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16-bit PNG tRNS key: convert("RGBA") misses the keyed pixel and makes another transparent

Abierto
  • ¿Algo no está bien?
#10,330 3 comentarios 0 reacciones 0 asignados Ver en GitHub

Los mantenedores suelen responder en 1 día

@laozidabo ya está trabajando en esto.

Desde el 27/10/2026.

  • #10338 de @laozidabo — abierto

Evaluación

Dificultad
3/5
Tiempo estimado
1-2 días
Aptitud para principiantes
58/100
Tipo de issue
Error
Claridad
Bien especificado
Estado de actividad
Activo
Stack tecnológico
python
Área
backend

Línea de trabajo

Start by reading the PNG decoder and transparency handling, then inspect the existing tests for 16-bit PNG and the related 2- and 4-bit gray case in #10320. Add regression coverage for 16-bit RGB and grayscale tRNS keys, checking that only matching pixels become transparent after convert("RGBA"). The issue provides a reproducer and expected alpha values; the fix is done when both cases behave as expected without breaking existing PNG tests.

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

Descripción

Conversion
What did you do?

I'm building an image-optimization tool that prepares screenshots and photos for PDFs: it picks the output format, carries the DPI so pages place at the right size, and checks that lossless steps keep every pixel. Its test suite generates each input type Pillow opens and runs it through the tool, which is where this came up. The tool reads transparency through convert("RGBA"), and on a 16-bit RGB PNG with a tRNS key the keyed pixels stayed opaque.

import io, struct, zlib
from PIL import Image

def png(width, depth, colour_type, row, trns):
    def chunk(kind, data):
        return struct.pack(">I", len(data)) + kind + data + struct.pack(">I", zlib.crc32(kind + data))
    ihdr = struct.pack(">IIBBBBB", width, 1, depth, colour_type, 0, 0, 0)
    return (b"\x89PNG\r\n\x1a\n" + chunk(b"IHDR", ihdr) + chunk(b"tRNS", trns)
            + chunk(b"IDAT", zlib.compress(b"\x00" + row)) + chunk(b"IEND", b""))

# 16-bit RGB: pixel 0 is the tRNS colour, pixel 1 is not
key = (0x1234, 0x5678, 0x9ABC)
row = struct.pack(">6H", *key, 0x3434, 0x7878, 0xBCBC)
with Image.open(io.BytesIO(png(2, 16, 2, row, struct.pack(">3H", *key)))) as im:
    print(im.mode, im.info["transparency"], [im.getpixel((x, 0)) for x in range(2)])
    print([im.convert("RGBA").getpixel((x, 0))[3] for x in range(2)])

# 16-bit greyscale: pixel 0 is the tRNS grey level, pixel 1 is not
row = struct.pack(">2H", 0x1234, 0x0034)
with Image.open(io.BytesIO(png(2, 16, 0, row, struct.pack(">H", 0x1234)))) as im:
    print(im.mode, im.info["transparency"], [im.getpixel((x, 0)) for x in range(2)])
    print([im.convert("RGBA").getpixel((x, 0))[3] for x in range(2)])
What did you expect to happen?

Alpha [0, 255] in both cases: the pixel equal to the tRNS value is transparent and the other is opaque, as for 8-bit files.

What actually happened?
RGB (4660, 22136, 39612) [(18, 86, 154), (52, 120, 188)]
[255, 0]
I;16 4660 [4660, 52]
[255, 0]

The keyed pixel stays opaque, and the pixel whose 8-bit value equals the key's low byte (0x34, 0x78, 0xBC) becomes transparent. 16-bit RGB opens as 8-bit RGB while info["transparency"] keeps the 16-bit tuple. #10320 covers the 2- and 4-bit gray case; this is the 16-bit one, and I'm not asking for 16-bit RGB support (#1888).

Related: #10329 (16-bit APNG frames merged on save).

What are your OS, Python and Pillow versions?
  • OS: Ubuntu 24.04 (Linux x86_64)
  • Python: 3.13.13
  • Pillow: 13.0.0 (also reproduces on 12.3.0)
--------------------------------------------------------------------
Pillow 13.0.0
Python 3.13.13 (main, May 10 2026, 19:26:54) [Clang 22.1.3 ]
--------------------------------------------------------------------
Python executable is ~/.venv/bin/python
Environment Python files loaded from ~/.venv
System Python files loaded from ~/.local/share/uv/python/cpython-3.13.13-linux-x86_64-gnu
--------------------------------------------------------------------
Python Pillow modules loaded from ~/.venv/lib/python3.13/site-packages/PIL
Binary Pillow modules loaded from ~/.venv/lib/python3.13/site-packages/PIL
--------------------------------------------------------------------
--- PIL CORE support ok, compiled for 13.0.0
--- TKINTER support ok, loaded 9.0
--- FREETYPE2 support ok, loaded 2.14.3
--- LITTLECMS2 support ok, loaded 2.19
--- WEBP support ok, loaded 1.6.0
--- AVIF support ok, loaded 1.4.2
--- JPEG support ok, compiled for libjpeg-turbo 3.2.0
--- OPENJPEG (JPEG2000) support ok, loaded 2.5.4
--- ZLIB (PNG/ZIP) support ok, loaded 1.3, compiled for zlib-ng 2.3.3
--- LIBTIFF support ok, loaded 4.7.2
--- RAQM (Bidirectional Text) support ok, loaded 0.11.0, fribidi 1.0.13, harfbuzz 14.5.0
*** LIBIMAGEQUANT (Quantization method) support not installed
--- XCB (X protocol) support ok
--------------------------------------------------------------------
Lenguaje dominante
Python
Estrellas
13.9k
Forks
2.6k
Merge medio
1 d 4 h
PR fusionados (30 d)
173

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