feature: auto-discovery of images in markdown (see #4744)

Author: cscheid

src/project/types/website/util/discover-meta.ts

@@ -79,6 +79,13 @@ export function findPreviewImgEl(doc: Document): Element | undefined {
       return img;
     }
   }
+
+  // as a last resort, just use the auto-discovered image from the lua
+  // filter chain, if it exists
+  const autoImg = doc.querySelector("img.quarto-discovered-preview-image");
+  if (autoImg) {
+    return autoImg;
+  }
 }
 
 // The general words per minute that we should use.

src/resources/filters/main.lua

@@ -148,6 +148,7 @@ import("./quarto-pre/table-captions.lua")
 import("./quarto-pre/table-colwidth.lua")
 import("./quarto-pre/table-rawhtml.lua")
 import("./quarto-pre/theorems.lua")
+import("./quarto-pre/discover_preview_images.lua")
 
 import("./customnodes/decoratedcodeblock.lua")
 
@@ -218,7 +219,8 @@ local quartoPre = {
   }) },
   { name = "pre-quartoPreMetaInject", filter = quartoPreMetaInject() },
   { name = "pre-writeResults", filter = writeResults() },
-  { name = "pre-projectPaths", filter = projectPaths()},
+  { name = "pre-projectPaths", filter = projectPaths() },
+  { name = "pre-discover_preview_images", filter = discover_preview_images() }
 }
 
 local quartoPost = {

src/resources/filters/quarto-pre/discover_preview_images.lua

@@ -0,0 +1,16 @@
+-- discover_preview_images.lua
+-- Copyright (C) 2023 Posit Software, PBC
+
+local set = false
+function discover_preview_images() 
+  return {
+    Image = function(el)
+      if set then
+        return nil
+      end
+      set = true
+      el.classes:insert("quarto-discovered-preview-image")
+      return el
+    end
+  }
+end