| Summary: | The JPEG standard allows four modes of operation. They are the hierarchical (HJPEG),
progressive (PJPEG), sequential (SJPEG), and lossless modes1: The HJPEG and PJPEG
modes inherently support progressive image coding. In HJPEG, an image is decomposed
into subimages of different resolution, each of which is then coded using one of the other
three modes of JPEG. Progressiveness within a resolution in HJPEG can be achieved
when each subimage is coded using PJPEG. An image coded using PJPEG consists of
scans, each of which contributes to a portion of the reconstructed image quality. While
SJPEG yields essentially the same level of compression performance for most encoder
implementations, the performance of PJPEG depends highly upon the designed encoder
structure. This is due to the flexibility the standard leaves open in designing PJPEG
encoders.
In this thesis, an efficient progressive image, coding algorithm is developed that is
compliant with the JPEG still image compression standard. The JPEG-compliant progressive
image encoder is a HJPEG encoder that employs a rate-distortion optimized
PJPEG encoding algorithm for each image resolution. Our encoder outperforms an op-
timized SJPEG encoder in terms of compression efficiency, substantially at low and high
bit rates. Moreover, unlike existing J P EG compliant encoders, our encoder can achieve
precise rate control for each fixed resolution. Such good compression performance at low
bit rates and precise rate control are two highly desired features currently sought for the
emerging JPEG-2000 standard.
1 Lossless JPEG algorithms are rarely used since their performance levels are significantly lower than
those of other lossless image compression algorithms, and are therefore not widely used.
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