An apparatus for decoding an encoded digital image signal having a data signal and an information signal, wherein the data signal includes a plurality of blocks of quantized transformed coefficients, each of the blocks including a DC coefficient and a predetermined number of AC coefficients, and said information signal includes each QS for each of the blocks, includes: an inverse quantizer for performing an inverse quantization on a subjective block with its corresponding QS to produce an inverse quantized DC coefficient and a plurality of inverse quantized AC coefficients; a candidate DC coefficient generator for determining a DC coefficient range and altering the value of the inverse quantized DC coefficient within the range to generate a predetermined number of candidate DC coefficients; a number of inverse discrete cosine transformer for performing inverse discrete cosine transformation on each of the candidate DC coefficients combined with the inverse quantized AC coefficients; and a comparator for comparing each of the results from the IDCT with previously performed IDCT results so as to enable the selection of an IDCT result producing a minimum blocking effect which occurs because of a luminance pixel difference between the previously performed IDCT results and the selected IDCT result.
An image processing apparatus reduces a distortion that occurs at a tile boundary (tile distortion) when an image is compressed or expanded on a tile-by-tile basis. It is equipped with a tone jump detecting section, a slope calculating section, and a slope correcting section. The tone jump detecting section calculates differences between the values of pixels on both sides of the tile boundary and detects a tone jump in the tile distortion according to the differences. The slope calculating section calculates slope-like correction values to be used for causing the tone jump to approach zero in a prescribed length ((prescribed length)<(tile length)), and adds the slope-like correction values to the values of pixels near the tile boundary. As a result, the steep tone jump due to the tile distortion is corrected into a gentle slope-like variation. Tile distortions are reduced properly by this processing.
According to the present invention circuitry is provided for processing digital data items. The circuitry comprises compression and decompression circuitry. The compression circuitry further comprises: a circuit for transforming M number of data items into N number of data items; a circuit for quantising P number of data items and producing Q number of data items; and a circuit for appropriately storing in memory and/or transferring R number of data items. The decompression circuitry comprises: a circuit for appropriately retrieving from memory and/or receiving S number of data items; a circuit for dequantising T number of data items and producing U number of dequantised data items; and a circuit for receiving and inverse transforming V number of data items into W number of data items, said W data items being representative of said M data items.
