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PDFCreator

repository·master·Indexed 43 days ago

https://github.com/pdfforge/pdfcreator

A printer driver that allows users to convert documents to PDF format from any printable application. The documentation includes environment setup for development and detailed technical references for the Ghostscript Interpreter API, covering instance management, stdio callbacks, parameter configuration via gsapi_set_param, and custom filing system implementation.

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What's inside PDFCreator

  1. Overview of the Ghostscript Core Library

    master

    The Ghostscript library provides C procedures to implement graphics and filtering capabilities used in PostScript and PDF. It is divided into two main layers:

    1. High-level API (gs_ prefix): Implements PostScript graphics operators directly (e.g., gs_moveto, gs_fill). This is the primary interface for most clients. It uses floating-point user coordinates and takes graphics state objects as arguments.
    2. Lower-level API (gx_ prefix): Provides higher-performance, less general graphics facilities. These interfaces are less stable and typically use device coordinates in an internal fixed-point representation.

    Note: Files prefixed with gz are internal and not intended for client use.

  2. Ghostscript PostScript Coding Guidelines Summary

    master

    To maintain a consistent and readable codebase in Ghostscript, follow these core principles:

    • Storage: Never store data into literals. Allocate storage (e.g., string, array, dict) instead.
    • Control Flow: Use loop to create blocks with multiple exits. For multi-way switches (more than 3 cases), use a dictionary or an array of procedures instead of nested ifelse statements.
    • File Structure: Every file must start with a copyright notice, the filename (RCS Id format), and a one-line summary.
    • Commenting:
      • Comment every procedure with its arguments, results, and function.
      • Comment stack contents, especially at the beginning of loop bodies.
      • Use section comments for well-defined functional areas.
    • Formatting:
      • Indent with 2 spaces.
      • Place { at the end of a line and } at the beginning of a line (unless the content is very short).
      • Always put spaces between adjacent tokens.
    • Naming:
      • Use only lower-case letters and digits, or 'Vienna style' (e.g., readSubrs).
      • Use an initial . for names intended to be used only within a single file.
    • Performance:
      • Avoid allocating objects (strings, arrays, etc.) in heavily used code to reduce garbage collection overhead.
      • Factor out repeated code (more than ~6 tokens) into procedures.

  3. Use GhostMono for C# GhostAPI integration on Linux

    master

    GhostMono is the C# interface for the GhostAPI library specifically designed for Linux systems. It serves as the Mono equivalent of GhostNET, providing Ghostscript capabilities without requiring a Windows environment. Developers use it to communicate with GhostAPI by preparing parameter payloads and executing tasks asynchronously.

    // GhostMono is the C# interface into the GhostAPI library developed for Linux systems.

  4. Overview of Ghostscript Color Management Architecture

    master

    Since version 9.0, Ghostscript's color architecture is primarily based on the ICC format. It is designed to be efficient in multithreaded environments and allows for easy interfacing with different Color Management Modules (CMMs).

    Key capabilities include:

    • Defining all color spaces in terms of ICC profiles.
    • Caching linked transformations and internally generated profiles.
    • Managing default profiles for DeviceGray, DeviceRGB, and DeviceCMYK.
    • Handling named colors (spots) via ICC named color profiles or proprietary formats.
    • Supporting various rendering intents (perceptual, colorimetric, saturation, absolute colorimetric) and black point compensation.
    • Allowing overrides of document-embedded ICC profiles with Ghostscript defaults.
    • Providing granular control over unique source/destination ICC profiles and rendering settings for specific object types (image, vector graphics, text).

  5. Overview of the ds32 32-bit CMYK downscaled device

    master

    The ds32 device is a sample 32-bit CMYK TIFF device designed for downscaled output from 32-bit CMYK internal rendering.

    Key Characteristics:

    • Downscaling Support: It supports downscaler options similar to the tiffscaled32 device, including DownScaleFactor, PostRenderProfile, TrapX, TrapY, and TrapOrder.
    • Memory-Centric Output: By default, no file is written to disk. This allows raster lines to be passed directly in memory to a user-defined function, making it useful for custom processing pipelines.
    • Debug Mode: It includes a debug mode that can write a PAM32 file for inspection.

  6. What is the GhostPDL Interpreter Framework?

    master

    GhostPDL is a modular interpreter framework designed to handle multiple input "languages" (formats) within a single build. It is intended to behave similarly to Ghostscript but is optimized for print processing by allowing different interpreters to be swapped in or out.

    Key characteristics:

    • Modular Design: New interpreters can be added or existing ones removed to customize capabilities.
    • Job Separation: Jobs must be separated by Universal End of Language (UEL) code sequences or PJL fragments (which implicitly start with a UEL marker).
    • Platform Support: Can be built as a DLL on Windows or a shared object on Linux, Unix, and MacOS X. It can also be built as a static library.
    • Single Instance Limitation: At this stage, GhostPDL does not support multiple instances of the interpreter within a single process.

  7. What is the Ghostscript Interpreter API?

    master

    The Ghostscript Interpreter API allows you to embed the Ghostscript interpreter directly into your application by loading it as a dynamic link library (DLL) or shared object.

    Key distinctions:

    • Interpreter vs. Graphics Library: This API is for the interpreter. It is distinct from the Ghostscript library, which provides a graphics library but lacks the interpreter logic.
    • Responsibility: The API does not handle windowing or UI. Your application (the host process) is responsible for providing all interaction with the windowing system, including image windows and text windows.
    • Concurrency Limitation: At this stage, Ghostscript does not support multiple instances of the interpreter within a single process.

  9. Configure image compression for Flate and JPEG

    master

    The pdfwrite device allows configuring image compression parameters via the (Gray|Color)ImageDict and (Gray|Color)ACSImageDict dictionaries. While Acrobat Distiller primarily supports JPEG/JPX, Ghostscript allows configuring Flate compression using these same dictionaries.

    Supported keys for Flate compression:

    • /Effort: Controls compression effort (default is 6, configurable at compile-time).
    • /Predictor: Controls the predictor (default is 15, which allows the encoder to select).

    The pdfwrite device automatically selects the appropriate parameters based on the chosen compression type.

  10. Understand Ghostscript measurement units

    master

    Ghostscript uses several standard units for dimensions and resolution. When configuring output devices or paper sizes, be aware of the following definitions:

    • Inches (in or "): 1 inch = 2.54 cm. Example: U.S. Letter is 8.5in × 11in.
    • Centimeters (cm) and Millimeters (mm): Used for ISO standard paper sizes (e.g., A4 is approximately 210 × 297 mm).
    • Points: The standard measure for PostScript and the printing trade. There are exactly 72 points per inch (approx. 28.35 per cm). Known paper sizes in Ghostscript are defined in terms of points in the gs_statd.ps initialization file.
    • Dots per inch (dpi): The standard measure for printing resolution.
    • Bits per pixel (bpp): Also known as 'bit depth' or 'pixel depth'; represents the number of digital bits used to represent the color of each pixel.

  11. Understand Ghostscript's architectural layers

    master

    Ghostscript is organized into four hierarchical layers. A layer at level $N$ can utilize facilities from any layer $M$ where $M ext{ extless= } N$.

    1. Substrate (Bottom Layer): Provides low-level facilities including memory management, streams, fixed-point arithmetic, and OS interfaces. Written primarily in C.
    2. Graphics Layer: Written in C, it consists of two parts:
      • Graphics Library: Manages graphics state and decomposes/renders 2-D images (PostScript, PDF, PCL 5e/5c/XL models).
      • Device Drivers: Called by the graphics library via virtual functions to produce actual output.
    3. Language Interpreters: The principal clients of the graphics layer. This includes the PostScript interpreter (written in C and PostScript) and PCL interpreters.
    4. PDF Interpreter (Top Layer): A client of the PostScript interpreter, written entirely in PostScript.

  12. Implement multi-way switches and conditionals in PostScript

    master

    When implementing logic with multiple exit points or cases, avoid deeply nested ifelse statements which degrade readability and indentation.

    Multi-way conditionals (using loop and exit)

    For blocks with more than 3 exit points, use a loop construct. This provides a common exit point and keeps indentation levels consistent.

    {
  ... { 
    ...1
    exit
  } if
  ... { 
    ...2
    exit
  } if
  ... { 
    ...3
    exit
  } if
  ...4
  exit
} loop

    Note: Always include the final exit to prevent the loop from actually looping.

    Switches (using Dictionaries)

    If you have more than 3 cases, use a dictionary or an array of procedures to implement a 'switch'. This is more efficient and cleaner than a series of conditionals.

    /xyzdict mark
  /a {...a} bind
  /b {...b} bind
  /c {...c} bind
.dicttomark readonly def
...
//xyzdict 1 index .knownget { 
  exch pop exec
} { 
  ...x
} ifelse
    {
  ... { 
    ...1
    exit
  } if
  ... { 
    ...2
    exit
  } if
  ... { 
    ...3
    exit
  } if
  ...4
  exit
} loop