Print processors are DLLs that handle the conversion between application data and printer-ready formats. The Windows printing subsystem relies on them to properly interpret and format print jobs before sending them to printers. The most common processor, WINPRINT.DLL, provides several data types including RAW and EMF.

WINPRINT with RAW data type is the default for most printers because:

// Typical printer driver configuration showing RAW as default
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Print\Printers\[PrinterName]
"PrintProcessor" = "winprint"
"Datatype" = "RAW"

This combination works well because RAW mode sends the print data directly to the printer with minimal processing, making it compatible with most PCL and PostScript printers.

Several scenarios require different print processors:

• Fax software: Often uses custom processors to handle fax-specific formatting
• Virtual printers: PDF creators may use processors like "msxpsinc" for XPS conversion
• Specialized hardware: Some plotters or industrial printers require custom processing

Symptoms of incorrect print processor selection include:

// Common error messages indicating processor issues
ERROR_PRINT_PROCESSOR_ALREADY_INSTALLED
ERROR_UNKNOWN_PRINTPROCESSOR
ERROR_PRINTPROCESSOR_NOT_INSTALLED

Real-world example: A user reported garbled output when printing from AutoCAD to a plotter. The solution was to change from WINPRINT/RAW to the manufacturer's custom processor that handled HPGL/2 commands properly.

The printing subsystem calls these key functions in a print processor:

// Typical print processor function exports
BOOL APIENTRY
PrintDocumentOnPrintProcessor(
    HANDLE hPrinter,
    LPWSTR pDocumentName,
    LPWSTR pOutputFile,
    LPWSTR pDatatype,
    DWORD dwNumberOfCopies,
    LPWSTR pParameters);

DWORD APIENTRY
ControlPrintProcessor(
    HANDLE hPrinter,
    LPWSTR pPrintProcessorName,
    DWORD Command,
    LPVOID pData,
    DWORD cbBuf);

The need for multiple print processors dates back to Windows NT architecture decisions:

• Separation of printer driver and processing logic
• Support for diverse printing hardware
• Flexibility in document conversion pipelines

This design persists in modern Windows versions, though WINPRINT/RAW remains the most stable choice for conventional printing.

At its most fundamental level, a print processor is a Windows component that handles the conversion of print job data into a format that a specific printer can understand. The WINPRINT.DLL (with RAW mode) has been the default workhorse since Windows NT, but alternative processors exist for specialized scenarios.

The evolution of print processors mirrors Windows' printing architecture changes:

• NT-based systems primarily used WINPRINT with RAW or EMF modes
• Windows 2000/XP introduced enhanced meta-file (EMF) processing
• Modern Windows maintains backwards compatibility while adding XPS support

// Typical registry entry showing available processors
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Print\Environments\
  Windows x64\Print Processors\
    winprint
    fax
    MSXPS
    hpzpp3bn

While WINPRINT/RAW works for most scenarios, consider alternatives when:

• Printing to virtual printers (PDF, FAX)
• Using printer-specific features like job accounting
• Processing XPS documents natively

When managing printers via code, you can specify the processor:

// PowerShell example to set print processor
$printer = Get-WmiObject -Class Win32_Printer -Filter "Name='YourPrinterName'"
$printer.PrintProcessor = "winprint"
$printer.Put()

The modern printing pipeline offers three main data types:

• RAW: Printer-ready data (fastest, least flexible)
• EMF Spooled GDI commands (portable but slower)
• XPS XML-based document format (modern, feature-rich)