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

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@1AbdullahButt ya está trabajando en esto.

Desde el 8/11/2026.

  • #1865 de @1AbdullahButt — abierto

Evaluación

Dificultad
3/5
Tiempo estimado
1-2 días
Aptitud para principiantes
78/100
Tipo de issue
Error
Claridad
Bien especificado
Estado de actividad
Activo
Stack tecnológico
javascript, node.js
Área
api, backend

Línea de trabajo

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.

Escrito por el modelo de indexación a partir del texto del issue.

Descripción

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/

Lenguaje dominante
JavaScript
Estrellas
10.7k
Forks
1.2k
Merge medio
19 h 16 min
PR fusionados (30 d)
25

Preparar el entorno

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  3. Haz un fork del repositorio y trabaja en una rama.
  4. Abre un pull request que haga referencia al número del issue.

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