抄録
Compound Images are mixed documents containing textual content along with continuous-tone or multi-level pictures. They are a very common form of documents found in textbooks, magazines, brochures, web sites etc. Because of the extremely distinct statistical nature of textual and multi-level images, compound image compression often involves multiple compression systems and a region segmentation scheme, such as the mixed raster content (MRC) approach. However, the new JPEG 2000 standard desires to have a coding system that is capable of compressing both continuous-tone and bi-level images progressively with similar system resources, and producing high quality images at low bit-rate, for example, below 0.25 bpp. Thus, in this paper, we search another approach, coding compound images with adaptive wavelet transform constructed by the lifting scheme. Though the wavelet transforms prove to be a powerful analysis tool for continuous-tone images, and provide superior performance for progressive transmission, their performance for compound and bi-level images is still not acceptable. The proposed adaptive discrete wavelet transform (ADWT), however, is able to reduce the entropy of bi-level region coefficients greatly while achieve nearly the same efficiency as the conventional transform for multi-level regions, by changing the wavelet basis adaptively according to the contents and embedding some non-linear processing to the transform. When evaluated by the JPEG 2000 verification model software version 0.0 (VM 0), the ADWT attains superior lossless compression ratio for compound and facsimile images versus conventional wavelet transforms, moreover, textual parts in a compound image are perfectly reconstructed at low bit-rate so that image quality of a compound image is improved at the early stage of transmission.
