FEATURE: Show a blurry preview when lazy loading images

This generates a 10x10 PNG thumbnail for each lightboxed image.
If Image Lazy Loading is enabled (IntersectionObserver API) then
we'll load the low res version when offscreen. As the image scrolls
in we'll swap it for the high res version.

We use a WeakMap to track the old image attributes. It's much less
memory than storing them as `data-*` attributes and swapping them
back and forth all the time.

app/assets/javascripts/discourse/lib/lazy-load-images.js.es6

@@ -2,25 +2,40 @@ const OBSERVER_OPTIONS = {
   rootMargin: "50%" // load images slightly before they're visible
 };
 
+const imageSources = new WeakMap();
+
+const LOADING_DATA =
+  "data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==";
+
 // We hide an image by replacing it with a transparent gif
 function hide(image) {
   image.classList.add("d-lazyload");
   image.classList.add("d-lazyload-hidden");
-  image.setAttribute("data-src", image.getAttribute("src"));
+
+  imageSources.set(image, {
+    src: image.getAttribute("src"),
+    srcSet: image.getAttribute("srcset")
+  });
+  image.removeAttribute("srcset");
+
   image.setAttribute(
     "src",
-    "data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw=="
+    image.getAttribute("data-small-upload") || LOADING_DATA
   );
+  image.removeAttribute("data-small-upload");
 }
 
-// Restore an image from the `data-src` attribute
+// Restore an image when onscreen
 function show(image) {
-  let dataSrc = image.getAttribute("data-src");
-  if (dataSrc) {
-    image.setAttribute("src", dataSrc);
-    image.classList.remove("d-lazyload-hidden");
+  let sources = imageSources.get(image);
+  if (sources) {
+    image.setAttribute("src", sources.src);
+    if (sources.srcSet) {
+      image.setAttribute("srcset", sources.srcSet);
     }
   }
+  image.classList.remove("d-lazyload-hidden");
+}
 
 export function setupLazyLoading(api) {
   // Old IE don't support this API

app/assets/stylesheets/common/base/lightbox.scss

@@ -1,6 +1,7 @@
 .lightbox-wrapper .lightbox {
   position: relative;
   display: inline-block;
+  overflow: hidden;
   background: $primary-low;
   &:hover .meta {
     opacity: 0.9;
@@ -9,7 +10,6 @@
 }
 
 .d-lazyload-hidden {
-  opacity: 0;
   box-sizing: border-box;
 }
 

app/models/optimized_image.rb

@@ -68,7 +68,7 @@ class OptimizedImage < ActiveRecord::Base
         Rails.logger.error("Could not find file in the store located at url: #{upload.url}")
       else
         # create a temp file with the same extension as the original
-        extension = ".#{upload.extension}"
+        extension = ".#{opts[:format] || upload.extension}"
 
         if extension.length == 1
           return nil
@@ -96,6 +96,7 @@ class OptimizedImage < ActiveRecord::Base
             url: "",
             filesize: File.size(temp_path)
           )
+
           # store the optimized image and update its url
           File.open(temp_path) do |file|
             url = Discourse.store.store_optimized_image(file, thumbnail)
@@ -173,7 +174,7 @@ class OptimizedImage < ActiveRecord::Base
   IM_DECODERS ||= /\A(jpe?g|png|tiff?|bmp|ico|gif)\z/i
 
   def self.prepend_decoder!(path, ext_path = nil, opts = nil)
-    extension = File.extname((opts && opts[:filename]) || ext_path || path)[1..-1]
+    extension = File.extname((opts && opts[:filename]) || path || ext_path)[1..-1]
     raise Discourse::InvalidAccess if !extension || !extension.match?(IM_DECODERS)
     "#{extension}:#{path}"
   end
@@ -189,10 +190,14 @@ class OptimizedImage < ActiveRecord::Base
     from = prepend_decoder!(from, to, opts)
     to = prepend_decoder!(to, to, opts)
 
+    instructions = ['convert', "#{from}[0]"]
+
+    if opts[:colors]
+      instructions << "-colors" << opts[:colors].to_s
+    end
+
     # NOTE: ORDER is important!
-    %W{
-      convert
-      #{from}[0]
+    instructions.concat(%W{
       -auto-orient
       -gravity center
       -background transparent
@@ -204,7 +209,7 @@ class OptimizedImage < ActiveRecord::Base
       -quality 98
       -profile #{File.join(Rails.root, 'vendor', 'data', 'RT_sRGB.icm')}
       #{to}
-    }
+    })
   end
 
   def self.resize_instructions_animated(from, to, dimensions, opts = {})
@@ -212,7 +217,7 @@ class OptimizedImage < ActiveRecord::Base
 
     %W{
       gifsicle
-      --colors=256
+      --colors=#{opts[:colors] || 256}
       --resize-fit #{dimensions}
       --optimize=3
       --output #{to}
@@ -248,7 +253,7 @@ class OptimizedImage < ActiveRecord::Base
     %W{
       gifsicle
       --crop 0,0+#{dimensions}
-      --colors=256
+      --colors=#{opts[:colors] || 256}
       --optimize=3
       --output #{to}
       #{from}

app/models/upload.rb

@@ -65,7 +65,8 @@ class Upload < ActiveRecord::Base
     opts = opts.merge(raise_on_error: true)
     begin
       OptimizedImage.create_for(self, width, height, opts)
-    rescue
+    rescue => ex
+      Rails.logger.info ex if Rails.env.development?
       opts = opts.merge(raise_on_error: false)
       if fix_image_extension
         OptimizedImage.create_for(self, width, height, opts)

lib/cooked_post_processor.rb

@@ -10,6 +10,8 @@ class CookedPostProcessor
 
   INLINE_ONEBOX_LOADING_CSS_CLASS = "inline-onebox-loading"
   INLINE_ONEBOX_CSS_CLASS = "inline-onebox"
+  LOADING_SIZE = 10
+  LOADING_COLORS = 32
 
   attr_reader :cooking_options, :doc
 
@@ -27,6 +29,8 @@ class CookedPostProcessor
     @doc = Nokogiri::HTML::fragment(post.cook(post.raw, @cooking_options))
     @has_oneboxes = post.post_analyzer.found_oneboxes?
     @size_cache = {}
+
+    @disable_loading_image = !!opts[:disable_loading_image]
   end
 
   def post_process(bypass_bump = false)
@@ -332,11 +336,19 @@ class CookedPostProcessor
           upload.create_thumbnail!(resized_w, resized_h, crop: crop)
         end
       end
+
+      unless @disable_loading_image
+        upload.create_thumbnail!(LOADING_SIZE, LOADING_SIZE, format: 'png', colors: LOADING_COLORS)
+      end
     end
 
     add_lightbox!(img, original_width, original_height, upload, cropped: crop)
   end
 
+  def loading_image(upload)
+    upload.thumbnail(LOADING_SIZE, LOADING_SIZE)
+  end
+
   def is_a_hyperlink?(img)
     parent = img.parent
     while parent
@@ -398,6 +410,10 @@ class CookedPostProcessor
       else
         img["src"] = upload.url
       end
+
+      if small_upload = loading_image(upload)
+        img["data-small-upload"] = small_upload.url
+      end
     end
 
     # then, some overlay informations

spec/components/cooked_post_processor_spec.rb

@@ -15,7 +15,7 @@ describe CookedPostProcessor do
       RAW
     end
 
-    let(:cpp) { CookedPostProcessor.new(post) }
+    let(:cpp) { CookedPostProcessor.new(post, disable_loading_image: true) }
     let(:post_process) { sequence("post_process") }
 
     it "post process in sequence" do
@@ -288,10 +288,22 @@ describe CookedPostProcessor do
           filesize: 800
         )
 
+        # Fake a loading image
+        OptimizedImage.create!(
+          url: 'http://a.b.c/10x10.png',
+          width: CookedPostProcessor::LOADING_SIZE,
+          height: CookedPostProcessor::LOADING_SIZE,
+          upload_id: upload.id,
+          sha1: SecureRandom.hex,
+          extension: '.png',
+          filesize: 123
+        )
+
         cpp = CookedPostProcessor.new(post)
 
         cpp.add_to_size_cache(upload.url, 2000, 1500)
         cpp.post_process_images
+        expect(cpp.loading_image(upload)).to be_present
 
         # 1.5x is skipped cause we have a missing thumb
         expect(cpp.html).to include('srcset="http://a.b.c/666x500.jpg, http://a.b.c/1998x1500.jpg 3x"')
@@ -379,7 +391,7 @@ describe CookedPostProcessor do
 
       let(:upload) { Fabricate(:upload) }
       let(:post) { Fabricate(:post_with_large_image) }
-      let(:cpp) { CookedPostProcessor.new(post) }
+      let(:cpp) { CookedPostProcessor.new(post, disable_loading_image: true) }
 
       before do
         SiteSetting.max_image_height = 2000
@@ -447,7 +459,7 @@ describe CookedPostProcessor do
 
       let(:upload) { Fabricate(:upload) }
       let(:post) { Fabricate(:post_with_large_image) }
-      let(:cpp) { CookedPostProcessor.new(post) }
+      let(:cpp) { CookedPostProcessor.new(post, disable_loading_image: true) }
 
       before do
         SiteSetting.create_thumbnails = true
@@ -462,7 +474,7 @@ describe CookedPostProcessor do
 
       it "crops the image" do
         cpp.post_process_images
-        expect(cpp.html).to match /width="690" height="500">/
+        expect(cpp.html).to match(/width="690" height="500">/)
         expect(cpp).to be_dirty
       end
 
@@ -472,7 +484,7 @@ describe CookedPostProcessor do
 
       let(:upload) { Fabricate(:upload) }
       let(:post) { Fabricate(:post_with_large_image) }
-      let(:cpp) { CookedPostProcessor.new(post) }
+      let(:cpp) { CookedPostProcessor.new(post, disable_loading_image: true) }
 
       before do
         SiteSetting.create_thumbnails = true
@@ -499,7 +511,7 @@ describe CookedPostProcessor do
 
       let(:upload) { Fabricate(:upload) }
       let(:post) { Fabricate(:post_with_large_image_on_subfolder) }
-      let(:cpp) { CookedPostProcessor.new(post) }
+      let(:cpp) { CookedPostProcessor.new(post, disable_loading_image: true) }
       let(:base_url) { "http://test.localhost/subfolder" }
       let(:base_uri) { "/subfolder" }
 
@@ -538,7 +550,7 @@ describe CookedPostProcessor do
 
       let(:upload) { Fabricate(:upload) }
       let(:post) { Fabricate(:post_with_large_image_and_title) }
-      let(:cpp) { CookedPostProcessor.new(post) }
+      let(:cpp) { CookedPostProcessor.new(post, disable_loading_image: true) }
 
       before do
         SiteSetting.max_image_height = 2000

spec/models/optimized_image_spec.rb

@@ -29,6 +29,21 @@ describe OptimizedImage do
       end
     end
 
+    describe ".resize_instructions" do
+      let(:image) { "#{Rails.root}/spec/fixtures/images/logo.png" }
+
+      it "doesn't return any color options by default" do
+        instructions = described_class.resize_instructions(image, image, "50x50")
+        expect(instructions).to_not include('-colors')
+      end
+
+      it "supports an optional color option" do
+        instructions = described_class.resize_instructions(image, image, "50x50", colors: 12)
+        expect(instructions).to include('-colors')
+      end
+
+    end
+
     describe '.resize' do
       it 'should work correctly when extension is bad' do
 
@@ -186,7 +201,6 @@ describe OptimizedImage do
   describe ".create_for" do
 
     it "is able to 'optimize' an svg" do
-
       # we don't really optimize anything, we simply copy
       # but at least this confirms this actually works
 
@@ -239,6 +253,11 @@ describe OptimizedImage do
           expect(oi.url).to eq("/internally/stored/optimized/image.png")
         end
 
+        it "is able to change the format" do
+          oi = OptimizedImage.create_for(upload, 100, 200, format: 'gif')
+          expect(oi.url).to eq("/internally/stored/optimized/image.gif")
+        end
+
       end
 
     end