BUG: dpi parameter not working properly when converting from svg in python

When parsing this svg to png:

temp.svg:

<?xml version="1.0" encoding="utf-8" ?>
<svg baseProfile="full" height="1000mm" version="1.1" width="1000mm" xmlns="http://www.w3.org/2000/svg" xmlns:ev="http://www.w3.org/2001/xml-events" xmlns:xlink="http://www.w3.org/1999/xlink">
<defs></defs>
<rect fill="#ffffff" height="1000mm" width="1000mm" x="0" y="0" />

with:

import pyvips
image = pyvips.Image.svgload("temp.svg", dpi=25.4)
image.write_to_file("output_test.png")

output_test's dimensions are 353mm x 353mm (and 353px x 353px). (Should be 1000mm x 1000mm and 1000px x 1000px)

also, intrestingly enough - with image = pyvips.Image.svgload("temp.svg", dpi=x) dimensions of the image scale roughly according to x^1.86

@Czoczaj

Hi @Czoczaj,

Thanks for reporting this. I agree, I think the scaling is wrong for non-pixel dimensions, it looks like we're applying the dpi scale twice in this case.

I'll make a quick PR.

I did some digging and I'm not certain this is easily fixable with the librsvg API.

The problem seems to be with the use of physical dimensions in SVG sizing. For eg. "1000mm" in the header to work, you need to call rsvg_handle_set_dpi() before load. This will then automatically scale the drawing for you nicely. However, if a pixel dimension is used (eg. "1000px") then librsvg does NOT use it to scale the header or drawing and you must apply a scale yourself during rendering.

The bug you're seeing is what happens when a SVG with physical dimensions in the header is rescaled: the dpi is applied twice, so you get something close to the square of dpi (the ^1.8 you saw).

I think perhaps the solution is to use the dpi= parameter to ONLY change the way physical dimensions in the header are interpreted (and the dpi recorded in the output image), and not the rendering scale. The scale= parameter could ONLY change the rendering scale.

I'll make an experimental branch so we can try this out. It looks likely to break some old code, unfortunately :(

(I think the other bug you opened is also fixed by this change)

I made a PR on libvips with some examples, and credited you. Let's move any discussion there.

Thanks for reporting this!

(edit: your other bug is different, sorry)

svgload: improve dpi= and scale= behaviour (#5205)

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BUG: dpi parameter not working properly when converting from svg in python #1424

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BUG: dpi parameter not working properly when converting from svg in python #1424

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jcupitt commented on Sep 2, 2026

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jcupitt commented on Sep 3, 2026

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