plotly / plotly/plotly.py

DPI metadata of PNG export should change with scale or be adjustable

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feature P3
Dominant language
Python
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Description

I export a figure as a png using write_image:

  fig.update_layout(width=300).write_image("test_1.png", scale=1)

Opening this image in another program, the width of the image is 3.125 inches corresponding to a DPI of 96.

If I instead export an image with a different scale, e.g. 2
fig.write_image("test_2.png", scale=2)
I get a png file with a scaled pixel width (e.g. 600).
But opening the image in another program, the width in inches is also doubled to 6.25.

The ideal behavior IMO would be that the width of the image in inches is unaffected by the scale parameter or having a dpi keyword argument that sets the DPI metadata directly.
A workaround (which others might find useful) is to use Pillow to fix the DPI metadata

  from PIL import Image
  scale = 2
  Image.open("test_2.png").save("test_2_fixed.png", dpi=(96 * scale, 96 * scale))

such that the image defaults to the same size in inches in other programs independent of the scale.

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Research direction

Start from the Python write_image export path and compare the PNG metadata produced with scale=1 and scale=2. Verify the resulting physical dimensions in another image program; done means scaling pixels no longer changes the physical size, or a dpi option provides the requested metadata control.

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
data-visualization
Issue type
Feature
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
45/100

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