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

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#10,330 コメント 3 件 リアクション 0 件 担当者 0 名 GitHub で見る

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@laozidabo がすでに取り組んでいます。

2026年10月27日 から。

  • #10338 @laozidabo による — オープン

評価

難易度
3/5
見積もり時間
1〜2日
初心者へのやさしさ
58/100
issue の種類
バグ
明瞭さ
明確に書かれている
活発さ
活発
技術スタック
python
領域
backend

調査の方向性

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.

索引モデルが issue の本文から書いたものです。

説明

Awaiting OP Action 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
--------------------------------------------------------------------
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