PNG colour-key transparency (tRNS for greyscale and truecolour) gives a wrong /Mask

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Dal 8/11/2026.

  • #1865 di @1AbdullahButt — aperta

Valutazione

Difficoltà
3/5
Tempo stimato
1-2 giorni
Idoneità per principianti
78/100
Tipo di issue
Bug
Chiarezza
Specificata chiaramente
Stato di attività
Attiva
Stack tecnologico
javascript, node.js
Ambito
api, backend

Direzione di ricerca

Start with lib/image/png.js and trace how it reads the tRNS bytes and builds the image XObject /Mask; the report also identifies png-js as the source of the raw grayscale and RGB values. Use the included grayscale, RGB, and 16-bit examples to check the generated mask. Done means each component has the expected min/max pair and the affected viewers handle the resulting transparency correctly.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

Bug Report

Description of the problem

For greyscale (colour type 0) and truecolour (colour type 2) PNGs with a tRNS colour key, the image XObject gets a wrong /Mask array. For 8-bit greyscale the key is always written as 0, so black pixels become transparent and the keyed grey stays. For RGB the array has 12 entries instead of 6, which viewers ignore (pdf.js, MuPDF, PDFium) or misread.

tRNS stores 2-byte big-endian samples for these colour types (one for grey, three for RGB, regardless of bit depth), and /Mask needs one min max pair per component (ISO 32000-1, 8.9.6.4). png-js keeps the chunk as raw bytes (for grey only read(chunkSize)[0], the high byte; for RGB all six bytes), and lib/image/png.js then pushes every byte as a pair (for (let x of rgb) mask.push(x, x)).

Expected output:

key-gray /Mask [128 128]
key-rgb /Mask [255 255 0 0 0 0]
key-rgb16 /Mask [65535 65535 0 0 0 0]

Actual output:

key-gray /Mask [0 0]
key-rgb /Mask [0 0 255 255 0 0 0 0 0 0 0 0]
key-rgb16 /Mask [255 255 255 255 0 0 0 0 0 0 0 0]

#1274 lists PngSuite transparency files among unsupported PNGs; this may be part of it.

Code sample

import zlib from "node:zlib";
import PDFDocument from "pdfkit";

const crc = (b) => {
  let c = 0xffffffff;
  for (const x of b) {
    c ^= x;
    for (let k = 0; k < 8; k++) c = c & 1 ? 0xedb88320 ^ (c >>> 1) : c >>> 1;
  }
  return (c ^ 0xffffffff) >>> 0;
};
const chunk = (type, data) => {
  const l = Buffer.alloc(4), td = Buffer.concat([Buffer.from(type), data]), c = Buffer.alloc(4);
  l.writeUInt32BE(data.length);
  c.writeUInt32BE(crc(td));
  return Buffer.concat([l, td, c]);
};
const png = (w, h, depth, ctype, rows, trns) => {
  const ih = Buffer.alloc(13);
  ih.writeUInt32BE(w, 0); ih.writeUInt32BE(h, 4); ih[8] = depth; ih[9] = ctype;
  return Buffer.concat([
    Buffer.from([137, 80, 78, 71, 13, 10, 26, 10]),
    chunk("IHDR", ih),
    chunk("tRNS", Buffer.from(trns)),
    chunk("IDAT", zlib.deflateSync(Buffer.concat(rows.map((r) => Buffer.from([0, ...r]))))),
    chunk("IEND", Buffer.alloc(0)),
  ]);
};

const cases = {
  "key-gray":  png(2, 1, 8, 0, [[128, 0]], [0, 128]),
  "key-rgb":   png(2, 1, 8, 2, [[255, 0, 0, 0, 0, 255]], [0, 255, 0, 0, 0, 0]),
  "key-rgb16": png(2, 1, 16, 2, [[255, 255, 0, 0, 0, 0, 0, 0, 0, 0, 255, 255]], [255, 255, 0, 0, 0, 0]),
};

for (const [name, buf] of Object.entries(cases)) {
  const doc = new PDFDocument({ size: [200, 100], margin: 0, compress: false });
  const out = [];
  doc.on("data", (d) => out.push(d));
  doc.image(buf, 0, 0, { width: 200, height: 100 });
  doc.end();
  await new Promise((r) => doc.on("end", r));
  console.log(name, Buffer.concat(out).toString("latin1").match(/\/Mask \[[^\]]*\]/)[0]);
}

Your environment

  • pdfkit version: 0.20.2 (with png-js 2.0.0), also a build of master at 91c54ef
  • Node version: 26.13.0
  • Browser version (if applicable): n/a
  • Operating System: Ubuntu Linux x86_64

Credit: https://ucsbplse.github.io/

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