QOI: QOI_OP_RUN does not update the previously seen pixels, corrupting qoi_test_images/edgecase.qoi

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@manit2004 がすでに取り組んでいます。

2026年11月7日 から。

  • #10453 @manit2004 による — オープン
  • #10454 @lianxiaozhi による — マージされずにクローズ

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難易度
2/5
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バグ
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明確に書かれている
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活発
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python
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backend

調査の方向性

QoiDecoder.decode() の QOI_OP_RUN 分岐と、issue で説明されている _add_to_previous_pixels() の呼び出しを読む。提示された 29 バイトの例で失敗を再現し、その後、test_op_index を含む QOI デコーダーのテストを確認する。最初の run ピクセルが後続の QOI_OP_INDEX から利用でき、edgecase.qoi の公式出力が edgecase.png と一致すれば完了。

索引モデルが issue の本文から書いたものです。

説明

What did you do?

Opened a QOI image that starts with a run of the initial pixel (0, 0, 0, 255) and later refers to that pixel with QOI_OP_INDEX.

The official test image edgecase.qoi from https://qoiformat.org/qoi_test_images.zip does this. The script below is a minimal 29-byte reproduction.

What did you expect to happen?

The third pixel should be (0, 0, 0, 255), and edgecase.qoi should match edgecase.png.

The specification says "Each pixel that is seen by the encoder and decoder is put into this array". The reference decoder updates the array after every chunk, including QOI_OP_RUN.

What actually happened?

The third pixel is (0, 0, 0, 0):

[(0, 0, 0, 255), (1, 2, 3, 4), (0, 0, 0, 0)]

For edgecase.qoi, 10272 of the 16384 pixels differ from edgecase.png.

The QOI_OP_RUN branch in QoiDecoder.decode() reaches continue before _add_to_previous_pixels() is called, so the pixel is never added to the array. For most pixels this does no harm, because the previous pixel is already in the array from the chunk that produced it. The initial pixel is the exception, because no chunk produced it.

This looks like a regression from #9008, first released in 11.3.0. That PR correctly stopped adding the initial pixel to the array up front, but the up-front add had been hiding this problem. I did not run older releases. On 13.0.0, restoring the removed line makes both the script below and edgecase.qoi decode correctly.

qoi.h's encoder does not add run pixels to its array, so it does not produce this pattern. Encoders that follow the specification literally can, as edgecase.qoi shows.

This change fixes it:

                     value = self._previous_pixel
+                    self._add_to_previous_pixels(value)
                     if bands == 3:

With this change, all eight official test images match their PNGs, and QOI_OP_INDEX as the first chunk still gives (0, 0, 0, 0), as test_op_index expects. I have not run the full test suite.

What are your OS, Python and Pillow versions?
  • OS: Fedora Linux 43
  • Python: 3.14.8
  • Pillow: 13.0.0. The QoiDecoder code on main is identical.
--------------------------------------------------------------------
Pillow 13.0.0
Python 3.14.8 (main, Oct  1 2026, 00:00:00) [GCC 15.3.1 20260722 (Red Hat 15.3.1-1)]
--------------------------------------------------------------------
Python executable is <venv>/bin/python
Environment Python files loaded from <venv>
System Python files loaded from /usr
--------------------------------------------------------------------
Python Pillow modules loaded from <venv>/lib64/python3.14/site-packages/PIL
Binary Pillow modules loaded from <venv>/lib64/python3.14/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 1.3.1.zlib-ng, compiled for zlib-ng 2.3.3
--- LIBTIFF support ok, loaded 4.7.2
--- RAQM (Bidirectional Text) support ok, loaded 0.11.0, fribidi 1.0.16, harfbuzz 14.5.0
*** LIBIMAGEQUANT (Quantization method) support not installed
--- XCB (X protocol) support ok
--------------------------------------------------------------------

from io import BytesIO

from PIL import Image

data = (
    b"qoif"
    + (3).to_bytes(4, "big")  # width
    + (1).to_bytes(4, "big")  # height
    + bytes([4, 0])  # channels, colorspace
    + bytes([0xC0])  # QOI_OP_RUN, length 1: repeats the initial (0, 0, 0, 255)
    + bytes([0xFF, 1, 2, 3, 4])  # QOI_OP_RGBA
    + bytes([53])  # QOI_OP_INDEX: 255 * 11 % 64 == 53, the slot of (0, 0, 0, 255)
    + bytes(7)
    + b"\x01"  # end marker
)

with Image.open(BytesIO(data)) as im:
    print([im.getpixel((x, 0)) for x in range(3)])
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