BITMAPCOREHEADER DIB resources decoded incorrectly (wrong color table stride, wrong row stride for 1-bit)

[elemental-admin-dev]

Bug

When loading raw DIB (device-independent bitmap) data embedded in Windows PE resource sections (e.g. cards.dll), Pillow misreads bitmaps that use the old OS/2 BITMAPCOREHEADER format (12-byte header, bcSize=12).

Two bugs

1. 4-bit paletted images

Color table is read as RGBQUAD (4 bytes/entry) instead of RGBTRIPLE (3 bytes/entry), shifting the pixel data offset by 16 bytes and producing wrong colors.

2. 1-bit monochrome images

Row stride calculation is wrong, producing a solid black stripe ~17px wide on the left of every decoded image.

Repro

Extract any bitmap resource from Windows XP cards.dll using pefile, write the raw bytes (without a BITMAPFILEHEADER) and open with:

from PIL import Image, ImageFile
ImageFile.LOAD_TRUNCATED_IMAGES = True
img = Image.open("raw_dib_resource.bmp")

Workaround

Manually parse the BITMAPCOREHEADER with struct and decode pixel data directly:

import struct
from PIL import Image

def decode_bitmapcoreheader(raw):
    bc_size, = struct.unpack_from("<I", raw, 0)
    assert bc_size == 12
    width, height, planes, bitcount = struct.unpack_from("<HHHH", raw, 4)
    num_colors = 2 ** bitcount if bitcount <= 8 else 0
    # RGBTRIPLE: 3 bytes each (NOT RGBQUAD)
    colors = []
    for i in range(num_colors):
        b, g, r = struct.unpack_from("<BBB", raw, 12 + i * 3)
        colors.append((r, g, b))
    px_offset = 12 + num_colors * 3
    row_stride = ((width * bitcount + 31) // 32) * 4
    px_data = raw[px_offset:]
    img = Image.new("RGB", (width, abs(height)))
    pixels = img.load()
    for row_idx in range(abs(height)):
        y = abs(height) - 1 - row_idx if height > 0 else row_idx
        for col in range(width):
            if bitcount == 1:
                byte_idx = row_idx * row_stride + col // 8
                bit = (px_data[byte_idx] >> (7 - col % 8)) & 1
                pixels[col, y] = colors[bit]
            elif bitcount == 4:
                byte_idx = row_idx * row_stride + col // 2
                nibble = (px_data[byte_idx] >> 4) & 0xF if col % 2 == 0 else px_data[byte_idx] & 0xF
                pixels[col, y] = colors[nibble]
    return img

Environment

• Pillow 11.x
• Python 3.14
• Linux x86_64
• Source: Windows XP cards.dll PE bitmap resources

[radarhere]

Would you be able to link to an image that demonstrates the problem for those of us who aren't on Windows XP?

In our test suite, we check https://github.com/python-pillow/Pillow/raw/refs/heads/main/Tests/images/bmp/g/pal8os2.bmp, which looks correct when I save it as a PNG using Pillow.