Use plot as blog post image

[cscheid]

Discussed in #2319

^{Originally posted by 11rchitwood September 6, 2022}
Let's say I have a post whose first code chunk is:

plot(cars)

How can I make this be the preview image for the post?

Being able to refer to easily refer to resources generated by the execution engines in our code is a good idea (I can imagine a similar feature being useful for the output of a dot or mermaid cell, or some arbitrary user shortcode)

[mcanouil]

To note, for blog post in Quarto website, if there is an image in the folder it was taken as the preview in the listing page.
So, it would only require to export the chunk as a raster image to the Qmd directory to have the preview.
This could easily be done at least for knitr with reusable code cell.

The above behaviour was observed at least until Quarto 1.0.36.
After I updated the yaml to explicitly list the image and added alt text.
See https://github.com/RLille/rlille.fr/tree/c6647dc8082caeadab3ec4ea32be3a4f013db192/posts/2022-01-24-golem.

[aammd]

I found this VERY challenging because the documentation states that this is possible

The image for this item, read from the image field of the front matter, or automatically discovered by taking the first of an image of class preview-image, an image with a file name starting with feature, cover, or thumbnail, or the first image to appear in the document.

[JonGretar]

Yeah that confuses me as well.

I found this VERY challenging because the documentation states that this is possible

[mcanouil]

Is this issue not fixed using the "preview-image" class as stated in the documentation? (I do not know since when)

```{r}
#| class-output: preview-image
plot(1)
```

[JonGretar]

No.
Nor does it indeed pick, if undefined, any plot as a preview image. It only picks the first static image it sees and leaves it previewless if none are found. I tried all the methods defined in the docs.

Is this issue not fixed using the "preview-image" class as stated in the documentation? (I do not know since when)

[dragonstyle]

starting in the next pre-release, this should be supported like so:

```{r}
#| classes: preview-image
plot(mtcars)
```

[cscheid]

(@dragonstyle I think I need to do this for mermaid charts, dot, etc)