causes motion estimating means  26  to omit the motion estimating process for the macro-block at the current position (at step  157 ). 
     EXAMPLE 
     Next, with reference to the accompanying drawings, an example of the present invention will be explained. 
     FIGS. 7A,  7 B,  7 C,  7 D,  8 A,  8 B,  8 C,  8 D, and  8 E show an example of a case that a frame with a scene change is compressed. In this example, a background is not changed, while a picture of the face of a person is replaced by a picture of the face of another person in the n-th frame. Since a scene change takes place in the n-th frame, macro-blocks from the n-th frame to the (n+2)-th frame are partly compressed in the special compressing process. Macro-blocks compressed in the special compressing process are those compressed in the intra-frame compressing process in the n-th frame. 
     FIG. 7A shows an example of the distribution of macro-blocks that are normally or specially compressed. In FIG. 7A, one frame is composed of 96 pixels in vertical direction by 128 pixels in horizontal direction. In other words, six macro-blocks in vertical direction by eight macro-blocks in horizontal direction are arranged in one frame. In FIG. 7A, it is assumed that macro-blocks in black are specially compressed. 
     FIG. 7B shows valid spatial frequency components of data that has been DCT processed after a scene change. As shown in FIG. 7B, it is assumed that in the first frame, 0-th to 14-th components are valid; in the second frame, 0-th to 28-th components are valid; in the third frame, 0-th to 53-rd components are valid; and in the fourth or later frame, all the spatial frequency components are valid. 
     FIGS. 7C and 7D show examples of encoding macro-blocks that are specially compressed in each frame. In FIG. 7C, in the n-th frame at a scene change, the intra-frame compressing process is performed where 0-th to 14-th spatial frequency components are validated. In the next (n+1)-th frame, the inter-frame compressing process is performed by using macro-frames in the n-th frame where 0-th to 28-th spatial frequency components are validated and macro-blocks of which preceding codes are decoded. 
     In the next (n+2)-th frame, the inter-frame compressing process is performed by using macro-blocks in the n-th frame where 0-th to 53-rd spatial frequency components are validated and macro-blocks of which preceding codes are decoded. However, for macro-blocks  11 ,  12 , and  45  shown in FIG. 7A, when the difference between a decoded macro-block and a macro-block having increased valid spatial frequency components is treated as 0, the normal inter-frame compressing process is performed rather than the special compressing process. 
     The encoded macro-blocks shown in FIG. 7D are slightly different from those shown in FIG.  7 C. For macro-blocks  11 ,  12 , and  45  shown in FIG. 7A, valid spacial frequency components are increased to those of (n+2)-th frame in the (n+1)-th frame because the difference between a decoded macro-block and a macro-block having increased valid spacial frequency components is treated as zero in the (n+2)-th frame. The inter-frame compressing process is performed with macro-blocks in the n-th frame where 0-th to 53-rd spatial frequency components are validated and macro-blocks of which the preceding codes are decoded. 
     Although only macro-blocks in a scene change frame which are judged to be specially compressed are specially compressed in the case of FIGS. 7C and 7D, all the macro-blocks in the scene change frame may be specially compressed. 
     Next, an example shown in FIGS. 8A,  8 B,  8 C,  8 D, and  8 E will be explained. FIG. 8A shows blocks of original frames. FIG. 8B shows macro-blocks P ix  of which higher-spatial-frequency components are deleted. FIG. 8C shows macro-blocks P′ ix  of a reference frame. In the n-th frame at the scene change, the macro-blocks P nA  in the n-th frame having 0-th to 14-th spatial frequency components are compressed in the intra-frame compressing process. 
     In the (n+1)-th frame, the inter-frame compressing process is performed by using the macro-block P′ nA  obtained from macro-block P nA  and the macro-block P nB  in the n-th frame where the 0-th to 28-th spatial frequencies are validated. In the next (n+2)-th frame, the inter-frame compressing process is performed by using the macro-block P′ nB  obtained from the macro-block PnB and the macro-block P nC  in the n-th frame where 0-th to 53-rd spatial frequency components are validated. In the next (n+3)-th frame, the inter-frame compressing process is performed by using the macro-block P′ nC  obtained from the macro-block PnC and the macro-block Pn+3 in the (n+3) -th frame. 
     Thus, as shown in FIG. 8D, the special intra-frame code of the macro-block P nA  which contains only lower-spatial-frequency components and is in the n-th frame is outputted in the n-th frame at the scene change. The special inter-frame code obtained from the macro-block P nB  having spatial frequency range extending higher than the preceding frame and the decoded macro-block P′ nA  is outputted in the (n+1)-th frame. The special inter-frame code obtained from the macro-block P nC  having spatial frequency range extending higher than the preceding frame and the decoded macro-block P′ nB  is outputted in the (n+2)-th frame. The inter-frame code obtained from the macro-block P n +3 in the (n+3)-th frame and the decoded macro-block P′ nC  is outputted in the (n+3)-th frame. 
     Thus, as shown in FIG. 8E, on the reproducing side, the decoded macro-block P′ nA  is displayed in the n-th frame, the decoded macro-block P′ nB  is displayed in the (n+1)-th frame, the decoded macro-block P′ nC  is displayed in the (n+2)-th frame, and the decoded macro-block P′ n+3  is displayed in the (n+3)-th frame. 
     In other words, a still picture of the face of the second person is displayed in three frames while gradually increasing resolution. Thereafter, the motion of the face emerges. 
     According to the present invention, the code amount of a frame at a scene change can be decreased because higher-special-frequency components in macro-blocks encoded by the special intra-frame compression process is decreased. In addition, picture quality quickly recovers after the scene change without need any special means at the reproducing side because a macro-block obtained by executing inverse DCT for spatial frequency components in gradually increasing range of a macro-block in the frame at the scene change is regarded as the current macro-block and the special inter-frame compression process is executed on the macro-block regarded as the current macro-block. 
     Although the present invention has been shown and explained with respect to the best mode embodiment thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions, and additions in the form and detail thereof may be made therein without departing from the spirit and scope of the present invention.