The present invention relates to a moving image coding apparatus and, more particularly, to a moving image coding apparatus for performing coding operation by performing pre-processing using an intraframe filter.
In moving image compression coding, a moving image coding scheme based on a combination of motion compensation and orthogonal transformation is often used. Image data input to a coding apparatus contains high-frequency components due to noise. To prevent a deterioration in coding efficiency due to such high-frequency components, pre-processing is performed by using an intraframe filter. The above intraframe filter has the effect of removing high-frequency components that impair spatial correlation. The removal of such high-frequency components, however, leads to a decrease in resolution and a deterioration in image quality in a still area, in particular. A scheme for solving this problem is disclosed in, for example, Japanese Patent Laid-Open No. 2-154588. In this scheme, the filter strength is changed for the still and moving areas of a moving image such that a weak filter is used for the still area, and a strong filter is used for the moving area.
FIG. 5 shows a conventional moving image signal coding apparatus. An input image 1 is processed in units of pixel blocks (e.g., 16xc3x9716). A moving area determination circuit 4 compares each pixel of the input image 1 with a corresponding pixel of the previous frame stored in a frame memory 3 to determine a moving or still area. A filter coefficient 19 used by an intraframe filter circuit 2 is output on the basis of the determination result and coding control data 17 output from a coding control circuit 15. The intraframe filter circuit 2 filters the input image 1 by using the filter coefficient 19 and sends the resultant data to a coding section 5.
A subtracter 6 subtracts a predicted image 18 motion-compensated by a motion compensation circuit 14 from the pixel blocks sent to the coding section 5. The resultant data is processed by a DCT (Discrete Cosine Transform) circuit 7 and a quantization circuit 8 and coded by a coding circuit 9. The coded data is stored in the buffer 16. The output from the quantization circuit 8 is processed by a de-quantization circuit 10 and an inverse DCT circuit 11 in the coding section 5 and is added to the predicted image 18 by an adder 12. The resultant data is stored in a frame memory 13.
In the above conventional apparatus, all the pixels even in an area having undergone accurate prediction based on motion compensation are filtered by the strong filter if the area is a moving area. This unnecessarily decreases the resolution of the image.
Furthermore, in the conventional apparatus, the strong filter sometimes works on even an accurately predicted area. The effect of this filter may increase the difference between an input signal and a predicted signal even in such an area where the difference should be sufficiently small, resulting in a decrease in coding efficiency.
The present invention has been made to solve the above problems in the prior art, and has as its object to provide a moving image signal coding apparatus which can improve the coding efficiency using an intraframe filter while minimizing a decrease in the resolution of an image.
In order to achieve the above object, according to the present invention, there is provided a moving image coding apparatus comprising motion compensation type moving image coding means for predicting a pixel block in a current frame from a preceding frame by a motion compensation scheme, and performing orthogonal transformation and quantization for a predicted error, thereby coding the predicted error, intraframe filter means for performing intraframe filtering for each pixel of an input frame, and means for adaptively determining and setting a filter characteristic of the intraframe filter means on the basis of a pixel of the input frame and a corresponding pixel of a predicted image or adaptively selecting an output from the intraframe filter means having an optimal filter characteristic.
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