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JPEG dpi read from EXIF uses XResolution for both axes #10331
Description
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. A JPEG whose resolution is only in EXIF, XResolution 300 and YResolution 150, came back square.
import io
from PIL import Image
exif = Image.Exif()
exif[0x011A] = 300.0 # XResolution
exif[0x011B] = 150.0 # YResolution
exif[0x0128] = 2 # ResolutionUnit: inches
buf = io.BytesIO()
Image.new("RGB", (16, 16)).save(buf, "JPEG", exif=exif)
# Pillow writes a JFIF APP0 segment with units 0; drop it so DPI must come from EXIF
data = buf.getvalue()
app0_length = int.from_bytes(data[4:6], "big")
assert data[2:4] == b"\xff\xe0"
data = data[:2] + data[4 + app0_length:]
with Image.open(io.BytesIO(data)) as im:
print(im.info["dpi"])What did you expect to happen?
(300.0, 150.0)
What actually happened?
(300.0, 300.0). _read_dpi_from_exif reads only 0x011A (XResolution) and sets (dpi, dpi), so YResolution never applies. The JFIF and TIFF paths report the two axes separately.
Related: #10332 (TIFF dpi with no resolution tags).
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
--------------------------------------------------------------------
Activity
radarhere commentedon Oct 27, 2026
Hi. Could you attach here an image that you found like this? It would be helpful to have real data to refer back to.
radarhere commentedon Oct 30, 2026
Investigating the history of this, I found that the Y Resolution was deliberately ignored. hugovk said
https://en.wikipedia.org/wiki/Exif says
"the X and Y resolutions may be unequal" in the TIFF specification, but it's hard to find anything official that clarifies what the EXIF specification meant.
So yes, if you have an image that demonstrates this, it would be helpful, and if you know how it was created, that would be even helpful.
brettdavies commentedon Oct 30, 2026
Thanks for looking into this. Closing.