318 PART IV / STANDARDS

Table 16.4 Selected Code Option Identification Key

Option n _< 8

8<n<16

16<n

u

Zero-Block 0000

Second-Extension 0001

FS 001

k -- 1 010

k=2 011

k = 3 100

k = 4 101

k=5 110

k=6

k---7

k--8

k=9 m

k=10

k=ll

k=12

k=13

k=14

k=15

9 o

9 o

k=29

No compression 111

00000 000000

00001 000001

0001 00001

0010 00010

0011 00011

0100 00100

0101 00101

0110 00110

0111 00111

1000 01000

1001 01001

1010 01010

1011 01011

1100 01100

1101 01101

1110 01110

01111

10000

11110

1111 11111

Note:

A dash indicates no applicable value.

16.4 PREPROCESSOR

The role of the preprocessor is to transform the data into samples that can be more efficiently

compressed by the entropy encoder. To ensure that compression is "lossless," the preprocessor

must be reversible. The FS and sample-split options work on non-negative integer values. The

most effective preprocessor will transform integer data values into all non,negative values with

a unimodal distribution resembling a one-sided Lapace function. The example in Section 16.3.1

shows that to achieve effective compression, we need a preprocessing stage that transforms the

original data so that shorter codewords occur with higher probability than longer codewords. If

there are several candidate preprocessing stages, we would like to select the one that produces

the shortest average codeword length.

In general a preprocessor that removes correlation between samples in the input data block

will improve the performance of the entropy coder. In the following discussion, we assume that

preprocessing is done by a predictor followed by a prediction error mapper. For some types of

data, more sophisticated transform-based techniques can offer improved compression efficiency,

at the expense of higher complexity.

16.4.1 Predictor

The decorrelation function of the preprocessor can be implemented by a judicious choice of

predictor for an expected set of data samples. The selection of a predictor should take into

account the expected data as well as possible variations in the background noise and the gain of

CHAPTER 16 / THE CCSDS RECOMMENDATION FOR SPACE APPLICATIONS 319

x(i- 1, j-

1)

x(i- 1, j)

x(i, j-

1)

x(i, j)

FIGURE 16.4

Typical pixel data arrangement.

scan line i- 1

scan line i

Prediction Error

Ai

Delay

A

Predictor

xi

FIGURE 16.5

Preprocessor using a unit-delay predictor.

Prediction

Error

Mapper

Preprocessed

Samples

8i

,,..._

the sensors acquiring the data. The predictor should be chosen to minimize the amount of noise

resulting from sensor non-uniformity.

Given the data arrangement in Fig. 16.4, the predictor value

2(i, j)

of input value

x(i, j)

can be

(a) A one-dimensional first-order predictor:

2(i, j) = x(i, j -

1), or

2(i, j) = x(i -

1, j).

(b) A two-dimensional predictor: ~(i, j) =

1/2[x(i, j -

1) +

x(i

- 1, j)].

(c) A two-dimensional predictor: ~(i, j) =

1/813x(i, j -

1) +

3x(i - 1, j)

+ 2x(i- 1, j-

1)].

Other types of weighted predictor can be devised depending on applications. For multispectral

data, another prediction technique is to use samples from one spectral band as an input to a higher

order prediction function for the next band.

One of the simplest predictive coders is a linear first-order unit-delay predictor shown in

Fig. 16.5. The output, Ai, will be the difference between the input data sample and the preceding

data sample.

16.4.2 Reference Sample

It is clear from Fig. 16.5 that for a reversible operation, a reference sample is needed to perform the

first prediction. A reference sample is an unaltered data sample upon which successive predictions

are based. Reference samples are required by the decoder to recover the original values from

difference values. In cases where a reference sample is not used in the preprocessor or where

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