Pixmap export 'unmultiply' option has no effect
Author: BenVosperCreated Sep 9, 2026Updated Sep 10, 2026
Labelsdocumentation issue
Description of the bug
Pixmap.pil_save and Pixmap.pil_tobytes docs both mention an unmultiply kwarg that seems to have no effect.
I noticed this when trying to do pil_save as webp using the example from the docs. By default the images this creates have the premultiplied alpha and toggling this kwarg doesn't change anything.
Let me know if I'm missing something. Thanks!
How to reproduce the bug
import inspect
from io import BytesIO
import pymupdf
from PIL import Image
print(f"PyMuPDF {pymupdf.__version__}")
print(f"pil_save{inspect.signature(pymupdf.Pixmap.pil_save)}")
print(f"pil_tobytes{inspect.signature(pymupdf.Pixmap.pil_tobytes)}")
# One page, one 50%-opacity red rectangle.
doc = pymupdf.open()
page = doc.new_page(width=100, height=100)
page.draw_rect(pymupdf.Rect(20, 20, 80, 80), fill=(1, 0, 0), fill_opacity=0.5)
pix = page.get_pixmap(alpha=True)
default = pix.pil_tobytes("PNG")
unmultiplied = pix.pil_tobytes("PNG", unmultiply=True)
print(f"\nunmultiply=True changes the output: {default != unmultiplied}")
pixel = (50, 50)
print(f"pixel {pixel}, premultiplied samples: {pix.pixel(*pixel)}")
print(f"pixel {pixel}, pil_tobytes(unmultiply=True): {Image.open(BytesIO(unmultiplied)).getpixel(pixel)}")
print(f"pixel {pixel}, Pixmap.tobytes('png'): {Image.open(BytesIO(pix.tobytes("png"))).getpixel(pixel)}")
# Red at any opacity should read back as (255, 0, 0, a).
# pil_tobytes returns (a, 0, 0, a): the premultiplied samples, unchanged.Running this prints:
PyMuPDF 1.28.2
pil_save(self, *args, **kwargs)
pil_tobytes(self, *args, **kwargs)
unmultiply=True changes the output: False
pixel (50, 50), premultiplied samples: (126, 0, 0, 126)
pixel (50, 50), pil_tobytes(unmultiply=True): (126, 0, 0, 126)
pixel (50, 50), Pixmap.tobytes('png'): (255, 0, 0, 126)PyMuPDF version
1.28.2
Operating system
Linux
Python version
3.13
Source: pymupdf/PyMuPDF