Add alt text to images added to Ppt via rmarkdown

[jsgosnell]

When using rmarkdown to add images to PowerPoint files, the alt text designation in the image tag becomes the title/caption. If using a web-based image, the url becomes the alt text. This isn't optimal for screen readers (the main reason I'm trying to add alt text to the images) (see answer provided to question on stackoverflow by Josh: https://stackoverflow.com/questions/66959628/how-do-you-add-alt-text-for-accessibility-to-images-in-powerpoint-files-create). Is it possible to update the code so the alt text truly becomes alt text and the caption is handled elsewhere (like a fig.cap argument)?

Example of issue:

---
title: "Making presentations in R"
author: "jsg"
date: "3/29/2021"
output: 
  powerpoint_presentation
---

## Issue example:  Puffin image

![This should be alt text and not caption](https://upload.wikimedia.org/wikipedia/commons/9/94/Puffin_Mrkoww.jpg)

---

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[apreshill]

I'm seeing in both cases (local image and url) that the alt text populates the image path:

[apreshill]

Source:

---

## Alt text does not work with urls

```
![look a puffin](https://upload.wikimedia.org/wikipedia/commons/9/94/Puffin_Mrkoww.jpg "Title: A proper puffin"){fig.alt="alt text: it is a very cute puffin."}
```

![look a puffin](https://upload.wikimedia.org/wikipedia/commons/9/94/Puffin_Mrkoww.jpg "Title: A proper puffin"){fig.alt="alt text: it is a very cute puffin."}

---

## Alt text does not work with local images either

```
![look a puffin](images/Puffin_Mrkoww.jpg "Title: A proper puffin"){fig.alt="alt text: it is a very cute puffin."}
```

![look a puffin](images/Puffin_Mrkoww.jpg "Title: A proper puffin"){fig.alt="alt text: it is a very cute puffin."}

[cderv]

Just for context for me later, there are several things in this issue.

This markdown syntax

![This should be alt text and not caption](https://upload.wikimedia.org/wikipedia/commons/9/94/Puffin_Mrkoww.jpg)

will be processed by Pandoc and not knitr or rmarkdown directly. So better support for alt text in the Markdown syntax is something to do on the Pandoc side.

Doc about the syntax is here: https://pandoc.org/MANUAL.html#images

A link immediately preceded by a ! will be treated as an image. The link text will be used as the image’s alt text:

![la lune](lalune.jpg "Voyage to the moon")

But

Extension: implicit_figures
An image with nonempty alt text, occurring by itself in a paragraph, will be rendered as a figure with a caption. The image’s alt text will be used as the caption.

![This is the caption](/url/of/image.png)

So implit_figures extentions could have an impact, and this is not clear which formats supports what.

I have looked into this while working on adding fig.alt in knitr for figures. So this is related also to support for non HTML output format: yihui/knitr#1967
Main issues to support fig.alt chunk option for DOCX / PPTX is because this is buggy in Pandoc.

With DOCX, the syntax would work (in some way) - see yihui/knitr#1965 (comment)

![A plot](test_files/figure-docx/unnamed-chunk-1-1.png "Description")
Will lead to having

• "A plot" as a image caption/legend
• Both "A plot" and "Description" as alternative text description in WORD I got

Description

A plot

Oddly we don't get that with PPTX and I think you've found a new bug in Pandoc @apreshill ! It should at least be the same as with DOCX. I'll report that to Pandoc repo maybe.

Also I am curious about using {fig.alt="alt text: it is a very cute puffin."} as an attributes on Pandoc figure. Did you find this syntax somewhere ? I was not aware about this - I guess you tried it with Powerpoint because of link_attributes pandoc's extension ?

[apreshill]

Thank you for filing that issue @cderv!

[apreshill]

I tried the syntax I did because JJ told me to try: ![figure caption goes here](img.png){fig.alt="alt text here"} in another issue.

"You can just put fig.alt on any pandoc image (or use as a chunk attribute). Alternatively you can just use the native pandoc syntax for alt (quoted text after the image path). include_graphics should definitely not be required"

[cderv]

Just to share that this will be taken into account in Pandoc. It has already been merged (see the issue in pandoc repo) and it will be available in a future Pandoc version.

[apreshill]

Here is the PR: jgm/pandoc#7507

[cderv]

Since Pandoc 2.14.2, It is now possible to pass a description text to inlucde in Powerpoint using the Title part of the syntax
https://pandoc.org/MANUAL.html#images

![used as caption](link "Description")

So with original example this should be

---
title: "Making presentations in R"
author: "jsg"
date: "3/29/2021"
output: 
  powerpoint_presentation
---

## Issue example:  Puffin image

![This is the caption](https://upload.wikimedia.org/wikipedia/commons/9/94/Puffin_Mrkoww.jpg "And here is more information in description")

This require a recent Pandoc which is included in RStudio current Daily version, and will be in next release.

Thanks for opening this issue!

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