Mumbai University > EXTC > Sem 7 > Data Compression and Encryption

Marks: 10 M

Year: May 2012

1.JPEG-LS is a new standard for the lossless (or near-lossless) compression of continuous tone images. JPEG-LS examines several of the previously seen neighbours of the current pixel, uses them as the context of the pixel, uses the context to predict the pixel and to select a probability distribution out of several such distributions , and uses that distribution to encode the prediction error with a Special Golomb code. There is also a run mode, where the length of a run of identical pixels is encoded.

*Figure 3.4.1 JPEG – LS *

2.The context used to predict the current pixel ‘x’ is shown below:

Figure 3.4.2

3.The encoder examines the context pixels and decides whether to encode the current pixel ‘x’ in the run mode or in the regular mode.

4.If the context suggests that the pixels y,z …. Following the current pixel are likely to be identical, the encoder selects the run mode, and otherwise it selects the regular mode. The rest of the encoding process depends on the mode selected.

5.In the regular mode, the encoder uses the values of context pixels a, b and c to predict pixel x, and subtracts the prediction from x to obtain the prediction error, denoted by Errval . This error is then corrected by a term that depends on the context (this correction is done to compensate for systematic biases in the prediction) and encoded with a Golomb Code.

6.The Golomb coding depends on all 4 pixels of the context and also on prediction errors that were previously encoded for the same context.

7.In the run mode, the encoder starts at the current pixel x and finds the longest run of pixels that are identical to context pixel a.

8.The encoder does not extend this run beyond the end of the current image row. Since all pixels in the run are identical to a (and a is already known to the decoder) only the length of the run needs to be encoded, and this is done with a 32-entry array denoted by J. If near-lossless compression is used, the encoder selects a run of pixels that are done to a within the tolerance parameter NEAR.

9.The decoder is not substantially different from the decoder, so JPEG-LS is nearly symmetric compression method. The compressed stream contains data segments, marker segments and markers.

10.A marker is a byte of all ones followed by a special code, signaling the start of a new segment. If a marker is followed by a byte whose most significant bit is 0, that byte is the start of a marker segment. Otherwise, that byte starts a data segment.

11.Advantages of JPEG-LS

• JPEG-LS is capable of lossless compression.
• JPEG-LS has very low computational efficiency.