JPEG 2000学习笔记 （1）

本文为学习笔记，内容取自JPEG 2000的相关论文。

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Overview of JPEG 2000

The operation flow of a typical JPEG 2000 encoder is shown in Figure 1.

Figure 1. Flowchart for JPEG 2000 Encoder

The encoder starts with a component and tile separation module. After this preprocessing, it applies a wavelet transform which yields a sequence of wavelet coefficients. The encoder next quantizes the wavelet coefficients which are regrouped to facilitate localized spatial and resolution access. The degrees off resolution are organized into subbands, which are divided into non-overlapping rectangular blocks. Three spatially co-located rectangles (one from each subband at a given resolution level) form a packet partition. Each packet partition is further divided into code-blocks, each of which is compressed by a subbitplane coder into an embedded bitstream with a rate-distortion curve that records the distortion and rate at the end of each level of subbitplane.

Main Differences between JPEG 2000 and JPEG

1. Transform module: wavelet versus DCT. JPEG uses 8×8 discrete cosine transform (DCT), while JPEG 2000 uses a wavelet transform with lifting implementation. The wavelet transform provides not only better energy compaction, but also the resolution scalability.

2. Block partition: spatial domain versus wavelet domain. JPEG partitions the image into 16×16 macroblocks in the spatial domain, and the applies the transform, quantization and entropy coding operation on each block separately. JPEG 2000 performs the partition operation in the wavelet domain. Coupled with the wavelet transform, there is no blocking artifacts in JPEG 2000.

3. Entropy coding module: run-level coefficient coding versus bitplane coding. JPEG encodes the DCT transform coefficients one by one. the resultant block bitstream can not be truncated. JPEG 2000 encodes the wavelet coefficients bitplane by bitplane. The generated bitstream can be truncated at any point with graceful quality degradation. It is the bitplane entropy coder in JPEG 2000 that enables the bitstream scalability.

4. Rate control: quantization module versus bitstream assembly module. In JPEG, the compression ratio and the amount of distortion is determined by the quatization module. in JPEG 2000, the compression ratio and distortion is determined by the bitstream assembly module.

待续