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Image data compressing apparatus
| Details |
Inventors: Watanabe, Tohru;
Assignee: Casio Computer Co., Ltd. (Tokyo, JP)
Primary Examiner: Britton; Howard W.
Assistant Examiner:
Attorney, Agent or Firm: Frishauf, Holtz, Goodman & Woodward
An image data compressing apparatus comprises a transform unit, such as a DCT computation unit, a quantization table memory for plural types of quantization tables, a quantizer which selectively uses the quantization tables, a coder for performing coding in accordance with a pre-given predicted value for the amount of generated codes of each block, memories for storing the amounts of codes for DC components and AC components generated for the individual blocks, a computation unit for computing the sum of the amounts of generated codes for DC components and the sum of the amounts of generated codes for AC components for each properly selected quantization table, and memories for storing the computed sums of the amounts of generated codes, wherein the apparatus uses a plurality of quantization tables to predict a change in the amount of generated codes caused by the alteration of the quantization table in the first compression, and compresses image data to the target amount of generated codes in the second compression. |
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DETAILED DESCRIPTION It is therefore an object of the present invention to provide an image data compressing apparatus and image data compressing method capable of controlling the amount of generated codes with less time.
There would be a difference (remaining bits) between the predicted value yielded in the conventional image data compressing apparatus 10 in the above-described scheme and the target amount of generated codes.
The reason for the remainder is as follows.
Actually, perfect division is not possible in the aforementioned computation, yielding a remainder.
When the numeral represented by the remaining bits is rounded, the resultant value may become larger than the actual value by some bits due to a rounding error and the predicted value as a whole would be larger than the intended value by some bits (or some bytes) as a consequence.
Since the predicted value exceeding the target value cannot be allowed, rounding is not actually performed and the remainder in the computation is basically cut off.
If the remainder is to be cut off, as there are over 10,000 blocks, with an average of five bits per block becoming the remainder, for example, over 50,000 bits for the entire blocks would be generated as the remainder.
The conventional image data compressing apparatus 10 is designed to cut off the remaining bits or collect them in the upper left block.
In the first case, this apparatus 10 cannot improve the image quality.
In the second case, if the amount of generated codes is smaller than predicted, the image quality is higher than that obtained in the first case.
If the amount of generated codes is larger than predicted, on the other hand, the amount of codes allocated for each block decreases, thus greatly deteriorating the image quality.
Accordingly, it is another object of the present invention to provide an image data compressing apparatus and image data compressing method capable of properly distributing the remainder of the amount of generated codes to improve the image quality
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Image Analysis 
