core/

This directory contains the internal C++ implementation of the PDF specification. These modules are for internal use and may change without notice.

Subdirectories

PDFium's core is divided into several subdirectories, each with a specific responsibility:

• fdrm/ (Rights Management): Provides cryptography primitives used for PDF security and encryption handling.
• fpdfapi/ (PDF spec implementation): The central module that understands the PDF format, covering parsing, logical page representation, rendering coordination, and document editing.
• fpdfdoc/ (Document Logic): Implements higher-level document features such as Bookmarks, Annotations, Interactive Forms (AcroForms), and Links.
• fpdftext/ (Text Extraction): Specialized logic for extracting text, identifying reading order, and performing text searches within a page.
• fxcodec/ (Codecs): Responsible for decoding various PDF stream formats. It depends on fxge for managing target bitmaps.
• fxcrt/ (Common Runtime): The foundational layer providing base types, string handling, custom containers, and memory management utilities. It has no dependencies on other core modules.
• fxge/ (Graphics Engine): Handles rendering primitives (paths, glyphs, bitmaps), font management abstractions (FreeType, Skia, AGG), and device drivers for different platforms.

Architectural Flow: Life of a Page

Building a rendered page involves several layers working in concert:

1. Parsing: fpdfapi/parser reads raw bytes and builds the PDF object graph (Dictionaries, Streams, etc.).
2. Decoding: As content is accessed, fxcodec decodes compressed data streams.
3. Logical Representation: fpdfapi/page interprets the objects into higher-level constructs like paths, text objects, and images.
4. Rendering: fpdfapi/render traverses the page objects and issues drawing commands to fxge.
5. Rasterization: fxge uses its rendering backend (e.g., AGG or Skia) to convert these primitives into final pixels in a device buffer.

Coordinate Systems

PDF documents use User Space, a coordinate system where the origin (0,0) is typically at the bottom-left of the page. Rendering devices (managed by fxge) usually use Device Space, where the origin is at the top-left.

core/ handles the complex affine transformations needed to map between these spaces, taking into account page rotation, scaling, and the specific transformation matrices (CTM) defined within content streams.