Force relative paths in knitr::include_graphics() - r

Question

I need the output to be portable so all the file paths need to be relative to the root directory of the project not absolute paths. I also need to set the figure size so I can't fall back on markdown and just use but knitr insists on creating an absolute path in my output. I've tried setting root.dir & base.dir and fig.path in various combinations but I keep getting an absolute path in my output. Setting root.dir = "/" does change the path in my output but setting it to "" or "./" still results in an abs path so this is not helping produce a portable output.
My rmarkdown::render is being run from the same directory as the Rmd file, the output is to this same directory and the graphics files are in a sub-directory of the one with the notebook. Is the a way I can make knitr just us the paths I give it and not try to transform them in any way?
Issues with knitr & paths seem to be a common problem and there are a lot of stub Q&As out there in search results which often don't satisfactorily resolve the problem, or if they do resolve it the questioner and/or answer seem unclear on why it worked.
I've referred to https://yihui.org/knitr/ & https://bookdown.org/yihui/rmarkdown/ and I still don't have a satisfactory understanding of how to control output paths with knitr could someone help me understand how to control this or point me in the direction of a resource that can?
OK so calmed down and I tried to make a reprex, I found some interesting behaviour.
The graphics folder is a symlink to a folder in the parent directory and this works fine if I use as basic html_document output but when I use the revealjs::revealjs_presentation output I get different results in the basics document the img tag contains:
src="data:image/jpeg;base64,/9j/4AAQSkZJRgABAQAAAQABAAD......."
and in the presentation contains:
src="/home/richardjacton/project/graphics_test/example.jpg"
Now this does not work because I need to be able to move the directory containing the presentation and it's assets to somewhere else and have it work once built.
If I go in with inspect element and edit it to src="./graphics/example.jpg" that works.
Also If I make a real directory instead of a symlink to the a directory in the parent then I get src="graphics/example.jpg" in the revealjs output
---
title: "Example"
author: "Richard J. Acton"
date: "`r Sys.Date()`"
# output:
# html_document:
# df_print: paged
output:
revealjs::revealjs_presentation:
theme: dark
highlight: zenburn
center: true
self_contained: false
reveal_plugins: ["notes"]
reveal_options:
slideNumber: true
previewLinks: true
fig_caption: true
---
# Example
```{r, echo=FALSE, include=FALSE, message=FALSE}
fs::dir_create("../graphics_test/")
download.file("https://i.ytimg.com/vi/0hMr5-05bFc/maxresdefault.jpg", "../graphics_test/example.jpg")
fs::link_create("../graphics_test", "graphics")
# fs::dir_create("graphics")
# download.file("https://i.ytimg.com/vi/0hMr5-05bFc/maxresdefault.jpg", "graphics/example.jpg")
```
```{r, out.width='80%', echo=FALSE}
knitr::include_graphics("graphics/example.jpg")
```
In my real case I'm creating a presentation from inside a {targets} project that's also has a {bookdown} project in it that produces a report based on the pipeline. So the presentation lives in a sub-directory so I can exclude it from the bookdown build. I explicitly built the presentation with an rmarkdown::render call as the knit button in the gui wants to try and render the book: rmarkdown::render("test.Rmd",envir = new.env()). Also this is all in a directory mounted to a docker container that's running my RStudio server instance so I have a portable and reproducible working environment for any non-R dependencies of my pipeline.
So this is why I was looking for a resource with a more in depth explanation of what exactly happens when I make a knitr::include_graphics call because this is the sort of weird edge case I seem to encounter a lot and get stuck trying to debug. Thanks for the response #YihuiXie I love your work and use it daily - sorry for offering you the opportunity to engage with this particular headache.