JPEG XL in PDF: Bringing JXL to PDFium with Rust

Decoding JPEG XL images embedded in PDF documents using the pure Rust decoder jxl-rs

Background

With JPEG XL landing in Chromium via the Rust decoder jxl-rs, it's natural to ask: what about PDF?

JPEG XL is poised to become the preferred format for HDR images in PDF (per ISO 32000-2:2020 discussions). While the PDF spec doesn't yet have a standardized filter name, this experiment uses /JXLDecode as a bring-up name to prove the concept works.

The Challenge

PDFium is Chromium's PDF renderer, but it's also used standalone (e.g., in other browsers, document viewers, server-side rendering). The challenge:

1. Rust integration — PDFium's standalone build needed Rust toolchain support
2. Alpha handling — JXL images can have alpha, but PDF expects a separate /SMask
3. Animation — JXL supports animation, but PDF image XObjects are static

Implementation

Approach

• Rust-only decoder: Uses jxl-rs via Chromium's CXX bridge wrapper
• No C++ libjxl: Memory-safe by design, aligns with Chromium's direction
• Synthesized SMask: Decoder extracts alpha and creates an internal soft mask
• Frame 0 only: For animated JXL, only the first frame is rendered

Build Integration

Added a new GN flag for standalone builds:

# In pdfium.gni
declare_args() {
  pdf_enable_rust_jxl = false
}

The decoder glue lives in core/fxcodec/jxl/:

core/fxcodec/jxl/
├── BUILD.gn
├── DEPS
├── jxl_decoder.cc
├── jxl_decoder.h
└── jxl_rs_stub.cc

Decoder Flow

// In cpdf_dib.cpp
bool CPDF_DIB::LoadJxlBitmap(pdfium::span<const uint8_t> data) {
  auto info = pdfium::jxl::ParseInfo(data);
  if (!info) return false;

  auto decoded = pdfium::jxl::DecodeFrame0(data, info->width, info->height);
  if (!decoded) return false;

  // Extract RGB
  // If has_alpha: synthesize SMask via jpx_inline_data_ mechanism

  return true;
}

The integration hooks into CreateDecoder() to handle the /JXLDecode filter:

if (decoder == "JXLDecode") {
  return LoadJxlBitmap(src_span) ? LoadState::kSuccess : LoadState::kFail;
}

Demo Artifacts

Sample PDFs with embedded JXL images, rendered by the patched PDFium:

Zoltan (RGB Photo)

Large RGB JPEG XL embedded in PDF.

Dice (Alpha)

RGBA JPEG XL with transparency. PDFium synthesizes an /SMask and composites over a checkerboard background (baked into the PDF to visualize alpha).

Animated Icons (Frame 0)

Animated RGBA JPEG XL. PDFium decodes only the first frame.

Code Changes

IN REVIEW CL 142070 — Add experimental JXLDecode support via Rust jxl-rs

Building

To build PDFium with JXL support:

# In args.gn
enable_rust = true
pdf_enable_rust_jxl = true

# Requires rust_revision pointing to a version with jxl-rs v0.3 wrapper

Related

• JPEG XL Returns to Chrome — The Chromium/Blink implementation
• jxl-rs — The pure Rust JPEG XL decoder
• PDFium — Google's PDF rendering library

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This is an experimental feature exploring JXL-in-PDF before standardization. The filter name /JXLDecode is a placeholder and may change.