org.apache.pdfbox.jbig2.decoder.arithmetic

Class CX

java.lang.Object

org.apache.pdfbox.jbig2.decoder.arithmetic.CX

public final class CX
extends Object

Arithmetic coding context used during JBIG2 bitstream decoding.

Represents a context in the arithmetic decoder that selects probability estimates and statistics used during decoding procedures, as defined in ITU-T Rec. T.88 (2000 E), ISO/IEC 14492:2001.

Context State:

• cx array: Stores the probability estimate index (0-127) for each context state
• mps array: Stores the most probable symbol (0 or 1) for each context state
• index: Current context index, selected based on the neighborhood pattern (template-specific arrangement of previously decoded pixels)

Usage: The index is set before each decision to select which context state to use. After decoding, the arithmetic decoder updates the selected context's probability estimate and MPS based on the decoded symbol (see Annex A, T.88).

Reuse: When arithmetic coding contexts are retained and reused across segments (§7.4.2.2), a copy() must be created to avoid sharing mutable probability state between decoders.

Constructor Summary

Constructors

Constructor and Description
CX(int size, int index) Creates a new context with the specified number of context states.

Method Summary

Modifier and Type	Method and Description
CX	copy() Creates and returns a deep copy of this CX object.
void	setIndex(int index) Sets the context index used for subsequent decoding decisions.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

CX

public CX(int size,
          int index)

Creates a new context with the specified number of context states. All probability estimates are initialized to 0, and all MPS values are initialized to 0.

Parameters:

size - the number of context states

index - the initial context index

Method Detail

setIndex

public void setIndex(int index)

Sets the context index used for subsequent decoding decisions. The index selects which context state's probability estimate and MPS will be used.

The index value is typically computed from the neighboring pixels in the template (§6.2.5.1 for generic regions, §6.4.7.1 for text regions).

Parameters:

index - the context index (0 to size-1)

copy

public CX copy()

Creates and returns a deep copy of this CX object. The new instance will have the same context values, probability estimates, and current index as this object, but will be a separate instance. Changes to the copied object will not affect the original, and vice versa.

This is required when reusing arithmetic coding contexts across segments, to avoid sharing mutable probability state between decoders.

Returns:

A new CX object with the same internal state as this object.