ImageSizingTreeprocessor for putting width and height attrs into images

README.adoc

@@ -97,6 +97,11 @@ Adds hardbreaks to the end of all non-empty lines that aren't section titles.
 HighlightTreeprocessor, link:lib/highlight-treeprocessor.rb[]::
 Highlights source blocks using the highlight command.
 
+ImageSizingTreeprocessor, link:lib/image-sizing-treeprocessor.rb[]::
+Embeds the dimensions of images and allows _either_ width or height scaling. 
+Solves the problem of the default stylesheet preventing height scaling (see related https://github.com/asciidoctor/asciidoctor/issues/1116[issue]).
+Assists with the existing problem that linking to an anchor inside a single html causes the undesirable behaviour where the page scrolls away from the anchor while the page is still rendering (see https://github.com/asciidoctor/asciidoctor/issues/820[issue]).
+
 ImplicitApidocInlineMacro, link:lib/implicit-apidoc-inline-macro.rb[]::
 Adds an inline macro for linking to the Javadoc of a class in the Java EE API.
 

lib/image-sizing-treeprocessor.rb

@@ -0,0 +1,7 @@
+RUBY_ENGINE == 'opal' ?
+  (require 'image-sizing-treeprocessor/extension') :
+  (require_relative 'image-sizing-treeprocessor/extension')
+
+Asciidoctor::Extensions.register do
+  treeprocessor ImageSizingTreeprocessor
+end

lib/image-sizing-treeprocessor/extension.rb

@@ -0,0 +1,73 @@
+require 'asciidoctor/extensions' unless RUBY_ENGINE == 'opal'
+require 'fastimage'
+
+include ::Asciidoctor
+
+class ImageSizingTreeprocessor < Extensions::Treeprocessor
+  def process document
+
+    images = document.find_by context: :image
+
+    images.each do |img|
+      adjust_image img, document
+    end
+
+    # we also need to image images in a| table cells
+    # this is not very elegant...
+    tables = document.find_by context: :table
+    tables.each do |t|
+      for acell in (t.rows.body + t.rows.foot).flatten.select {|c| c.attr? 'style', :asciidoc }
+        for img in acell.inner_document.find_by context: :image
+          adjust_image img, document
+        end
+      end
+    end
+
+  end
+
+  def adjust_image(img, document)
+
+    attrs = img.attributes
+
+    # is this necessarily constant? could it be removed out of the loop or passed in?
+    basedir =  (document.attr 'outdir') || ((document.respond_to? :options) && document.options[:to_dir])
+
+    uri = img.image_uri attrs['target']
+    uri_starts = ['http://', 'https://', 'ftp://']
+    # absolute path for local files
+    if ! uri.start_with? *uri_starts
+      uri = File.join basedir, uri
+    end
+
+    width_w, height_w = nil
+
+    width_w = (attrs['2'] if attrs.key? '2') \
+                 || (attrs['width'] if attrs.key? 'width')
+
+    height_w = (attrs['3'] if attrs.key? '3') \
+                 || (attrs['height'] if attrs.key? 'height')
+
+    # this might fail due to e.g. no network connection
+    # potentially timeout parameters might be useful?
+    begin
+      width, height = FastImage.size uri
+    rescue
+      $stderr.puts('Unable to get image parameters for: ' + uri)
+      return
+    end
+
+    # this always uses the image aspect ratio and any any user input
+    # width (obviously) dominates
+    if width_w
+      attrs['width'] = width_w
+      attrs['height'] = width_w.to_f/width*height
+    elsif height_w
+      attrs['width'] = height_w.to_f/height*width
+      attrs['height'] = height_w
+    else
+      attrs['width'] = width
+      attrs['height'] = height
+    end
+
+  end
+end

lib/image-sizing-treeprocessor/sample.adoc

@@ -0,0 +1,44 @@
+= Image Scaling
+
+Imagine we have lots of images
+
+Local images would work nicely too but to prevent repo bloat we will link to them.
+
+If we open this referring to the fifth section with something like `+++sample.html#_section_5+++` 
+with and without the extension we see quite different behaviour
+
+== Section 1
+
+image::https://openclipart.org/download/275473/FireDiamond.svg[]
+
+== Section 2
+
+image::https://openclipart.org/download/284400/GlitchSimplifiedFire.svg[]
+
+== Section 3
+
+image::https://openclipart.org/download/226036/fire-maze-mobile-casino.svg[]
+
+== Section 4
+
+image::https://openclipart.org/download/223224/fire-anthropomorphism.svg[]
+
+== Section 5
+image::https://openclipart.org/download/189740/Fire-Extinguisher-Sign--Arvin61r58.svg[]
+
+
+== Section 6
+
+These guys are the same height!
+
+image::https://openclipart.org/download/275473/FireDiamond.svg[height=50]
+
+image::https://openclipart.org/download/284400/GlitchSimplifiedFire.svg[height=50]
+
+image::https://openclipart.org/download/226036/fire-maze-mobile-casino.svg[height=50]
+
+image::https://openclipart.org/download/223224/fire-anthropomorphism.svg[height=50]
+
+image::https://openclipart.org/download/189740/Fire-Extinguisher-Sign--Arvin61r58.svg[height=50]
+
+