Image sub-sampling apparatus and method

Abstract: A quantization coefficient is inputted to an inverse quantization circuit. The coefficient is multiplied by a quantization step size stored in a quantization table. Resultant data is delivered to an inverse discrete cosine transformation circuit. The quantization step size is attained by replacing each value of a high-frequency portion of the quantization table with "0" by masking means. In consequence, each value of inversely quantized data associated with the portion is set to "0". Data inversely transformed according to a transformation expression is fed to a sub-sampling circuit. Based on an n specification signal, the data is sub-sampled such that a dot is extracted for every n dots, thereby generating a compressed image signal. In accordance with the present invention, without using an expensive low-pass filter, the aliasing noise can be presented thanks to the masking of the quantization table. In addition, the sub-sampling ratio can be selected.

Description: BACKGROUND OF THE INVENTION The present invention relates to an image sub-sampling sub-sampling apparatus, and in particular, to an image apparatus for use in an operation to receive and to reproduce an image encoded according to a discrete cosine transformation and a linear quantization for decoding the image. The invention further relates to a sub-sampling method. Description of the Related Art Heretofore, data represented by image signals to be transmitted or to be stored on a storage medium such as a memory card or a magnetic disk is commonly compressed to have a reduced data capacity in consideration of a limited capacity of the storage device or the like. In this case, the image data compression is carried out as follows. An image is subdivided into a plurality of blocks such that data of pixels constituting each subdivided block is transformed, for example, through a discrete cosine transformation. Transformation coefficients resultant from the operation are finally quantized so as to encode the data. The image data thus encoded is subjected, in reception and reproduction phases thereof, to an inverse quantization and an inverse discrete cosine transformation so as to be decoded into the original signal. In the decoding step, to obtain a minimized image signal, there is achieved an operation to sub-sample signals constituting the image. As the conventional method of sub-sampling image signals, there has been used a method in which the sub-sampling operation is effected in the unit of n dots after the inverse discrete cosine transformation, a method in which the n-dot sub-sampling operation is conducted after the signals are passed through a low-pass filter, or a method in which only the direct-current (dc) components of the encoded data are subjected to the inverse quantization so as to produce an output signal without achieving the inverse discrete cosine transformation. Referring now to Figs. 1 to 3, description will be given of image sub-sampling apparatuses respectively of these three types. Fig. 1 shows a first example of the conventional image sub-sampling system. The apparatus of Fig. 1 conducts only the n-dot sub-sampling operation above. As can be seen from this diagram, the system includes an inverse quantization circuit 11, a quantization table 12, an inverse discrete cosine transformation circuit 13, and a sub-sampling circuit 14. The circuit 11 receives as an input thereto a quantization coefficient Sq configured in an 8-element by 8-element matrix to inversely quantize the coefficient Sq. In the operation, a quantization step size Q stored in the quantization table 12 is read therefrom to be multiplied by the coefficient Sq, thereby producing data R. The obtained data R is inputted to the circuit 13, which then conducts an inverse discrete cosine transformation for the data R to thereby create output data r. The data r is fed to the circuit 14 to be sujected to an n-dot sub-sampling operation, thereby generating conpressed image signal S 2 . In the apparatus constituted as above, the quantization coefficient Sq (V,U) in the form of an (8×8) matrix is first delivered to the circuit 11. The quantization step size Q(V, U) registered to the table 12 is multiplied by each element of the matrix so as to produce inversely quantized data R(V, U). The data R(V, U) is then supplied to the circuit 13 to undergo an inverse transformation such that the resultant data is inputted to the circuit 14. The data r(V, U) produced from the circuit 13 is subjected an n-dot sub-sampling operation in the circuit 14 thereby creating compressed image data S 2 . For example, assuming n = 4, there are produced four items r(0, 0), r(0, 4), r(4, 0), and r(4, 4). Fig. 2 shows a second example of the image sub-sampling device of the prior art. In this system, data is delivered to a low-pass filter such that the resultant data is subjected to the n-dot sub-sampling operation. This configuration is different from that of Fig. 1 in that a low-pass filter is disposed between the circuits 13 and 14. The filter 16 reduces the size of a frequency band of the data r(V, U) to 1/n thereof. The system of Fig. 2 operates substantially in the same fashion as the apparatus of Fig. 1 excepting that the data r(V, U) is passed through the filter 16. Fig. 3 shows a third example of the conventional image sub-sampling device. As can be seen from the diagram, in this system, the inversely quantized data produced from the circuit 11 is fed to a dc component extracting circuit 17 to extract dc components therefrom so as to accomplish the image sub-sampling operation without achieving the inverse transformation. This constitution is different from that of Fig. 1 in that the dc extracting circuit 17 is connected to an output terminal of the circuit 11 so that the circuit 17 produces a compressed image signal S 2 . In this operation, only a dc component R(0, O) is extracted from the coefficient R(V, U) so as to deliver the obtained data as the compressed image S 2 . Of these apparatuses, the apparatus of Fig. 1 achieving only the n-dot sub-sampling operation is attended with an aliasing noise. Moreover, the system of Fig. 2 includes the low-pass filter to prevent the aliasing noise so as to achieve the n-dot sub-sampling operation after the data is passed through the filter 16. This consequently leads to a drawback that the apparatus is expensive due to adoption of the filter 16. In addition, in the device of Fig. 3, only the dc components are extracted from the inversely quantized data, thereby achieving the sub-sampling operation without conducting the inverse transformation. The system however has a disadvantage that the sub-sampling ratio is fixed. Furthermore, the apparatus is advantageous in that the operation time is reduced when software processing is employed for the operation. However, in a large scale integrated circuit including a circuit dedicated to the discrete cosine transformation and the inverse discrete cosine transformation circuit, the processing time cannot be reduced and hence the above advantage is obtained even when the configuration of Fig. 3 is adopted. EP-A-0,447,269 discloses an image data processing system. Inverse quantizing is therein performed by a dequantizer which determines the number of significant questions in a block and outputs an insignificant block signal zero to a DCT unit if no significant coefficient exists. For significant coefficients inverse quantizing is as usual performed by multiplying the quantized coefficients by corresponding values provided by a quantization table. JP-A-04,097,678 discloses a picture decoding system. In this system transformation coefficients corresponding to high-frequency components are made zero and inverse conversion is then implemented by a same block size in the case of coding and low pass filtering. Finally the output is reduced by subsampling. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an image sub-sampling apparatus capable of preventing an aliasing noise without necessitating such an expensive device as a low-pass filter, as well as a sub-sampling method. Another object of the present invention is to provide an image sub-sampling apparatus capable of selecting an image sub-sampling ratio. In accordance with the present invention, there is provided an image sub-sampling apparatus as defined in independent claim 1, Claims 2 to 3 define particular embodiment thereof. The method is described in claim 4. BRIEF DESCRIPTION OF THE DRAWINGS The objects and features of the present invention will become more apparent from the consideration of the following detailed description taken in conjunction with the accompanying drawings wherein: Fig. 1 is a schematic block diagram showing a first example of the conventional image sub-sampling apparatus; Fig. 2 is a block diagram illustratively showing a second example of the image sub-sampling apparatus of the prior art; Fig. 3 is a schematic block diagram showing a third example of the conventional image sub-sampling apparatus; and Fig. 4 is a block diagram showing an embodiment of the image sub-sampling apparatus in accordance with the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 4 schematically shows the constitution of an embodiment of the image sub-sampling apparatus in accordance with the present invention. As can be seen from the diagram, the system includes an inverse quantization circuit 21, a quantization table 22 containing values of a quantization step size in the format of a table for an inverse quantization, masking means 23 for masking the table 22, an inverse discrete cosine transformation circuit 24, and a sub-sampling circuit 25. The circuit 21 references the contents of the table 22 to conduct a linear inverse quantization for a quantization coefficient Sq so as to produce inversely quantized data R. The means 23 replaces each value of a portion of the table 22, primarily, a high-frequency portion with "0", thereby achieving the masking operation. As a result, of the data R produced from the circuit 21, each value of data corresponding to the portion masked by the table 22 is set to "0". Moreover, the data inversely quantized by the circuit 21 is fed to the circuit 24 to undergo an inverse discrete cosine transformation, thereby generating a restored image signal S 1 . The signal S 1 is then fed to the circuit 25. According to an n specification signal, the signal S 1 is subjected to an n-dot sub-sampling operation in the circuit 25. Namely, in each of the vertical and horizontal directions, the data is sub-sampled such that every n dots are reduced to one dot, thereby producing a compressed image signal S 2 . In operation. a quantization coefficient Sq(V, U) in the form of an (8 ×8) matrix is delivered to the circuit 21. A quantization step size A(V, U) stored in the table 22 is multiplied by each element of the matrix to create inversely quantized data R(V, U). The size Q(V, U) of the table 22 is obtained by substituting each value of the high-frequency portion of the table 22 for "0" by the masking means 23. Each value of the inverse quantization data corresponding to the portion replaced by "0" becomes to be "0". The obtained data R(V, U) is inputted to the circuit 24 to undergo an inverse transformation according to the following expression (1) such that the resultant data is fed to the sub-sampling circuit 25. r(i,j)= 1 4 ΣΣC( u )C( v )·cos (2i+1) u π 16 ·cos (2j+1) v π 16 · R( u , v ) where, c( u )= 1/√2 for u = 0 and c( u )= 1 for u ≠ 0, c( v )= 1/√2 for v = 0 and c( v )= 1 for v ≠ 0, r(i, j) indicates an output pixel, and R( u , v ) denotes a DCT coefficient. In the circuit 25, the data r(V, U) is sub-sampled depending on the n specification signal. Namely, in the vertical and horizontal directions, one dot is acquired as an output for every n dots. For example, when the value of n is two, there are extracted from the data r(V, U) data items represented as V = 2M and U = 2N (M, N, = 0, 1, 2, 3). Resultantly, the reduced image signal S 2 is created from the circuit 25. Description will now be given of the masking means 23 of the embodiment. In this means 23, according to the n specification signal supplied thereto, each value of the data area beyond 1/n of the original size of the table 22 in the vertical and horizontal directions is replaced with "0". For example, in a case where n = 2, each value of a portion of Q(V, U) represented with V ≧ 4 and U ≧ 4 is substituted for "0". With this provision, the operation procedure to delete the high-frequency components can be altered according to the sub-sampling ratio. As above, in the apparatus, a portion of the quantization table 22 is masked to set each value of a high-frequency portion of the table 22 to "0" such that the inverse quantization is carried out according to the contents of the table 22, thereby preventing occurrence of the aliasing noise. In consequence, without using such an expensive device as a low-pass filter, which has been necessitated in the prior art, it is possible to prevent the aliasing noise from being generated. Furthermore, in the conventional system shown in Fig. 3, the dc components are extracted from the inversely quantized data to be outputted without conducting the inverse transformation. Consequently, the image sub-sampling ratio cannot be selected. However, according to the image sub-sampling apparatus of the present invention, either one of the sub-sampling ratios 1/2, 1/4, and 1/8 can be selected according to the n specification signal. While the present invention has been described with reference to the particular illustrative embodiment, it is not to be restricted by the embodiment but only by the appended claims.

Claims: An image sub-sampling apparatus operative in an encoding operation of image data encoded by a discrete cosine transformation and a linear quantization for sub-sampling the image data, comprising: inverse quantization means (21) for linearly and inversely quantizing a quantized coefficient according to a quantization table (22) and thereby producing inversely quantized data; inverse discrete cosine transformation means (24) for conducting an inverse discrete cosine transformation for the data created by the inverse quantization means (21); and sub-sampling means (25) for sub-sampling the data generated from the inverse discrete cosine transformation means (24) to obtain a dot for every n dots in each of the vertical and horizontal directions of the data and thereby producing an image signal, the quantization table including a high-frequency portion, characterized by further comprising: masking means (23) for masking said portion of said quantization table to set each value of the portion to "0", wherein the masking means masks, according to a specification signal inputted thereto, an area beyond the n-th column and the n-th row of the quantization table (22) in the vertical and horizontal directions of the table. thereby setting each value of the area to "0". An apparatus as claimed in claim 1, wherein the sub-sampling means (25) receives a sub-sampling ratio specification signal to conduct the sub-sampling operation according to the sub-sampling ratio specification signal, thereby producing the image signal. An apparatus as claimed in claim 1, wherein the sub-sampling means (25) is responsive to the input of the sub-sampling ratio specification signal for selecting either one of sub-sampling ratios 1/2, 1/4, and 1/8. A method of sub-sampling image data encoded by a discrete cosine transformation and a linear quantization, said method comprising the steps of: providing a quantization table (22); linearly and inversely quantizing a quantized coefficient according to said quantization table (22) and thereby producing inversely quantized data; conducting an inverse discrete cosine transformation for the data created by the inverse quantization; and sub-sampling the data generated by the inverse discrete cosine transformation to obtain a dot for every n dots in each of the vertical and horizontal directions of the data and thereby producing an image signal, the quantization table including a high-frequency portion characterized by the steps of: masking a portion of said quantization table to set each value of said portion to "0", wherein the masking is performed according to a specification signal which specifies that an area beyond the n-th column and the n-th row of said quantization table is to be set to "0".

Inventors: KINOUCHI SHIGENORI, SAWADA AKIRA, KINOUCHI, SHIGENORI, SAWADA, AKIRA
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1993
Main classification: H04N7
IPC codes: G06T9, H04N7, H03M7, H04N1