Better handling for Figures in ODT output?

[iandol]

I always have trouble with ODT not sizing images to the page width as a default. Given this simple markdown and only pandoc -t test.[docx|odt] (xkcd.png has 300PPI set, xkcd2.png has no PPI info):

Text.

![A Caption](xkcd.png)

![A Caption](xkcd2.png)

Text.

We get very different outputs between Word which scales to document size:

…and ODT:

The two images are identical in size (2953x970px) apart from the PPI metadata.

When the same image is manually imported in LibreOffice its default is to scale it to page width (both the with-PPI and no-PPI variants):

Pandoc's XML for the ODT looks like:

<text:p text:style-name="Text_20_body">Text.</text:p>
<text:p text:style-name="FigureWithCaption">
<draw:frame draw:name="img1" svg:width="711.0903010033445pt" svg:height="233.57859531772576pt"><draw:image xlink:href="Pictures/0.png" xlink:type="simple" xlink:show="embed" xlink:actuate="onLoad" /></draw:frame></text:p>
<text:p text:style-name="FigureCaption">A Caption</text:p>
<text:p text:style-name="FigureWithCaption">
<draw:frame draw:name="img2" svg:width="2953.0pt" svg:height="970.0pt"><draw:image xlink:href="Pictures/1.png" xlink:type="simple" xlink:show="embed" xlink:actuate="onLoad" /></draw:frame></text:p>
<text:p text:style-name="FigureCaption">A Caption</text:p>
<text:p text:style-name="Text_20_body">Text.</text:p>

The manually imported ODT looks like:

<text:p text:style-name="P1">
	<draw:frame draw:style-name="fr1" draw:name="Image1" text:anchor-type="char"
		svg:width="17.59cm" svg:height="5.777cm" draw:z-index="0">
		<draw:image xlink:href="Pictures/1000000000000B89000003CA2C349683.png"
			xlink:type="simple" xlink:show="embed" xlink:actuate="onLoad"
			draw:mime-type="image/png" />
	</draw:frame>
</text:p>

LibreOffice obviously knows the document size and can adjust the width and height appropriately. I assume the problem is Pandoc doesn't know the document size. For DOCX the size info is simply removed and Word chooses appropriately, but perhaps ODT demands a size and the problem is how to choose one? Is there any way to improve this from Pandoc's side?

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Here is the image (PPI removed) I used for testing if you want to reproduce this testcase.

[iandol]

As a workaround, a python macro for LibreOffice that resizes all too-big images to document width (thanks to ChatGPT though it took several rounds to get it to work):

https://codeberg.org/iandol/dotfiles/src/branch/main/bin/FitImagesToPageWidth.py

[iandol]

I checked the ODF spec and you can at least get relative scaling to the containing frame's width with style:rel-width="100%" style:rel-height="scale":

https://docs.oasis-open.org/office/v1.2/os/OpenDocument-v1.2-os-part1.html#__RefHeading__1420124_253892949

In fact Pandoc does already support this when setting an explicit image link attribute with {width=100%}:

<text:p text:style-name="FigureWithCaption">
  <draw:frame draw:name="img2" style:rel-width="100.0%" style:rel-height="scale" svg:width="3000.0pt" svg:height="985.0pt">
    <draw:image xlink:href="Pictures/1.png" xlink:type="simple" xlink:show="embed" xlink:actuate="onLoad" />
  </draw:frame>
</text:p>

...but at least my writing editor does not add link attributes by default so this doesn't help me. The easiest solution therefore is a Lua filter. This Lua filter sets images > 600px¹ to 100% width if no width attribute has been set:

-- if no image width passed set images > 600px to 100% width

function Image(im)
	local sizes = nil
	local mediatype, content = pandoc.mediabag.fetch(im.src)
	if mediatype and mediatype:match("^image/") then
		sizes = pandoc.image.size(content)
	end

	if sizes and sizes.width > 600 and not im.attributes.width then
		im.attributes.width = "100%"
		im.attributes.height = nil
	end
	return im
end

Having 100% set as the default of ODT conversions would certainly be better for me (most academic images fill the container width), but I recognise this would change existing behaviour and be detrimental for people who include many small images in their work, so Lua filters rock!

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Footnotes

1. 600px is very roughly 16cm for 96DPI display assumption, ignoring the DPI of the image. ↩