Using Pandoc with Org Mode to Generate PowerPoint Slides

Introduction

To generate HTML content I rely on org mode's HTML export functionality.
I had used pandoc (from John Macfarlane) for converting content in other formats to org mode
For a bigger project, I needed Microsoft PowerPoint slides
As I mostly use org mode for writing, I evaluated pandoc to generate PowerPoint pptx files. Documentation:

Summary

Using pandoc, you can generate Powerpoint slides easily
pandoc supports a core set of slide types
- title slide, divider slide
- bulleted lists
- tables
- "stand alone" images and images with text, possibly bulleted
You might want to use the latest pandoc version (I used 3.1), as there had been quite some improvements and bug fixes
More detail at https://web222.webclient5.de/doc/swdev/emacs/orgmode/pandoc
The test files can be found in a zip file org2pptx.zip

Telling pandoc How the Resulting pptx File Shall Look

Pandoc has two main mechanisms to influence the look of the result document:

Templates e.g. for HTML output
Reference documents e.g. for docx and pptx output

The difference seems to be that templates can also have content that will be copied to the output document, whereas reference documents only provide "styles" that will influence the look of the output document.

By default, pandoc looks for the files /home/user/.local/share/pandoc/reference.docx or /home/user/.local/share/pandoc/reference.pptx as reference document. However, if you provide a --reference-doc=file command line argument, pandoc will look for file in the "resource-path" and use file as the reference document instead. The resource-path is the working directory by default. You can give an absolute path as file. There is also the possibility to specify the resource-path.

Options to Run the Conversion

I use org-babel to run pandoc directly from the org source file and invoke PowerPoint to show the result
For more complex projects, I use a Makefile

Observations

Structure of the Presentation

Whether or not to generate "divider" slides, is described in the org manual under "Structuring the slide show"
To map from org-mode headings to slides, pandoc has the concept of "slide level"
- this is the org-mode heading level that maps to the slide headers

Localised Master Slide Names Need To Be Renamed 1/2

Per slide, pandoc picks the appropriate template slide from the slide master in the template file
pandoc looks for a standard english name of the template slide
When using a non-English localised version of PowerPoint, you have to rename the slide master templates slides to:
- Title Slide
- Title and Content
- Section Header
- Two Content
- Content with Caption
- Blank
- Comparison

Localised Master Slide Names Need To Be Renamed 2/2

If the names do not match, pandoc will not find the master template slide, complain and use some default
In older pandoc versions, master slides were picked by pandoc by position not by name
Usually there are some more master template slides. The 2016 de version of PowerPoint 2016 has
- Nur Titel
- Bild mit Überschrift
- Titel und vertikaler Text
- Vertikaler Titel und Text
These would just not be used by pandoc and hence do not need renaming

Showing Source Code or Similar Content

Code blocks (i.e. with colon, #+begin_src, #+begin_example) are indented. This might be configurable on the pandoc side

Tables

pandoc uses the default table layout that you define in the PowerPoint template file
org-mode automatically right aligns columns with figures in Emacs, whereas pandoc needs a first row with <r> type hints
Columns in HTML tables have variable width. pandoc generates PowerPoint table columns with all the same widths

Images - Shape Types

An org-mode link to a JPEG file becomes in PowerPoint a shape of type msoPicture (13), not msoLinkedPicture (11). I.e. the image becomes embedded in the PPTX file
Similarely, a linked SVG file becomes msoGraphic (28), not msoLinkedGraphic (29)

Images - Scaling

PowerPoint
- uses DPI meta information from the image file when you insert an image
- uses a default of 96 dpi for SVG images, which do not contain DPI information
pandoc does not seem to use the DPI meta information. It scales images so that they fully fill the content area
- The aspect ratio of the images is kept
- For some SVG files, the aspect ratio got changed a bit by pandoc. It turned out, these SVG files had an UTF-8 BOM (byte order mark) of 0xef 0xbb 0xef as the first three bytes in the SVG file. This will be fixed in a future version of pandoc

Images - Captions and Links to Images

Image captions provided in markdown input work, but do not work from org mode input in V3.1. It did work in V2.9.2.1 though.
If an image link contains a link text, the image will not be embedded. Rather a link will be embedded, with an underlined link text

Images - Limitations

You cannot have multiple images on the same slide
If there is text right before before an image link, the image does not appear in the PPTX
Text after an image link is put onto another slide. This might be unavoidable because there seems to be no notion of in-line images in PowerPoint

Slide Footer

The PowerPoint output contains the footer information as given in the PowerPoint template file:

Slide numbers are set automatically.
The date field is filled with the org-mode #+DATE: field, even if the date was set to "manual" in the template
The footer text field is not filled by pandoc. It will have the content as was provided by the PowerPoint template file, if any. Footer information does not seem to be part of pandoc's internal data structure. There has been discussion about this on the Internet, related to docx output. This might apply to pptx output as well.

Covered Functionality

Scope

Two factors determine, what PowerPoint functionality can be used from org mode via pandoc conversion:

What PowerPoint functionality is accessible from pandoc
What pandoc functionality is accessible from org mode

Regards the PowerPoint output format, the pandoc documentation elaborates about

The needed template slide layouts
"fenced_code_attributes" Extension
A markdown code example
Speaker notes

Markdown and Org Mode

On the Internet, there are often markdown examples to convert using pandoc
The pandoc manual itself seems to be written in markdown
pandoc cannot access Emacs/org mode settings in init.el
pandoc might access in-buffer org-mode settings

Because of this, my first attempt to scope the supported functionality was to convert the provided markdown example to org mode using pandoc.

What about Generating DOCX Files?

Generating Microsoft Word docx files using pandoc follows the same mechanics as outlined above for pptx.

However I noticed the following issue (pandoc version 3.1.8):

The same org file referencing an SVG graphics would be processed by pandoc OK and properly show the graphics in the resulting pptx. However for docx, pandoc would show a warning:

Could not convert image process file.svg:
check that rsvg-convert is in the path.

When opening with MS Word 2016, the resulting docx file did show the graphics. But when opened with LibreOffice Writer version 7.4.7.2, the graphics did not show, rather an empty frame with some place holder grapics.

rsvg-convert is a command line utility to convert SVG to PNG. In Debian Bookworm, it is contained in package librsvg2-bin.

On the other hand, this LibreOffice version does support inserted SVG graphics.