LZW Compression Support

Overview

HotPDF now includes comprehensive LZW (Lempel-Ziv-Welch) compression support for PDF streams. LZW is a lossless data compression algorithm that's particularly effective for text and simple graphics, commonly used in PDF documents for content stream compression.

Key Features

Full LZW decompression support for PDF streams
Configurable predictor support for enhanced compression
Multiple fill order options (top-to-bottom, bottom-to-top)
Early code change support for compatibility
Memory-efficient stream processing
PDF parameter integration (Predictor, Colors, BitsPerComponent, Columns)

Technical Implementation

The LZW compression support is implemented through the HPDFLZW.pas unit, which provides:

TPDFLZWDecompressor: Main decompression class
TPDFLZWParms: Parameter structure for PDF-specific settings
TPDFLZWFillOrder: Enumeration for bit order processing

Class Reference

TPDFLZWDecompressor

The main class for LZW decompression operations.

Properties:

FillOrder: Specifies bit order (foBottom, foTop)
EarlyChange: Controls early code size changes
InitialCodeSize: Initial code size for decompression

Methods:

Decompress(Input: AnsiString): AnsiString - Basic decompression
Decompress(Input: AnsiString; Parms: TPDFLZWParms): AnsiString - Decompression with PDF parameters

Usage Examples

Basic LZW Decompression


// Delphi example - Basic LZW decompression
procedure DecompressLZWData;
var
  Decompressor: TPDFLZWDecompressor;
  CompressedData: AnsiString;
  DecompressedData: AnsiString;
begin
  Decompressor := TPDFLZWDecompressor.Create;
  try
    // Configure decompressor
    Decompressor.FillOrder := foTop;
    Decompressor.EarlyChange := True;
    Decompressor.InitialCodeSize := 9;

    // Decompress data
    DecompressedData := Decompressor.Decompress(CompressedData);

    // Use decompressed data
    ProcessDecompressedData(DecompressedData);
  finally
    Decompressor.Free;
  end;
end;

Advanced LZW Decompression with PDF Parameters


// Delphi example - Advanced LZW decompression with PDF parameters
procedure DecompressLZWWithParms;
var
  Decompressor: TPDFLZWDecompressor;
  CompressedData: AnsiString;
  DecompressedData: AnsiString;
  Parms: TPDFLZWParms;
begin
  Decompressor := TPDFLZWDecompressor.Create;
  try
    // Configure PDF parameters
    Parms.Predictor := 2;           // Horizontal differencing predictor
    Parms.Colors := 3;              // RGB color space
    Parms.BitsPerComponent := 8;    // 8 bits per component
    Parms.Columns := 100;           // Image width in pixels
    Parms.ExpandedTo8Bit := True;   // Expand to 8-bit components
    Parms.ColorSpace := 'DeviceRGB';

    // Decompress with parameters
    DecompressedData := Decompressor.Decompress(CompressedData, Parms);

    // Process the decompressed image data
    ProcessImageData(DecompressedData, Parms);
  finally
    Decompressor.Free;
  end;
end;

PDF Parameter Support

TPDFLZWParms Structure

Predictor: Predictor function (0=none, 2=horizontal differencing)
Colors: Number of color components (1=grayscale, 3=RGB, 4=CMYK)
BitsPerComponent: Bits per color component (1, 2, 4, 8, 16)
Columns: Number of samples per row
ExpandedTo8Bit: Whether to expand to 8-bit components
ColorSpace: Color space identifier

Algorithm Details

The LZW implementation includes:

Dictionary-based Compression: Builds and maintains a dynamic dictionary
Variable Code Length: Supports code lengths from 9 to 12 bits
Clear Code Handling: Proper handling of clear and end-of-information codes
String Table Management: Efficient string table for pattern recognition

Performance Characteristics

Memory Efficient: Optimized for large data streams
Fast Decompression: Highly optimized decompression algorithms
Predictable Performance: Consistent performance across different data types
Low Memory Footprint: Minimal memory overhead during processing

Common Use Cases

PDF content stream decompression
Image data decompression in PDF files
Text stream decompression
Form data decompression
PostScript stream decompression

Error Handling

The LZW decompressor includes robust error handling for:

Invalid code sequences
Corrupted data streams
Memory allocation failures
Dictionary overflow conditions

Standards Compliance

PDF 1.2+ LZWDecode filter compliance
PostScript Level 2 LZW compatibility
TIFF LZW compression compatibility
Adobe LZW implementation compatibility

See Also