Fast scaling of JPEG images

Details Fast scaling of JPEG images Fast scaling of JPEG images

1997-09-11

2000-05-23

Tran, Phuoc

Image analysis

Image compression or coding

Transform coding

382233, 382299, 382300, G06K 936, G06K 946

Patent

active

060673842

ABSTRACT:
An image scaling system that enlarges images during JPEG encoding (400) and reduces images during JPEG decoding (500). The image scaling system uses N-point forward and inverse one-dimensionial scaled discrete cosine transforms (DCTs), where N is selected from among 1, 2, 3, 4, and 6. When encoding a source image (110) to an enlarged JPEG image (422), the system partitions the source image into N.times.N blocks. Each N.times.N block is transformed using the N-point scaled DCT (410). The system modifies quantization tables (418) to account for the scale of the transform and the increase in size of the image and quantizes (412) the blocks of N.times.N scaled cosine coefficients using the modified quantization tables. The resulting N.times.N blocks are enlarged (414) to 8.times.8 blocks by padding each block with coefficients having values of zero. When decoding a JPEG image (510) to a reduced output image (518), the system retrieves 8.times.8 blocks of quantized cosine coefficients from the JPEG image. The system reduces (512) each block to an N.times.N block of quantized cosine coefficients. The system modifies the quantization tables (520) retrieved from the JPEG image to account for the scale factor and the decrease in size of the image, then quantizes (514) the N.times.N blocks to produce N.times.N blocks of scaled cosine coefficients. The system performs an N-point scaled inverse DCT (516) on each block of scaled cosine coefficients. The results of the inverse DCT form the reduced output image (518). The forward and inverse DCTs are performed using efficient processes that require relatively few calculations to achieve the desired result.

REFERENCES:
patent: 5708732 (1998-01-01), Merhav et al.
William B. Pennebaker and Joan L. Mitchell; "JPEG Still Image Data Compression Standard"; 1993; pp. 1-638.

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