Selecting the don't care bits in JPEG2000 ROI coding

Region of interest coding is an important feature in JPEG2000 and it is accomplished by de-emphasizing the wavelet coefficients associated with the non-ROI regions of the image. In the general scaling-based method, a number of extra bits appear below the least significant bit of the ROI samples after the shifting process. These bits need be coded at the time of bit-plane coding, but they are discarded by the decoder. The usual procedure is for the encoder to set these "don't care" bits to zero. In this paper, we propose a method that exploits the state of the JPEG2000 entropy coder to set the values of these bits in a more intelligent way. The method has been observed to reduce the number of bits required to represent ROI code-block by up to 7%, with the bit-stream remaining JPEG2000 compliant.